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Chemistry & Biochemistry Graduate Theses &
Dissertations
Chemistry & Biochemistry
Spring 1-1-2014
Laboratory Studies of Heterogeneous Ice
Nucleation: Implications for Cirrus Cloud
Formation
Gregory Park Schill
University of Colorado Boulder, [email protected]
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LABORATORY STUDIES OF HETEROGENEOUS ICE NUCLEATION:
IMPLICATIONS FOR CIRRUS CLOUD FORMATION
by
GREGORY PARK SCHILL
B.S., Gonzaga University, 2008
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in patrial fulfillment
of the requirements for the degree of
Doctor of Philosophy
Department of Chemistry and Biochemistry
2014
The thesis entitled:
Laboratory Studies of Heterogeneous Ice Nucleation:
Implications for Cirrus Cloud Formation
written by Gregory Park Schill
has been approved by the Department of Chemistry and Biochemistry
________________________________________
Margaret A. Tolbert
________________________________________
Barbara Ervens
Date ________________
The final copy of this thesis has been examined by the signatories, and we
find that both the content and the form meet acceptable presentation standards
of scholarly work in the above mentioned discipline
Schill, Gregory Park (Ph.D., Chemistry)
Laboratory Studies of Heterogeneous Ice Nucleation: Implications for Cirrus Cloud
Formation
Thesis directed by Distinguished Professor Margaret A. Tolbert
ABSTRACT
Cirrus clouds, composed of water ice, cover 30% of the global surface and affect climate
by scattering incoming solar radiation and scattering and absorbing outgoing terrestrial radiation.
Despite their ubiquity, ice clouds are not accurately represented in global climate models and
provide one of the largest uncertainties in predictions of climate change. This is, in part, because
our understanding of ice particle formation processes in the atmosphere is low. Recent evidence
has shown that pure ice clouds are dominated by heterogeneous ice nucleation, and, therefore,
are dependent on presence of rare (<1 in 105) ice freezing nuclei. While it is known that mineral
dust aerosol is an efficient ice nucleus, much less is known about organic-inorganic species,
which composed a significant fraction of cirrus ice residuals.
In this study we have probed the ice nucleation efficiencies of organic, organic-sulfate,
and organic-sea-salt species using a Knudsen cell flow reactor and a Raman microscope coupled
to an environmental cell. Specifically, we have probed the ice nucleation efficiency of thin films
of crystalline monocarboxylic acids from 180 to 200 K, liquid-liquid phase separated particles
containing ammonium sulfate and organic polyols from 210 to 235 K, simulated secondary
organic aerosol derived from aqueous processes with and without ammonium sulfate from 215 to
230 K, and synthetic sea-salt particles with and without a proxy for dissolved organic carbon
from 215 to 225 K. The hygroscopic phase transitions of the above particles were also explored
up to 260 K. From these experiments we find that certain subsets of organic-inorganic particles
iii
may be very efficient heterogeneous ice nuclei in the upper troposphere. These subsets include
crystalline organics with a high O:C ratio, sugar-like glassy organics particles or coatings/shells,
and effloresced ammonium sulfate or sea-salt with liquid organic/inorganic coatings.
iv
To Mom, Grandma, Andrew, and Hallie.
Thank you for all of your love and support.
ACKNOWLEDGEMENTS
First and foremost, I would like to acknowledge my advisor, Maggie Tolbert, whose
guidance and support throughout my graduate school career has been truly invaluable. I would
also like to acknowledge the entire Tolbert Group, past and present, for their support throughout
my graduate school experience. In particular, Kelly Baustian and Matt Wise designed and
developed the Raman Microscope system for these ice nucleation experiments and entrusted it to
me in excellent working order. Finally, I would also like to acknowledge my collaborator David
De Haan for introducing me to and providing expertise in an area of research I could not have
explored own. This work was supported by grants from the National Science Foundation’s
Divisions of Atmospheric Sciences and Atmospheric and Geophysical Sciences.
vi
TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION .................................................................................... 1
OVERVIEW OF THE RESEARCH .............................................................................. 1
THE UPPER TROPOSPHERE AND CIRRUS CLOUDS ............................................ 1
CLIMATIC IMPACT OF CIRRUS CLOUDS............................................................... 2
ICE NUCLEATION MECHANISMS ............................................................................ 3
BACKGROUND AEROSOL AT CIRRUS ALTITUDES AND CIRRUS ICE
RESIDUALS............................................................................................................................... 5
THESIS FOCUS ............................................................................................................. 7
REFERENCES ............................................................................................................. 10
CHAPTER 2: DEPOSITIONAL ICE NUCLEATION ON MONOCARBOXYLIC
ACIDS: EFFECT OF THE O:C RATIO ................................................................................. 12
INTRODUCTION ........................................................................................................ 12
EXPERIMENTAL ........................................................................................................ 13
Knudsen Cell. ............................................................................................................ 13
Organic Film Preparation. ......................................................................................... 16
Ice Nucleation Experiment. ...................................................................................... 16
Chemicals. ................................................................................................................. 18
RESULTS AND DISCUSSION ................................................................................... 18
ATMOSPHERIC IMPLICATIONS ............................................................................. 27
vii
SUMMARY AND CONCLUSION ............................................................................. 30
REFERENCES ............................................................................................................. 31
CHAPTER
3:
HETEROGENEOUS
ICE
NUCLEATION
ON
PHASE-
SEPARATED ORGANIC-SULFATE PARTICLES: EFFECT OF LIQUID VS. GLASSY
COATINGS ................................................................................................................................. 36
INTRODUCTION ........................................................................................................ 36
EXPERIMENTAL ........................................................................................................ 40
Sample Preparation. .................................................................................................. 40
Raman Microscopy and Environmental Cell. ........................................................... 40
Deliquescence, Liquid-Liquid Phase Separation, and Efflorescence Experiments. . 43
Ice Nucleation Experiments. ..................................................................................... 47
RESULTS ..................................................................................................................... 48
Low Temperature Deliquescence/Efflorescence and Liquid-Liquid Phase Separation
of Organic-Sulfate Particles. ................................................................................................. 48
Particle Morphology of Effloresced Organic-Sulfate Particles. ............................... 50
Ice Nucleation Experiments. ..................................................................................... 54
Ice Nucleation on Solid Amorphous Organic Particles above the Glass Transition
Line. ...................................................................................................................................... 63
ATMOSPHERIC IMPLICATIONS ............................................................................. 67
CONCLUSIONS........................................................................................................... 70
REFERENCES ............................................................................................................. 72
viii
CHAPTER 4: HETEROGENEOUS ICE NUCLEATION ON SIMULATED
SECONDARY ORGANIC AEROSOL .................................................................................... 76
INTRODUCTION ........................................................................................................ 76
EXPERIMENTAL ........................................................................................................ 78
Raman Microscope. .................................................................................................. 78
Sample Preparation. .................................................................................................. 79
Impact-Flow Experiments. ........................................................................................ 80
Ice Nucleation Experiments. ..................................................................................... 81
RESULTS AND DISCUSSION ................................................................................... 82
ATMOSPHERIC IMPLICATIONS ............................................................................. 93
REFERENCES ............................................................................................................. 99
CHAPTER 5: HETEROGENEOUS ICE NUCLEATION ON SIMULATED SEASPRAY AEROSOL USING RAMAN MICROSCOPY ........................................................ 102
INTRODUCTION ...................................................................................................... 102
EXPERIMENTAL ...................................................................................................... 105
Raman Microscope. ................................................................................................ 105
Materials. ................................................................................................................ 105
Particle Generation.................................................................................................. 106
Deliquescence and Ice Nucleation Experiments. .................................................... 106
Water Uptake Experiments. .................................................................................... 108
ix
Quality Assurance/Quality Control......................................................................... 110
RESULTS AND DISCUSSION ................................................................................. 110
ATMOSPHERIC IMPLICATIONS ........................................................................... 121
REFERENCES ........................................................................................................... 125
CHAPTER 6: SUMMARY AND CONCLUSION..................................................... 128
CHAPTER 7: BIBLIOGRAPHY ................................................................................ 132
CHAPTER 8: APPENDIX ........................................................................................... 143
x
TABLES
Table 1. Physical Constants, Average Surface Concentrations (O/nm2), and O:C ratios
for the Organics Studied in This Work. ........................................................................................ 22
xi
FIGURES
Figure 1.1. Radiative forcing estimates in 2011 relative to 1750 and aggregated
uncertainties for the main drivers of climate change ...................................................................... 4
Figure 1.2. Ice nucleation mechanisms within ice supersaturation versus temperature
space ................................................................................................................................................ 6
Figure 1.3. Flight tracks of ice cloud–residual measurements for four aircraft campaigns
spanning a range of geographic regions and seasons...................................................................... 8
Figure 2.1. Schematic of the Knudsen cell flow reactor setup. ....................................... 14
Figure 2.2. Typical FTIR-RAS spectra of butyric acid immediately before and after ice
nucleation ...................................................................................................................................... 17
Figure 2.3. Saturation ratioand integrated absorbance of the ice libration peak as a
function of time for a typical ice nucleation experiment on butyric acid at 180 K ...................... 19
Figure 2.4. Critical saturation ratios required to nucleate ice on thin organic films as a
function of temperature and O:C Ratio......................................................................................... 20
Figure 2.5. Calculated effective contact angles as a function of O:C ratio for the films
used in this study........................................................................................................................... 25
Figure 2.6. Oxygen per square nanometer as a function of O:C ratio for the organic films
used in this study........................................................................................................................... 28
Figure 3.1. Schematic representation of the Raman microscope setup ........................... 41
Figure 3.2. Experimental trajectories for deliquescence/efflorescence and ice nucleation
shown in relative humidty and ice saturation ratio space ............................................................. 44
Figure 3.3. Raman spectra of the center of deliquesced C6/AS, the edge of deliquesced
C6/AS, the core of a C6/AS particle after LLPS, the shell of a C6/AS particle after LLPS, the
xii
core of a C6/AS particle after efflorescence, and the shell of a C6/AS particle after efflorescence
....................................................................................................................................................... 45
Figure 3.4. Hygroscopic phase transitions of C6/AS and C6/C10/AS ............................ 49
Figure 3.5. Two-dimensional Raman spectral map of a phase-separated, effloresced
C6/AS particle at 298 K ................................................................................................................ 51
Figure 3.6. Possible morphologies of particles after liquid-liquid phase separation and
efflorescence ................................................................................................................................. 53
Figure 3.7. Scrit as a function of temperature for homogeneous freezing of pure C6 and
immersion freezing of C6 by the AS core .................................................................................... 55
Figure 3.8. Optical microscope images of ice nucleating on the effloresced AS core of a
C6/AS particle that has undergone LLPS at 215 K ...................................................................... 56
Figure 3.9. Optical microscope image and corresponding Raman line map of
C6/AS/H2O-ice at 215 K............................................................................................................... 58
Figure 3.10. Scrit as a function of temperature for pure C6/C10 and C6/C10/AS ............ 59
Figure 3.11. Optical microscope images of ice nucleating on the shell of a C6/C10/AS
particle that has undergone LLPS at 215 K .................................................................................. 61
Figure 3.12. Optical microscope image and corresponding Raman line map of
C6/C10/AS/H2O-ice at 215 K ...................................................................................................... 62
Figure 3.13. The glass transition temperatures of C6, C10, and 1:1 C6/C10 shown as a
function of water activity .............................................................................................................. 64
Figure 4.1. 50x image of methylglyoxal + methylamine aqSOA particles freshly dried
and immediately after impact........................................................................................................ 83
xiii
Figure 4.2. A typical image series of a flow experiment on a shattered methylglyoxal +
methylamine aqSOA particle ........................................................................................................ 85
Figure 4.3. The Start of Flow RH and onset ice saturation ratios for depositional
nucleation on methylglyoxal + methylamine aqSOA as a function of temperature ..................... 86
Figure 4.4. 50x images of ice depositionally nucleating on a methylglyoxal +
methylamine aqSOA particle, and the immersion freezing of aqSOA by AS islands .................. 88
Figure 4.5. A typical image series of a deliquescence experiment on methylglyoxal +
methylamine aqSOA + AS ........................................................................................................... 90
Figure 4.6. The deliquescence RH of AS islands in
a liquid methylglyoxal +
methylamine aqSOA matrix and the onset ice saturation ratios for immersion mode nucleation of
methylglyoxal + methylamine aqSOA by AS island as a function of temperature ...................... 92
Figure 4.7. The onset ice saturation ratios for heterogeneous ice nucleation on single and
complex amorphous organic (semi-)solids and heterogeneous ice nucleation on methylglyoxal +
methylamine aqSOA and immersion freezing of aqSOA by AS islands...................................... 95
Figure 5.1. Raman spectra and 50x gray-scaled and binary contrast enhanced images
from a typical water uptake experiment along the 215 K experimental trajectory ..................... 109
Figure 5.2. The full DRH and onset ice saturation ratios for heterogeneous ice nucleation
on effloresced SSS particles as a function of temperature .......................................................... 111
Figure 5.3. Water uptake relative humidities for pure, effloresced SSS particles from 225
to 255 K....................................................................................................................................... 113
Figure 5.4. 50x images of an SSS particle dry, during a typical ice experiment, at Sice ~ 1,
and after it has nucleated ice at 215 K ........................................................................................ 114
Figure 5.5. 100x line map of the SSS particle that has nucleated ice in Figure 5.4d ..... 116
xiv
Figure 5.6. The full deliquescence RHs and onset ice saturation ratios for heterogeneous
ice nucleation on effloresced SSS:sucrose particles as a function of temperature ..................... 117
Figure 5.7. 50x images of an SSS:sucrose particle dry, during a typical ice experiment, at
Sice ~ 1, and after it has nucleated ice at 215 K ........................................................................... 119
Figure 5.8. 100x line map of the SSS:sucrose particle that has nucleated ice in Figure
5.6d.............................................................................................................................................. 120
Figure A.1. 50x images of an effloresced LiCl particle before and after impact........... 143
Figure A.2. A 50x image, 100x Raman 2D maps, and a 100x Raman spectrum taken
from the center of a representative aqSOA + AS particle at 298 K and 0% RH ........................ 144
xv
CHAPTER 1: INTRODUCTION
OVERVIEW OF THE RESEARCH
The focus of this thesis was to probe the heterogeneous ice nucleation properties of
organic-inorganic aerosol relevant to the atmosphere in the laboratory setting. The results
outlined in this study have potentially important roles in the formation of cirrus clouds in the
atmosphere. In turn, the details surrounding the formation of cirrus clouds may affect the extent
of their radiative effects. To help put the context of this thesis into a better perspective, this
chapter will outline the general characteristics of the upper troposphere and cirrus clouds, the
climatic impact of cirrus clouds, and the role of heterogeneous ice nucleation in the formation of
these high level clouds.
THE UPPER TROPOSPHERE AND CIRRUS CLOUDS
The Earth’s atmosphere contains several distinct layers that are determined largely by
their temperature structure. The lowest, and the one in which we reside, is the troposphere, which
extends from the Earth’s surface to the tropopause. The height of the tropopause depends both on
the latitude and time of the year, and ranges from approximately 8 km at the poles to 18 km at
the equator [Seinfeld and Pandis, 2006]. The upper troposphere, which this work defines as
anywhere between 4 km and the tropopause, represents the coldest regions of the troposphere
(typically 253-203 K). Further, the upper troposphere is also marked by low aerosol number
densities (typically 102-103 cm-3) with modes in the number distributions corresponding to 0.01
and 0.25 µm, respectively. Under these cold conditions, clouds containing ice particles can form.
Below 0 °C, clouds can be pure liquid, pure ice, or both; below approximately -38 °C, clouds
exist only as pure ice [Koop et al., 2000]. These clouds generally form from the updraft of
1
humidified air parcels, which expand and cool as they rise, creating supersaturated conditions.
The principal mechanisms for generating upper tropospheric ice clouds include the detrainment
from deep convective clouds and in situ formation from synoptic motions [Waliser et al., 2009].
The microstructure of clouds has been well studied in the past [Cziczo and Froyd, 2014;
Pruppacher and Klett, 1996]; from these studies, it is apparent that ice particles in cirrus span a
variety of non-spherical shapes from plates and columns to bullet rosettes and aggregates,
typically range between <1 to 100 L-1 in concentration, and have dimensions of 1-8000 µm.
CLIMATIC IMPACT OF CIRRUS CLOUDS
Clouds cover over two-thirds of the Earth’s surface and affect climate by scattering
incoming solar radiation and scattering and absorbing outgoing terrestrial radiation [Baker,
1997]. Clouds also regulate the atmospheric hydrological cycle, transporting latent heat upwards
and modifying atmospheric circulation [Levin and Cotton, 2008]. About 40-50% of all clouds are
high-level ice clouds, according to instruments most sensitive to thin cirrus [Stubenrauch et al.,
2013]. Because of their location in the Earth’s atmosphere, these high level clouds absorb
longwave terrestrial radiation, but emit radiation at very low temperatures compared to clear-sky
conditions, providing a net positive cloud radiative forcing [McFarquhar et al., 2000]. In fact,
the longwave radiative effect of all clouds is primarily due to high level clouds [Boucher et al.,
2013]; thus the effect of these clouds on the earth’s radiation budget can be significant. In
addition to their cloud radiative effects, thin cirrus found also serve to dehydrate air ascending in
the tropical tropopause layer, effectively regulating the stratospheric humidity [Jensen and
Pfister, 2004]. The relative influence of these effects, however, depends strongly on their height,
thickness, and optical properties [Waliser et al., 2009]. These properties have their origins in
cloud microphysical properties, which, in turn, depend on the ability of upper tropospheric
2
aerosol particles to initiate ice. Unfortunately, ice forming particles in the free troposphere are
extremely rare [<1 in 105 particles nucleate ice [Rogers et al., 2001]] and, therefore, present an
extremely difficult measurement challenge. Because of this measurement challenge, laboratory
studies of ice nucleation have proven useful in providing empirical frameworks of ice nucleation
of atmospheric aerosol [Niemand et al., 2012]. Currently, however, simulations of climate
forcing by clouds are of questionable validity when ice processes are involved because of the
overly simplified parameterizations that are used to represent the formation of ice crystals
[Baumgardner et al., 2012]. Indeed, in their Fifth Assessment Report, the International Panel for
Climate Change has indicated that cloud adjustments due to aerosols are the only main driver of
climate change whose radiative forcing estimate has a low level of confidence (Figure 1.1).
ICE NUCLEATION MECHANISMS
For an ice particle to form spontaneously from the vapor phase, supersaturations as high
as several hundred percent are required, which are not attainable in the natural troposphere
[Pruppacher and Klett, 1996]. Thus, ice particles in the atmosphere must form on preexisting
aerosol particles in the upper troposphere. In the simplest case, called homogeneous freezing, ice
embryos nucleate directly out of the liquid phase. For supercooled, dilute liquid droplets this
occurs at about -38 °C; for liquid droplets with higher concentrations of solutes, the freezing
temperature will be depressed, and homogeneous freezing will require even lower temperatures.
Previously, it was thought that homogeneous nucleation would dominate cirrus cloud
formation; however, recent field [Cziczo et al., 2013; Froyd et al., 2010] and modelling [Jensen
et al., 2010] evidence suggests that a heterogeneous mechanism better explains cloud numbers
and size distributions. There are four (or more) main sub-mechanisms of heterogeneous ice
3
Figure 1.1. Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for
the main drivers of climate change. Figure SPM.5 from IPCC [2013].
4
nucleation as defined by Vali [1985]. Depositional nucleation is the formation of ice by vapor
deposition directly onto the surface of a heterogeneous ice nucleus (IN). Immersion freezing is
initiated by an IN suspended within or surrounding a supercooled liquid droplet. Condensation
freezing involves the activation of liquid water onto a cloud condensation nucleus, which
immediately induces freezing of the condensate in the immersion mode. Finally, contact freezing
occurs when the surface of a supercooled liquid droplet comes in contact, either outside-in or
inside-out, with an IN.
These sub-mechanisms can be seen more clearly in Figure 1.2. Here, each mechanism is
shown within ice supersaturation versus temperature space. This study focuses on depositional
and water sub-saturated immersion freezing mechanisms below 240 K, which are, by definition,
the mechanisms that are most likely responsible for heterogeneous nucleation of cirrus clouds.
BACKGROUND AEROSOL AT CIRRUS ALTITUDES AND CIRRUS ICE RESIDUALS
One of the most common methods used for separating cloud elements from
background aerosol is with a counter-flow virtual impactor (CVI, [Cziczo and Froyd, 2014]).
This technique uses a counter-flow of air as an inertial barrier to smaller aerosol particles, but
allows more massive ice particles to into an inlet where the ice particle will be sublimed and the
resultant ice residual can be analyzed for composition. The two most comment methods for
compositional analysis are the collection of ice residuals on an electron microscopy grid for
offline analysis and single particle mass spectrometry for in situ analysis. Previous studies of
upper tropospheric aerosol have determined that background aerosol are dominated by sulfates
and organics [Froyd et al., 2009; Murphy et al., 2006]. Ice residual populations, however, differ
from the background aerosol populations, which indicate that a heterogeneous process dictates
5
Figure 1.2. Ice nucleation mechanisms within ice supersaturation versus temperature space. The
solid line refer to water and ice saturation respectively and the dashed line represents the ice
saturation ratios required for homogeneous nucleation of an aqueous droplet [Koop et al., 2000].
6
cirrus formation. This is better seen in Figure 1.3, which quantifies the background aerosol, ice
residuals, and flight tracks of 4 separate aircraft campaigns sampling cirrus produced with and
without convection [D. J. Cziczo et al., 2013]. As shown, mineral dust/metallic particles are the
dominant heterogeneous IN, however, 9-16% of IN across all flights are composed of sulfateorganic type particles. Further, for subvisible cirrus forming above 14 km, both background
aerosol and ice residuals are dominated by sulfate-organic type particles; here, a heterogeneous
mechanism was implied from ice number densities and cloud particle size distributions [Froyd et
al., 2010]. Additionally, it can be seen in Figure 1.3 that sea-salt aerosol often comprise the
second-most abundant IN behind mineral dust. Despite their abundances in cirrus ice residuals,
the heterogeneous ice nucleation efficiency of both organic-sulfate and organic-sea-salt aerosol
are vastly understudies when compared to mineral dust [Hoose and Moehler, 2012].
THESIS FOCUS
This thesis focuses on the heterogeneous ice nucleation properties of laboratory
proxies of organic-sulfate and organic-sea-salt particles. By accurately knowing what
compositions and morphologies produce efficient ice nuclei, it is hoped that a better
understanding of cirrus cloud formation and its subsequent cloud radiative effects will be
attained.
Within each chapter, the following format is used to describe each set of experiments and
results. A brief introduction discusses the motivation for the research in the context of this
introduction. Next, an experimental section outlines the specific details of the experimental
apparatus and the techniques. Results of the experiment are then given and discussed
7
Figure 1.3. Flight tracks of ice cloud–residual (IR) measurements for four aircraft campaigns
spanning a range of geographic regions and seasons. The composition of cirrus IRs and nearcloud aerosol, on a number basis, are summarized for each campaign. Ice-crystal separation was
accomplished with a counterflow virtual impactor inlet, and composition was determined by
single-particle mass spectrometry. IR particles were also collected and analyzed with electron
microscopy coupled to energy-dispersive x-ray microanalysis. Figure 1 from Cziczo et al.
[2013].
8
simultaneously, followed by a short atmospheric implications/conclusions section. For clarity,
each chapter possesses its own self-contained reference list. Further, to assist the reader,
abbreviations are redefined in each chapter.
Specifically, Chapter 3 describes the depositional freezing behavior of a series of thin
films of monocarboxylic acids from 180-200 K. Chapter 4 describes the deliquescence,
efflorescence, liquid-liquid phase separation (LLPS) behavior of particles containing ammonium
sulfate and a liquid organic or a highly-viscous (semi-)solid organic from 240-265 K.
Additionally, ice nucleation on each of the organics as well as their corresponding LLPS
particles was probed from 210-235 K. Chapter 5 discusses the ice nucleation behavior and
humidity induced glass transition of secondary organic aerosol derived from aqueous processes
with and without internally-mixed ammonium sulfate from 215-250 K. Chapter 6 discusses the
heterogeneous ice nucleation behavior of synthetic sea-salt particles with and without sucrose
from 215 to 235 K. Sucrose was used as a proxy for dissolved organic carbon. Finally, this thesis
ends a brief summary of all experiments in Chapter 7.
9
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Baker, M. B. (1997), Cloud microphysics and climate, Science, 276(5315), 1072-1078.
Baumgardner, D., et al. (2012), IN SITU, AIRBORNE INSTRUMENTATION
Addressing and Solving Measurement Problems in Ice Clouds, Bulletin of the American
Meteorological Society, 93(2), E529-E534.
Boucher, O., et al. (2013), Clouds and Aerosols, in Climate Change 2013: The Physical
Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change, edited by T. F. Stocker, D. Qin, G. K. Plattner, M.
Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley, Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA.
Cziczo, D. J., and K. D. Froyd (2014), Sampling the composition of cirrus ice residuals,
Atmospheric Research, 142, 15-31.
Cziczo, D. J., K. D. Froyd, C. Hoose, E. J. Jensen, M. Diao, M. A. Zondlo, J. B. Smith,
C. H. Twohy, and D. M. Murphy (2013), Clarifying the Dominant Sources and Mechanisms of
Cirrus Cloud Formation, Science, 340(6138), 1320-1324.
Froyd, K. D., D. M. Murphy, P. Lawson, D. Baumgardner, and R. L. Herman (2010),
Aerosols that form subvisible cirrus at the tropical tropopause, Atmospheric Chemistry and
Physics, 10(1), 209-218.
Froyd, K. D., D. M. Murphy, T. J. Sanford, D. S. Thomson, J. C. Wilson, L. Pfister, and
L. Lait (2009), Aerosol composition of the tropical upper troposphere, Atmospheric Chemistry
and Physics, 9(13), 4363-4385.
Hoose, C., and O. Moehler (2012), Heterogeneous ice nucleation on atmospheric
aerosols: a review of results from laboratory experiments, Atmospheric Chemistry and Physics,
12(20), 9817-9854.
IPCC, (2013): Summary for Policymakers, in: Climate Change 2013: The Physical
Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change, edited by T. F. Stocker, D. Qin, G. K. Plattner, M.
Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley, Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA.
Jensen, E., and L. Pfister (2004), Transport and freeze-drying in the tropical tropopause
layer, Journal of Geophysical Research-Atmospheres, 109(D2).
Jensen, E. J., L. Pfister, T. P. Bui, P. Lawson, and D. Baumgardner (2010), Ice nucleation
and cloud microphysical properties in tropical tropopause layer cirrus, Atmospheric Chemistry
and Physics, 10(3), 1369-1384.
Koop, T., B. P. Luo, A. Tsias, and T. Peter (2000), Water activity as the determinant for
homogeneous ice nucleation in aqueous solutions, Nature, 406(6796), 611-614.
10
Levin, Z., and W. R. Cotton (2008), Aerosol Pollution Impact on Precipitation: A
Scientific Review, Springer.
McFarquhar, G. M., A. J. Heymsfield, J. Spinhirne, and B. Hart (2000), Thin and
subvisual tropopause tropical cirrus: Observations and radiative impacts, Journal of the
Atmospheric Sciences, 57(12), 1841-1853.
Murphy, D. M., D. J. Cziczo, K. D. Froyd, P. K. Hudson, B. M. Matthew, A. M.
Middlebrook, R. E. Peltier, A. Sullivan, D. S. Thomson, and R. J. Weber (2006), Single-particle
mass spectrometry of tropospheric aerosol particles, Journal of Geophysical ResearchAtmospheres, 111(D23).
Niemand, M., et al. (2012), A Particle-Surface-Area-Based Parameterization of
Immersion Freezing on Desert Dust Particles, Journal of the Atmospheric Sciences, 69(10),
3077-3092.
Pruppacher, H. R., and J. D. Klett (1996), Microphysics of clouds and precipitation,
Kluwer Academic Publishers.
Rogers, D. C., P. J. DeMott, and S. M. Kreidenweis (2001), Airborne measurements of
tropospheric ice-nucleating aerosol particles in the Arctic spring, Journal of Geophysical
Research-Atmospheres, 106(D14), 15053-15063.
Seinfeld, J. H., and S. N. Pandis (2006), Atmospheric chemistry and physics: from air
pollution to climate change, Wiley.
Stubenrauch, C. J., et al. (2013), Assessment of Global Cloud Datasets from Satellites:
Project and Database Initiated by the GEWEX Radiation Panel, Bulletin of the American
Meteorological Society, 94(7), 1031-1049.
Vali, G. (1985), NUCLEATION TERMINOLOGY, Bulletin of the American
Meteorological Society, 66(11), 1426-1427.
Waliser, D. E., et al. (2009), Cloud ice: A climate model challenge with signs and
expectations of progress, Journal of Geophysical Research-Atmospheres, 114.
11
CHAPTER 2: DEPOSITIONAL ICE NUCLEATION ON MONOCARBOXYLIC ACIDS:
EFFECT OF THE O:C RATIO
INTRODUCTION
Cirrus clouds, composed of water ice, cover approximately 30% of the Earth’s surface
and have a predominant warming effect on climate [Chen et al., 2000]. Despite their ubiquity in
the atmosphere, ice clouds are not accurately represented in current global climate models and
provide one of the largest uncertainties in predictions of climate change [Forster, 2007]. This
uncertainty is due, in part, to an incomplete understanding of ice formation mechanisms.
Ice nucleation in the atmosphere has been shown to take place via homogeneous or
heterogeneous nucleation as reviewed by Cantrell and Heymsfield [2005]. Homogeneous
nucleation occurs when ice-like clusters, or nucleation embryos, within an aqueous solution
droplet reach a critical size and stimulate ice formation. In contrast, heterogeneous ice nucleation
occurs at the surface of solid ice-nuclei (IN), which act as a catalytic surface for the formation of
ice embryos. Traditionally, it has been thought that homogeneous ice nucleation is the dominant
mechanism governing cirrus cloud formation [Jensen et al., 2005; Karcher and Strom, 2003].
However, recent field [Froyd et al., 2010; Prenni et al., 2007; Strom et al., 2003] and modeling
[Jensen et al., 2010] studies have underlined the importance of heterogeneous nucleation.
In the past, extensive work has been done to study the efficacy of various heterogeneous
IN. It is now generally accepted that mineral dust [Archuleta et al., 2005; Kanji and Abbatt,
2006] as well as bacteria and pollen [Mohler et al., 2007; Pratt et al., 2009] are all exceptionally
good IN. However, recent field studies have shown that aerosols at cirrus altitudes are dominated
by sulfates and organics [Froyd et al., 2010; Froyd et al., 2009]. In addition, sub-visible cirrus
12
residues are enhanced in sulfates and organics and depleted in mineral dust and bacteria. While
ammonium sulfate has been shown to be a sufficient IN [Abbatt et al., 2006], single particle
measurements have determined that up to 80% of the dry aerosol mass can be attributed to
organic material [Murphy et al., 2006]. Thus, it is clear that heterogeneous ice nucleation on
organic species cannot be ignored. Previously, it has been shown that organics suppress ice
nucleation [Cziczo et al., 2004; Mohler et al., 2008; Prenni et al., 2001]; some laboratory and
modeling studies, however, indicate that organic species can also be efficient IN [Darvas et al.,
2011; Kanji et al., 2008; Murray et al., 2010; Shilling et al., 2006; Zobrist et al., 2006].
Furthermore, Knopf et al. [2010] have shown that anthropogenic particles collected in Mexico
City are predominantly organic in nature and can nucleate ice as efficiently as ammonium sulfate
at temperatures relevant to cirrus formation. Given the diversity, complexity, and mutability of
organics in the atmosphere, it is no longer sufficient to bin organics as either generally “good” or
“poor” IN, nor can studies of organic IN focus solely on likely nucleation candidates. Systematic
experiments to unveil general trends in organic IN efficacy may provide useful data and offer
predictive ability.
Here, we have studied the depositional nucleation of ice on a series of thin films of
monocarboxylic acids between 180 and 200 K as a first attempt to parameterize organic IN.
Carboxylic acids were chosen as they are expected to be an important part of aged aerosol
[Heald et al., 2010].
EXPERIMENTAL
Knudsen Cell. A schematic of the high vacuum Knudsen cell used in these studies is shown in
Figure 2.1. This setup has been described in detail previously [Hudson et al., 2002].
13
Figure 2.1. Schematic of the Knudsen cell flow reactor setup.
14
Briefly, the apparatus consists of a single stainless steel chamber that houses a vertically
mounted gold substrate (d = 2.34 cm) upon which thin organic films are deposited. The
temperature of the substrate is controlled by resistively heating against a differentially pumped
liquid nitrogen cryostat. Temperature is monitored by three T-type thermocouples, which are
calibrated by referencing the measured ice vapor pressure at the frost point to literature data
[Marti and Mauersberger, 1993]; this results in a temperature error < 0.5%. A Teflon seal
surrounds the substrate and separates the cell from the cryostat, ensuring that the substrate is the
only cold surface in the chamber. A Teflon cup, mounted on a pneumatic linear translator, allows
the surface to be isolated from, or exposed to, the chamber atmosphere without significantly
altering the chamber volume. When the cup is open, the condensed phase is monitored using
Fourier-transform infrared reflectance absorbance spectroscopy (FTIR-RAS). Here, an IR beam
from a Nicolet Magna 550 spectrometer passes into the vacuum chamber where it is reflected
from the gold substrate at a grazing angle of 87° from the surface normal. The beam exits the
chamber and is detected using a mercury/cadmium telluride (MCT-A) detector. Up to three leak
valves are used to introduce gas phase species into the chamber. Vapors are drawn from bulbs
containing water, propanoic, butyric, valeric, and hexanoic acids, all subjected to several freezepump-thaw cycles. Chamber pressures are monitored over several pressure regimes using an
ionization gauge and an MKS Baraton absolute capacitance manometer (690A), while gas-phase
constituents are analyzed by an electron-impact ionization quadrupole mass spectrometer (MS,
UTI 100C). Partial pressures of the organic species are determined by calibrating the MS signals
of representative ions (i.e. m/z 60 for butyric acid) to the ion gauge and absolute capacitance
manometer. Water partial pressures are read directly using the absolute capacitance manometer.
15
Organic Film Preparation. In a typical experiment, the gold substrate is first cooled to the
desired temperature. With the Teflon cup closed, a C3-C6 monocarboxylic acid is leaked into the
chamber until a steady pressure of ~1x10-4 Torr is achieved. The cup is then retracted and the
organic species is allowed to condense onto the substrate. Condensation is noted by a decrease in
the calibrated MS signal. Monolayer coverage was calculated by integrating the area under the
uptake curve to determine the number of molecules lost to the surface and dividing by the
surface area of the gold substrate. Coverage was converted into monolayer units by dividing by
the monolayer density as described by Hudson et al. [2002]. In every experiment, at least 10
monolayers are deposited; this ensures that the entire gold surface is covered and does not take
part in the nucleation process. The growth and stability of the films are monitored by FTIR-RAS.
In separate experiments, FTIR-RAS was also used to determine that these organic films are
crystalline rather than amorphous. A sample spectrum of butyric acid is shown in Figure 2.2a.
The lack of scattering in the high energy end of the spectrum indicates that the film is optically
flat and smooth on the scales of IR light. Film stability is determined by monitoring the C=O
stretching vibration. This region at ~1720 cm-1 is stable to within 0.2% over a time period of 900
s. Once a stable film has been achieved, the cup is extended over the film, the chamber is
pumped down to ~10-7 Torr, and the ice nucleation experiment begins.
Ice Nucleation Experiment. The experiment measures the critical ice saturation ratio, Scrit, as a
function of temperature for each film. Scrit is defined as
Scrit = PH2O/VPice,
(1)
where PH2O is the water partial pressure at the temperature when ice formation is observed and
VPice(T) is the equilibrium vapor pressure of water over ice at the same temperature. Thus, Scrit
16
Figure 2.2. Typical FTIR-RAS spectra of butyric acid immediately before (a) and after (b) ice
nucleation. After nucleating ice, the organic film shows absorbance features in the –OH stretch
(3230 cm-1) and ice libration (870 cm-1) regions that are characteristic of crystalline ice. Spectra
have been offset for clarity.
17
can also be simply defined as the ice saturation ratio (Sice) at which ice nucleates. Figure 2.3
shows the results from a typical ice nucleation experiment on butyric acid as a function of time.
Here water partial pressure is set just below the ice frost point (Sice < 1) and incrementally
increased every 100 s, causing a concomitant increase in Sice. IR spectra are collected every 2.0 s,
and the area under the ice libration peak (1000-870 cm-1) is integrated. Prior to nucleation, the
integrated IR signal remains constant, further denoting the stability of the organic film as this
window includes organic peaks. Ice nucleation is noted at ~1525 s by a simultaneous decrease in
water partial pressure and increase in the integrated IR absorbance. A typical spectrum of ice,
which has nucleated on a thin butyric acid film, can be seen in Figure 2.2b. After nucleation, the
film is allowed to grow so that a frost-point calibration can be performed. VPice(T) is determined
by adjusting the water pressure until the ice film is neither growing nor desorbing.
Chemicals. All chemicals were purchased from Sigma-Aldrich (St. Louis, MO). Listed are the
chemicals and corresponding purities used in these studies: water (HPLC grade), propanoic
(99.5%), butanoic (butyric, 99%), pentanoic (valeric, 99%), and hexanoic acids (99.5%).
RESULTS AND DISCUSSION
The Scrit values for the heterogeneous nucleation of ice on thin films of propanoic,
butyric, valeric, and hexanoic acids as a function of temperature are shown in Figure 2.4a. Each
point on the graph is the average of at least three ice nucleation experiments performed at that
temperature and each experiment was conducted on a freshly deposited film. Errors in S crit are
reported as one standard deviation. Also shown is previously published data for maleic acid from
the same experimental setup. As indicated in Figure 2.4a, the Scrit values for each film exhibit
18
Figure 2.3. Saturation ratio (Sice, left axis, solid line) and integrated absorbance of the ice
libration peak (right axis, dotted line) as a function of time for a typical ice nucleation
experiment on butyric acid at 180 K. The vertical dashed line at ~ 1525 s denotes the onset of ice
nucleation. For clarity, a horizontal dashed line has been drawn corresponding to an Sice value of
1.
19
Figure 2.4. Critical saturation ratios required to nucleate ice on thin organic films as a function
of temperature (a) and O:C Ratio (b). Also shown in plot B are ice nucleation data for α-pinene
SOA19 and organic aerosol collected in Mexico City [Knopf et al., 2010]; O:C values and ranges
are taken from Aiken et al. [2010], see text for more detail. As seen in both plots, increasing the
O:C ratio decreases the Scrit value at each temperature explored. Error bars for monocarboxylic
acid Scrit values are reported as one standard deviation. Maleic acid data from Shilling et al.
[2006].
20
strong temperature dependences. Such behavior has been noted and discussed in previous
publications [Shilling et al., 2006; Trainer et al., 2009]. Perhaps more significant is that Scrit also
displays a strong dependence on the chemical nature of the film. Amongst the monocarboxylic
acids explored, Scrit increases as the number of carbon atoms in the acid backbone increases at
each of the temperature probed. Taking into account the maleic acid data by Shilling et al.
[2006], this work suggests that, for organic acids, the ice nucleation efficiency increases as the
O:C ratio increases. This is shown more clearly in Figure 2.4b; here, Scrit is plotted as a function
of O:C ratio at each temperature explored.
To confirm that O:C ratio was a better predictor of ice nucleation capability than other
well-tabulated molecular properties, we investigated the relationship between several physical
properties of the organic acids used in this study and their Scrit values. As shown in Table 2.1,
there are no discernible relationships between Scrit and solubility, pKa, melting point, or
molecular weight for these species. Furthermore, although molecular dipole moment shows a
positive correlation with Scrit, it is also a poor indicator of ice nucleation ability, since films with
very different ice nucleation capabilities have the same dipole moment within the given
uncertainties.
We interpret the effect of the O:C ratio on the ice nucleation capabilities of organic films
under the framework of classical nucleation theory, which states that heterogeneous ice
nucleation begins with the formation of a critical ice embryo assuming the shape of a spherical
cap on a planar ice-active surface. For this gas-to-crystalline nucleation, the angle of contact (θ)
between the crystalline ice embryo and the solid surface is neither observable nor measureable;
however, it provides a useful parameter to subsume all specific ice nucleation effects of an
individual ice nucleus. Thus, we term θ as the effective contact angle, where θ = 0° implies a
21
Table 1. Physical Constantsa, Average Surface Concentrations (O/nm2), and O:C ratios for the
Organics Studied in This Work.
pKa
melting
point
(°C)
MW
(g/mol)
density
(g/mL)
cross
sectional
area (10-15
cm2)
O/nm2
O:C
ratio
Scrit at
200
K
789
1.92
139
116.073
1.59b
6.35
6.30
1.00
1.18
1.75 ± 0.09
miscible
4.87
- 20.5
74.079
0.9882
6.46
3.09
0.67
1.21
butyric
1.65 ± 0.03
miscible
4.83
-5.1
88.106
0.9528
7.43
2.69
0.50
1.29
valeric
1.61 ± 0.03
45.2
4.83
-33.6
102.132
0.9339
8.31
2.41
0.40
1.41
hexanoic
1.13 ± 0.03
10.2
4.85
-3
116.158
0.9212
9.14
2.19
0.33
1.57
organic
acid
dipole
moment
(Debye)
solubility
(g/L)
maleic
3.176
propanoic
a
CRC Handbook 2010-2011; all values taken at 25 °C unless otherwise noted
b
20 °C
22
perfect match between ice and the ice nucleus and θ = 180° means that the free energy of ice
formation is not reduced at all by the presence of the nucleus, equivalent to homogeneous
nucleation. Differences in surface chemistry between planar films will alter this effective contact
angle between the spherical ice embryo and the surface, thereby changing their Scrit values; those
films which are more efficient ice nuclei will have lower effective contact angles.
The equations and assumptions used to effective calculate contact angles under classical
nucleation theory have been described previously in detail [Fortin et al., 2003]. Briefly, the
heterogeneous nucleation rate Jhet is defined by Pruppacher and Klett [1996] as
Jhet = A ∙ exp[-ΔFhet/(kT)],
(2)
where A is a preexponential factor, ΔFhet is the free energy of formation of the ice embryo, k is
the Boltzmann factor, and T is the temperature. ΔFhet is related to the experimentally determined
Scrit by the following expression
ΔFhet = (16πM2σiv3)/[3(RTρScrit)2] ∙ f(m),
(3)
where M is the molecular weight of water, σiv is the surface tension of the ice/vapor interface, R
is the universal gas constant, and ρ is the density of ice. The matching function, f(m), is defined
as
f(m) = (2+m)(1-m)2/4 ,
(4)
m = cos(θ).
(5)
where
23
To determine θ, ΔFhet was calculated using Equation (2) and the approximations Jhet = 1
cm-2 s-1 and A = 1025 cm-2 s-1; such approximations are justified by the fact that θ is not sensitive
to either term. For example, changing either Jhet or A by one order of magnitude changes θ by
~0.3°. Then, θ was calculated using Equations 3, 4, and 5 with M = 18.015 g mol -1, σiv = 106 mJ
m2, and ρ = 0.92 g cm-3. These values, as well as Jhet and A, were chosen to allow direct
comparison with the parameterization of Wang and Knopf [2011]. The effective contact angle
for the films used in this study as a function of O:C can be seen in Figure 2.5. At each
temperature, the effective angle of contact decreases as the O:C value increases, as expected.
Also seen in Figure 2.5, the effective contact angles calculated for all films at all temperatures in
this study display excellent agreement with Equation 8 in Wang and Knopf [2011]. It should be
noted, however, that our points are systematically lower than the parameterization; we believe
this is the result of a lower temperature used relative to the temperatures sampled for the
parameterization.
For depositional ice nucleation, one aspect that has been proposed to decrease the
effective contact angle is particle wettability [Hobbs, 1974]. Here, the foreign surface physically
adsorbs a layer of amorphous, metastable water prior to nucleation; this provides a nucleation
template similar to the crystal structure of ice and, therefore, facilitates the formation of critical
ice embryos. This is aligned with the theory of preactivation, a phenomenon where the effective
freezing temperature of some ice nucleators increases after having already catalyzed the phase
transition. Originally thought to be caused by persistent ice remnants in cracks, crevices, or other
small cavities, Edwards and Evans [1971] have shown that preactivation is primarily dictated by
an ice-like layer of water adsorbed on the nucleation substrate.
24
Figure 2.5. Calculated effective contact angles as a function of O:C ratio for the films used in
this study. At each temperature, increasing the O:C ratio decreases the angle of contact. Error
bars are reported as one standard deviation. Maleic acid data from Shilling et al. [2006] Shown in
red are calculated effective contact angles from the parameterization found in Wang and Knopf
[2011].
25
Previously, Gorbunov et al. have shown that, for both metal oxide [1994] and soot [2001]
particles, increasing the surface concentration of hydroxyl and/or carbonyl groups (#/nm2)
concomitantly decreases effective contact angles. Based on these results, they conclude that
surface hydrophilicity is directly related to depositional ice nucleation activity. Specifically, the
authors note that the ice forming activity of a particle depends on the surface concentration of
chemical groups that can hydrogen bond to water molecules. Combining these results with those
of Edwards and Evans, it is reasonable to assume that as the number of hydrogen bonding groups
on the heterogeneous surface is increased, the better the surface is able to adsorb a semi-ordered,
ice-like layer of water. Here, however, the ordered water layer is not retained from the previous
nucleation event, but adsorbed directly from the vapor phase prior to ice formation. Furthermore,
although it has not been quantified for these organic acids, it has been shown that a monolayer of
water can be retained on solid surfaces at relative humidities relevant to this work. For example,
it is well known that a monolayer of water subsists on crystalline NaCl at ~40% RH and on αFe2O3 as low as 10%-15% RH [Cwiertny et al., 2008; Finlayson-Pitts, 2003].
For organic IN, we hypothesize that the O:C ratio has a direct effect on the surface
hydrophilicity. Here, by increasing the O:C ratio, the average surface concentration of hydrogen
bonding groups is increased. To a first order approximation, this can be calculated by modeling
the organic molecules as spheres. Using the density and molecular weight, the effective radius of
a spherical molecule can be found and used to determine the cross-sectional area of the molecule.
Dividing the number of hydrogen bonding groups by the cross-sectional area of the molecule
gives the abovementioned average surface concentration. The molecular weight, density, cross
sectional area, and average surface concentration for each monocarboxylic acid are included in
Table 2.1. As shown, there is a positive correlation between the O/nm2 and the O:C ratio for the
26
organic molecules used in this study. This is better seen in Figure 2.6; the solid line represents a
best fit to the data forced to pass through the zero point, since a molecule with an O:C ratio of
zero would necessarily have a O/nm2 of zero. It is important to note, however, that this simple
correlation breaks down when structural effects skew the relationship between O:C ratio and
surface concentration of hydrogen bonding groups. A good example of this is the efficient
nucleation of ice by Langmuir films of long chain alcohols on supercooled water drops
[Popovitzbiro et al., 1994; Zobrist et al., 2007]. Here, while the O:C ratio of these molecules is
very low, the surface concentration of hydrogen bonding groups is high due to their twodimensional crystalline structure as determined by grazing incidence X-ray diffraction.
Furthermore, McIntire et al. have shown that terminally unsaturated self-assembled monolayers
exposed to O3 form aggregates with polar functional groups buried inside a hydrophobic shell
[McIntire et al., 2010; McIntire et al., 2005]. Here, it is shown that increasing the number of
hydrogen bonding groups does not necessarily indicate an increase in surface hydrophilicity.
Finally, as only carboxylic acids were used in this study, future studies that probe the variations
between functional groups and the impact each has on surface hydrophilicity would be desirable.
Indeed, several modeling studies by Picaud and co-workers have indicated that not only the type
of hydrogen bonding group is important for water adsorption, but also their distribution, both
absolute and in relation to each other [Hantal et al., 2010; Moulin et al., 2007; Picaud et al.,
2008].
ATMOSPHERIC IMPLICATIONS
To extend this work to the atmosphere, Scrit data at temperatures relevant to this study for
secondary organic aerosol (SOA) particles generated by ozonolysis of α-pinene [Mohler et al.,
2008] and predominantly organic particles collected during the MILAGRO 2006 campaign at the
27
Figure 2.6. Oxygen per square nanometer as a function of O:C ratio for the organic films used in
this study. The solid line represents a best fit to the data constrained to pass through the zero
point.
28
Mexico City T0 site [Knopf et al., 2010] were compared to the monocarboxylic data (Figure 4b.)
The O:C ratios for each are given by Aiken et al. [2008], who used on-line mass spectrometry to
determine the elemental ratios. The α-pinene SOA O:C ratios were taken from chamber
experiments done at UC Riverside under similar conditions and mass loadings to the ice
nucleation experiments. The range of O:C ratios for the Mexico City organic aerosol are onehour values averaged over six days and were sampled at the T0 site during the MILAGRO
campaign. Only the O:C values for hours coincident with aerosol collection for ice nucleation
experiments are considered. These results show excellent agreement with our data despite the
fact that these particles are both multi-component organic aerosols and most likely liquid or
amorphous (semi-)solids [Koop et al., 2011; Marcolli et al., 2004] instead of crystalline films.
Using the O:C ratio as a proxy for heterogeneous ice nucleation efficacy is particularly
appealing for several reasons. First, the O:C ratio of ambient aerosol can be measured in situ and
as a function of time, [Aiken et al., 2008] and does not require the chemical deconvolution of the
complex organic fraction. Second, efforts have already been undertaken to incorporate O:C ratio
into simple aerosol models [Jimenez et al., 2009], correlate O:C ratios to photochemical aging
and the widely-used hygroscopicity parameter κ [Massoli et al., 2010], as well as relate O:C
ratios to aerosol optical properties [Cappa et al., 2011]. Finally, Bertram et al. [2011] have
shown that the O:C ratio in conjunction with the organic-to-sulfate ratio can be used to predict
the liquid-liquid separation, efflorescence, and deliquescence relative humidities of mixed
particles containing ammonium sulfate, organic material, and water.
29
SUMMARY AND CONCLUSION
We have presented measurements of the depositional ice nucleation ability of thin C3-C6
monocarboxylic acid films between 180 and 200 K. It was shown that the ice nucleation ability
of these organic films was dependent on their O:C ratio. This behavior was attributed to an
increase in the average surface hydrophilicity as the O:C ratio was increased. Comparison of this
data to α-pinene SOA and ambient organic aerosol collected from Mexico City was also made.
Within experimental uncertainties, both the environmental chamber and field data agree well
with the results from this work.
Recently, there has been evidence that atmospheric particles dominated by organic
material can act as potential IN for cirrus cloud formation. Given that organic fraction of
atmospheric aerosol has been estimated to comprise up to 100,000 compounds [Goldstein and
Galbally, 2007] that are undergoing constant chemical alteration [Robinson et al., 2007], it is
clear that organic IN provide a considerable barrier to the effective modeling of cirrus clouds.
The present results, however, suggest that systematic ice nucleation studies on organics can
alleviate this barrier by providing both insightful information about the mechanism of
heterogeneous ice nucleation and simple parameterizations of organic ice nucleation efficacy.
30
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35
CHAPTER 3: HETEROGENEOUS ICE NUCLEATION ON PHASE-SEPARATED
ORGANIC-SULFATE PARTICLES: EFFECT OF LIQUID VS. GLASSY COATINGS
INTRODUCTION
Cirrus clouds are ubiquitous in the upper troposphere and are known to have a large
effect on the Earth’s climate. For example, it has been shown that sub-visible cirrus clouds are
present in the tropical tropopause layer (TTL) 50-80% of the time [Jensen et al., 2010]. These
clouds are of atmospheric importance because they can dehydrate air ascending through the TTL
and, therefore, effectively regulate stratospheric humidity. Despite their ubiquity in the upper
troposphere, ice cloud formation remains one of the least understood processes in the atmosphere
and one of the largest uncertainties in the prediction of climate change [Forster, 2007]. This is in
large part due to the fact that particles available for ice nucleation are complex and that ice
number concentrations are low (~1 in 105 particles in the upper troposphere nucleate ice [Rogers
et al., 2001]), making in-situ measurements difficult. Recently, single particle measurements in
the upper troposphere have shown that background aerosols as well as sub-visible residue are
enhanced in sulfates and organics and have little aerosol typically assumed to be excellent ice
nuclei (IN), like mineral dust [Froyd et al., 2010; Froyd et al., 2009]. Furthermore, it was shown
that the average organic fraction of these particles ranged from 30—80% by mass in the
convective region as well as the TTL [Froyd et al., 2009]. These measurements, however, do not
provide any information about the phase state and/or the distribution of chemicals within these
organic-sulfate particles. Thus, the identity of the ice nuclei in this region of the atmosphere is
still uncertain.
36
The hygroscopic phase transitions of organic-sulfate aerosol can influence the ice
nucleation mechanism. If the particles are in an aqueous liquid phase, they are expected to
nucleate ice homogeneously. Here, a critical ice germ is formed within the aqueous matrix of a
supercooled liquid droplet. Homogeneous nucleation has been studied extensively in the past and
has been well parameterized [Koop et al., 2000]. Conversely, if the organic-sulfate particles are
crystalline or amorphous solids, then they can act as heterogeneous ice nuclei. Here a foreign
surface acts as a catalyst for ice germ formation. Previously, it was thought that homogeneous
nucleation dominated cirrus cloud formation; however, combined field [Kramer et al., 2009;
Lawson et al., 2008] and modeling studies [Jensen et al., 2010] have indicated that ice number
and ice size distributions in the upper troposphere are sometimes better explained by
heterogeneous nucleation.
Recently, it has been shown that mixed organic-sulfate particles can undergo another
hygroscopic phase transition termed liquid-liquid phase separation (LLPS) [Ciobanu et al., 2009;
Marcolli and Krieger, 2006]. Here the inorganic component ammonium sulfate effectively salts
out the less soluble organic components. Bertram et al. [2011] have shown that if the organic
component of mixed organic-sulfate particles has an O:C ratio < 0.7, then it will generally phase
separate; moreover, it has been shown that complex organic-sulfate particles studied by Song et
al. [2012], consisting of three separate dicarboxylic acids, always phase separated, as long as
their combined O:C ratio was < 0.7. LLPS has also been observed with real-world organicsulfate particles collected in the Atlanta, GA region [You et al., 2012]. Finally, it has been
shown that particles that have undergone LLPS minimally affect the deliquescence and
efflorescence behavior of ammonium sulfate [Bertram et al., 2011; Smith et al., 2011].
37
Past work has shown that phase-separated particles containing two liquids can assume a
number of partially engulfed morphologies [Kwamena et al., 2010; Reid et al., 2011]. Partial
engulfment has also been shown for mixed organic-sulfate particles that have undergone LLPS
and subsequently effloresce [Song et al., 2012]. However, it has been shown from 2D projections
that many phase-separated, effloresced organic-sulfate systems assume a fully engulfed coreshell morphology, with an evenly thick organic coating and a crystalline ammonium sulfate core
[Bertram et al., 2011; Ciobanu et al., 2009; Song et al., 2012]. There have been many studies
looking at the effects of organic coatings on the efficiency of ice nucleation. Mohler et al. [2008]
found that coating Arizona test dust (ATD) and illite dust particles with secondary organic
material (SOM) produced by the ozonlysis of α-pinene required a higher supersaturation to
nucleate ice than for the bare dust particles at temperatures between 205 and 210 K. Koehler et
al. [2010] observed similar results for coating ATD particles with SOM from the ozonlysis of αpinene for temperatures between 218 and 233 K. In contrast, Wise et al. [2010] have shown that
coating ammonium sulfate with palmitic acid minimally affected the ice nucleation efficacy of
ammonium sulfate over a similar temperatures range to Koehler et al. [2010]. Raman analysis of
these particles, however, revealed that this behavior could be attributed to uneven coating by the
organic material. Thus, partial coatings are shown to minimally affect the ice nucleation behavior
of their cores; it is not clear, however, if full organic coatings would inhibit ice nucleation by
ammonium sulfate.
Generally, ambient organic material has been anti-correlated with ice nucleation efficacy
[Baustian et al., 2012; Cziczo et al., 2004]; some crystalline subsets, however, of organic aerosol
may very well be efficient ice nuclei [Kanji et al., 2008; Schill and Tolbert, 2012; Zobrist et al.,
2006]. Real organic aerosol will most likely be composed of thousands to hundreds of thousands
38
of species [Goldstein and Galbally, 2007], and likely be in an amorphous liquid, not crystalline
state [Marcolli et al., 2004]. Recently, there has been evidence that organic aerosol may in some
cases be in a semi-solid or solid amorphous state such as rubbers, gels, or glasses [Koop et al.,
2011; Mikhailov et al., 2009; Virtanen et al., 2010; Zobrist et al., 2008]. The work of Murray and
coworkers has shown that, in general, highly oxygenated glassy organic aerosols are good
heterogeneous ice nuclei, and can heterogeneously nucleate ice up to temperatures as high 218.5
K [Murray et al., 2010; Wilson et al., 2012]. Further, the work of Knopf and coworkers suggests
that chamber generated SOM can heterogeneously nucleate ice to temperatures as high as 240 K
[Wang et al., 2012b] and that glassy organic coatings may be responsible for the efficient
nucleation of ice by ambient particles collected in Mexico City and the Los Angeles Region
[Wang et al., 2012a]. Laboratory work in our group has shown glassy sugars are excellent
heterogeneous ice nucleators [Baustian et al., 2013]. These combined works suggest that
(semi-)solid amorphous organic coatings may enhance, not deactivate, the ice nucleation ability
of phase-separated particles.
In the present work, we studied the low temperature deliquescence, liquid-liquid phase
separation, and efflorescence behavior of mixed organic-sulfate particles using 1,2,6-hexanetriol
or an equal mass ratio of 1,2,6-hexanetriol/2,2,6,6-tetrakis(hydroxymethyl) cycohexanol as the
organic species. Additionally, we studied the heterogeneous ice nucleation behavior of the pure
organics and mixed organic-sulfate particles to help elucidate the role of phase-separated
particles in ice cloud formation.
39
EXPERIMENTAL
Sample Preparation. Aqueous solutions containing organics and organic-sulfate mixtures were
prepared by dissolving pure substances in high purity water. Pure 1,2,6-hexanetriol (C6,
molecular weight = 134.17 g/mol) was made as a 1% solution. C6 was chosen because it has
been previously shown to phase separate at 273 K for various organic-to-sulfate ratios [Bertram
et al., 2011] and the conditions under which it forms a glass have been quantified [Zobrist et al.,
2008]. The other pure organic system consisted of a 1:1 mixture of C6 and 2,2,6,6tetrakis(hydroxymethyl) cyclohexanol (C10, molecular weight = 220.26 g/mol, melting point =
123-127° C), and was also made as a 1% solution. C10 was chosen because the conditions under
which is forms a glass are also known [Zobrist et al., 2008]. Further, C10 was chosen to act as a
vitrifier in the C6/C10 system because of its high molecular weight and high glass transition
temperature (Tg, 258.6 ± 5.2 K) in relation to C6 (Tg = 193.5 ± 1.3 K). Each of these organic
systems was also mixed with ammonium sulfate (AS) in a 2:1 organic-to-sulfate ratio. It should
be noted that the O:C ratio of pure C6 as well as the C6/C10 mixture was 0.5.
Raman Microscopy and Environmental Cell. A schematic of the experimental system used to
probe deliquescence, liquid-liquid phase separation, efflorescence, and ice nucleation is shown in
Figure 3.1. This system has been described in detail previously [Baustian et al., 2010; Wise et
al., 2010]. Briefly, a Nicolet Almega XR Dispersive Raman spectrometer has been outfitted with
a Linkham THMS600 environmental cell and a Buck Research CR-1A chilled-mirror
hygrometer. Coupled to the spectrometer is an Olympus BX51 research grade optical microscope
with 10x, 50x, and 100x magnification abilities.
40
Figure 3.1. Schematic representation of the Raman microscope setup.
41
Particles were deposited and viewed on a hydrophobically treated quartz disc (1 mm
thick). To generate particles, a solution sample was aspirated into a Meinhard TR-50 glass
concentric nebulizer. Nebulized droplets were directed at the quartz disc and allowed to
coagulate into supermicron droplets. The sample disc was then transferred into the
environmental cell and exposed to a low humidity environment. This evaporated the water,
resulting in organic/AS droplets ranging from 1 to 40 µm in lateral diameter.
The temperature (T) of the particles was controlled with a Linkham TMS94 automated
temperature controller. The temperature controller was calibrated against the melting point of
ice, the deliquescence relative humidity (DRH) of NaCl, and the ferroelectric phase transition of
ammonium sulfate as described by Baustian et al. [2010], which yielded an uncertainty in T of <
± 0.2 K. Water vapor inside the cell was controlled by continuously flowing dry and humidified
N2. At the exit of the cell, the CR-1A hygrometer measures dew point with an accuracy of ± 0.15
K. The relative humidity with respect to water and ice experienced by the particles was derived
from T and the dew point employing the parameterizations of Murphy and Koop [2005]. A Gast
Mechanical diaphragm pump ensured that the flow through the system was kept constant at 1 L
min-1.
In the present study, both individual Raman spectra and spectral maps were acquired on a
point-by-point manner. A frequency-doubled Nd:YAG laser operating at 532 nm was used for
molecular excitation. Spectra were obtained using 50x and 100x objectives, which have
diffraction limited spot sizes of 1.3 and 1.1 µm, respectively [Everall, 2010]. Spectra were
collected from 150—4000 cm-1, with a typical spectral resolution of ~2-4 cm-1. Raman mapping
was utilized to gather information about the spatial distribution of chemicals within individual
particles. Raman line and 2D maps were acquired by taking a spectrum every 1 µm in the Y and
42
XY directions, respectively. An automated mapping program and high-precision motorized stage
ensured accurate particle mapping.
Deliquescence, Liquid-Liquid Phase Separation, and Efflorescence Experiments. Figure
3.2a shows the experimental trajectory of a typical deliquescence experiment. A typical
deliquescence experiment was started by exposing wet impacted particles (point A) to ~0%
relative humidity (RH) at 298 K for at least 10 minutes to ensure that all particles were
effloresced and completely dry (point B). In each experiment, a collection of 10 to 20 random
particles were monitored visually under 50x magnification using the video output from a CCD
camera mounted on the optical microscope. The relative humidity experienced by the particles in
the environmental cell was controlled by keeping a constant dew point and steadily decreasing T.
Specifically, after the dew point was stable for 10 minutes, T was decreased at a rate of 10 K
min-1 from room temperature until RH ≈ 60% (point C); the sample was then cooled at a rate of
0.1 K min-1, corresponding to a change in RH of approximately 0.6% min -1, until deliquescence
was observed visually (point D). An example of a deliquesced particle is shown in Figure 3.3.
Raman spectroscopy was used to verify deliquescence had occurred. Spectra were taken at the
center and edge of particles to ensure that the particles were well-mixed. Typical spectra of a
deliquesced C6/AS particle can be seen in Figure 3.3a and Figure 3.3b. The main features of the
spectrum are a sharp peak at 972 cm-1 (due to the symmetric stretching mode of SO42-), a broad
band centered at 2900 cm-1 (due to the C-H stretching vibrations of C6), and a broad band
centered at 3400 cm-1 (due to the symmetric and anti-symmetric O-H stretching modes of liquid
water). The spectra taken at the center and edge of the particle are nearly identical, verifying that
the particle is one homogeneous aqueous phase.
43
Figure 3.2. Experimental trajectories for (a) deliquescence/efflorescence and (b) ice nucleation
shown in relative humidty and ice saturation ratio (Sice) space, respectively. Each trajectory line
has been labeled with an experimental heating/cooling rate. The solid and dashed black lines in
Figure 3.2a correspond to the deliquescence and efflorescence RH of ammonium sulfate,
respectively. The thick solid, dashed, and thin solid black lines in Figure 3.2b correspond to
water saturation, the Sice values for homogenous nucleation of an aqueous droplet, and ice
saturation, respectively. The capital letters in the panels indicate points that are of special
interest; see text for details.
44
Figure 3.3. Raman spectra of (a) the center of deliquesced C6/AS, (b) the edge of deliquesced
C6/AS (c) the core of a C6/AS particle after LLPS, (d) the shell of a C6/AS particle after LLPS,
(e) the core of a C6/AS particle after efflorescence, and (f) the shell of a C6/AS particle after
efflorescence. The mixed particle used for Raman analysis is shown on the right. All
measurements were taken at 260 K. The size bar in the optical microscope image corresponds to
10 µm.
45
Liquid-liquid phase separation experiments (not shown in Figure 3.2) started from
deliquesced particles. The sample was warmed at a rate of 0.1 K min-1 until liquid-liquid phase
separation occurred. Initial phase separation occurred in the form of schlieren [Ciobanu et al.,
2009], which coalesce to form AS inclusions that diffuse into each other to eventually form two
distinct phases. Raman spectroscopy was utilized to verify spectrally that the two distinct phases
had formed. An image of a phase-separated particle and its subsequent spectra can be seen in
Figure 3.3c and 3.3d. As shown, the inside phase is enhanced in ammonia, sulfate, and water
peaks, while the outside phase is enriched in organic peaks. After two distinct phases had formed
and were verified spectrally, the sample was cooled at a rate of 0.1 K min-1 to determine the
separation relative humidity by increasing the RH. These particles then underwent similar visual
and spectral analysis to ensure that the particle had merged back into one aqueous phase.
Figure 3.2a also shows the experimental trajectory of a typical effloresce experiment.
Efflorescence experiments were conducted on particles that had already undergone
deliquescence (point E). The particles were cooled at a rate of 10 K min-1 until RH was ~45%
(point F). Then, the particles were warmed at a rate of 0.1 K min-1 until efflorescence was
observed visually (point G) and verified spectrally (Figure 3.3e and 3.3f). Inside/outside spectra
were taken to ensure that the particles remained in a core-shell configuration after efflorescence.
An additional, broad peak centered at 3175 cm-1 (due to the N-H stretching vibrations of the
ammonium ion), previously masked by the broad water peak, also appears in the spectra of the
effloresced core.
For all deliquescence, liquid-liquid phase separation, and efflorescence experiments, at
least three experiments were performed at each temperature on at least two independent sets of
particles. Error bars in RH and temperature are reported as one standard deviation.
46
Ice Nucleation Experiments. Figure 3.2b shows the experimental trajectory of a typical ice
nucleation experiment. Each ice nucleation experiment started by exposing wet impacted
particles (point A) to ~0% relative humidity to ensure all particles were effloresced and
completely dry (point B). The ice saturation ratio (Sice = PH2O/VPice) value over the particles in
the environmental cell was controlled by keeping a constant dew point and steadily decreasing T.
Specifically, after the dew point was held stable for at least 10 minutes, T was decreased from
room temperature until Sice ≈ 0.88 (point C). The sample was then cooled at a rate of 0.1 K min-1,
corresponding to a change in Sice of approximately 0.01 min-1, until the onset of ice (point D)
was observed at the critical ice saturation ratio (Scrit). This experimental procedure is essentially
the same as the deliquescence experiment, but at lower temperatures; thus, in these experiments,
depositional nucleation is only possible before deliquescence occurs. Ice nucleation was
monitored at 10x magnification. Great care was taken to ensure that the first ice nucleation event
was observed; if multiple events were observed, those points were omitted and the experiment
repeated.
After the first ice event was observed, the 50x objective was used to verify the existence
of ice both visually and spectrally. The presence of water-ice was marked by the emergence of a
broad band centered near 3250 cm-1. Next, the ice was sublimed by increasing T by 10 K (point
E) and then a line map of the resultant ice nuclei was taken. Finally, to test whether ice
preactivation influenced these results, the sample was warmed to 298 K and dried to ~0% RH
before the next experimental run. In our threshold freezing experiments, the same particle was
never observed to be the first particle to nucleate ice more than once.
47
For all ice nucleation experiments at least three experiments were performed at each
temperature on at least two independent sets of particles. Error bars in RH and temperature are
reported as one standard deviation.
RESULTS
Low Temperature Deliquescence/Efflorescence and Liquid-Liquid Phase Separation of
Organic-Sulfate Particles. The results from the deliquescence/efflorescence and liquid-liquid
phase separation experiments are shown in Figure 3.4. Also shown are lines corresponding to the
DRH (RH = 80%) and ERH (RH = 35%) of pure AS [Martin, 2000]. Comparing the
deliquescence and efflorescence results from this work to those of pure AS, we find that liquid
organic coatings minimally affect the deliquescence and efflorescence behavior of ammonium
sulfate. This has been noted in the past for particles from 273 to 298 K [Bertram et al., 2011;
Smith et al., 2011]; however these results extend this general behavior to below the melting point
of ice. We believe that this behavior can be explained by the fast diffusion of water vapor
through the liquid organic coating. Differential scanning calorimetry experiments by Zobrist et
al. [Zobrist et al., 2008] have shown that C6 is expected to be in a liquid state at the DRH and
ERH of AS for the temperatures explored in this work. The diffusion coefficient for a small
molecule like water diffusing through a liquid organic matrix is ~10-5 to 10-7 cm2 s-1 [Ciobanu et
al., 2009; Shiraiwa et al., 2011]. Thus, a small molecule like water should be able to diffuse
through a 1 µm liquid organic coating on the order of milliseconds to hundreds of milliseconds.
Similar to the data shown in Figure 3.3c and 3d for C6/AS, the C6/C10/AS mixtures were
also found to undergo liquid-liquid phase separation at the temperatures explored in this study.
Figure 3.4 includes the liquid-liquid phase separation RH (SRH) for these mixtures as a function
48
Figure 3.4. Hygroscopic phase transitions of C6/AS and C6/C10/AS. For comparison, the DRH
(RH = 80%, solid line) and ERH (RH = 35%, dashed line) lines for pure AS have been plotted.
Experimental results from Bertram et al. [2011] for C6/AS in a 2.127 organic-to-sulfate ratio
have also been plotted (open squares). Error bars in RH and temperature are reported as one
standard deviation.
49
of temperature. To the authors’ knowledge, this is the first example of liquid-liquid phase
separation at these temperatures; previous work was conducted at 273 to 298 K. These results are
in agreement with Bertram et al. [2011], who have reported that binary organic and sulfate
mixtures will generally phase separate if the O:C ratio of the organic species is < 0.7.
Furthermore, the SRH of the mixtures in this study show no temperature dependence within
error; indeed the SRH of the C6/AS mixture in a 2:1 organic-to-sulfate ratio in this work at 245
K is consistent with the SRH measurements of Bertram et al. [2011] taken at 273 K for C6/AS in
a 2.127:1 organic to sulfate ratio (Figure 3.4).
Particle Morphology of Effloresced Organic-Sulfate Particles. Previously, Wise et al. [2010]
showed that mixed organic-sulfate particles are efficient at nucleating ice due to incomplete
organic coatings. There, it was concluded that any small portion of uncoated AS could act as the
catalyst for new ice formation. To ensure that our particles were fully coated laterally, we
utilized 2D Raman mapping to investigate several particles. An example 2D Raman map of an
effloresced, phase-separated C6/AS particle can be seen in Figure 3.5. Here, the sulfate
containing region is mapped using the intensity of the SO42- stretch (972 cm-1) in the Raman
spectrum where warmer colors correspond to higher intensities. Similar analysis was done for the
organic containing region using the intensity of the C-H stretch (2900 cm-1), however, the
organic color map was greyscaled to differentiate between the organic and sulfate containing
regions. Thus, for a phase-separated nine micron particle we see that it will consist of an
approximately 6 µm AS core surrounded by ~1-3 µm of an organic coating at all points.
Unfortunately, the present experimental setup gives only the top view of the particles and
yields no quantitative information about the side view. To determine the three-dimensional
morphology of the particle and ensure that no portion of the effloresced ammonium sulfate core
50
Figure 3.5. Two-dimensional Raman spectral map of a phase-separated, effloresced C6/AS
particle at 298 K. The sulfate-containing region is shown as a color map where warmer colors
refer to a higher intensity of the SO42- peak (972 cm-1). The organic-containing region has
undergone a similar analysis using the C-H peak (2900 cm-1); however, this map has been greyscaled for clarity. As shown, the particle is fully coated laterally.
51
was exposed, we used a volume-geometry analysis similar to Ciobanu et al. [2009] to reconstruct
the particle shape on the experimental substrate. To do so, we determined the contact angle of
several 2 µL C6 droplets on the silanized quartz disc using ImageJ software [Schneider et al.,
2012] at 298 K. The average contact angle measured for these droplets was 77 ± 3°. Using this
value we were able to calculate the height of any droplet by measuring its lateral dimensions;
however, this does not give us information about the configuration of the AS core. Thus, we
assumed several possible arrangements put forth by Ciobanu et al. [2009] for two immiscible
liquids (Figure 3.6a-d). Two of these arrangements, spherical calotte in a spherical calotte
(Figure 3.6c) and half of an ellipsoid in a spherical calotte (Figure 3.6d), constrained the AS core
to have the same height as the organic shell; here a small portion of the AS core could be
exposed. In the remaining two configurations, half-sphere in a half-sphere (Figure 3.6a) and halfsphere in a spherical calotte (Figure 3.6b), the AS core was fully engulfed in an organic shell. In
all cases, the contact angle of the outer spherical calotte was assumed to be 77°. To determine
which of these configurations was most likely, we measured the radius of the AS core and C6
shell to calculate the ratio of the volume of AS (VAS) to the total volume (Vtotal) for each
arrangement. These calculations were done for ten particles ranging from 10-35 µm in diameter
(Figure 3.6e), which were diameters typical of ice nuclei in this work. We then calculated the
experimental ratio VAS/Vtotal using the 2:1 organic-to-sulfate mass ratio of the bulk solution and a
bulk density of 1.106 and 1.77 g/cm3 for pure C6 and effloresced AS, respectively. As shown in
Figure 3.6e, the bulk density calculation VAS/Vtotal matches best with the half-sphere in a
spherical calotte configuration and is inconsistent with both configurations that constrain the
inner and outer particles to the same height. Using this half-sphere in a spherical calotte
morphology, the difference in heights between the top of the C6 calotte and the top of the
52
Figure 3.6. Possible morphologies of particles after phase separation and efflorescence from
Ciobanu et al. [2009]: (a) half-sphere in a half-sphere, (b) half-sphere in a spherical calotte
(contact angle of spherical calotte = 77°), (c) spherical calotte in a spherical calotte (same height,
contact angle of outer spherical calotte = 77°), and (d) half of an ellipsoid in a spherical calotte
(same height, contact angle of outer spherical calotte = 77°). Also shown (e) are the calculated
ratios of effloresced AS volume to the total volume for each particle morphology compared to
bulk volume ratio for particles from 10-35 µm diameter used in the present study.
53
effloresced AS half-sphere was determined for the particles in Figure 3.6e and ranged from 1.3
to 3.6 µm. Thus, from these measurements, we conclude the particles used in this study are
effloresced AS cores fully engulfed in organic shells.
Ice Nucleation Experiments. The ice nucleation behavior of the pure C6 and C6/AS system can
be seen in Figure 3.7. Also shown is a parameterization for depositional ice nucleation on pure
AS from Baustian et al. [2010]. As seen, pure C6 is a poor ice nucleus at all temperatures
explored. Indeed, at the lower temperatures, C6 tends to nucleate ice near or even above the
Koop homogeneous freezing line. This is expected, as pure C6 is expected to be a liquid at each
of the temperatures and RH probed in this study and the Koop line is for homogeneous
nucleation from aqueous solutions. It should be noted that the slope of our homogeneous
nucleation points is slightly skewed from the line of Koop et al. [2000] for a homogenous
nucleation rate of 5x109 cm-3 s-1. This could be the result of our lowest temperature points being
slightly outside of our temperature calibration curve as the lowest point in our calibration curve
is the ferroelectric phase transition of ammonium sulfate at 223.1 K; however, groups studying
the homogeneous nucleation of sulfuric acid [Mohler et al., 2003] and organics [Wilson et al.,
2012] using the AIDA chamber have observed slopes similar to ours.
In contrast, for the C6/AS system we find that these phase-separated particles are very
efficient ice nuclei. In fact, these mixed organic-sulfate particles tend to nucleate ice nearly as
efficiently as pure AS despite their full organic coatings. To elucidate why these phase-separated
particles are such efficient ice nuclei, we took several videos at 50x magnification of ice
nucleating on a larger particle. Select frames from one example video are shown in Figure 3.8
and the entire video is available in the supplementary DVD (Movie A.1). The particle before ice
nucleation is shown at 0 s. At 20 s, ice nucleates inside of the coated particle at the AS-organic
54
Figure 3.7. Scrit as a function of temperature for homogeneous freezing of pure C6 and
immersion freezing of C6 by the AS core (C6/AS). The thick and thin solid lines refer to water
and ice saturation respectively. The dashed line represents the Scrit values for homogeneous
nucleation of an aqueous droplet [Koop et al., 2000]. Also included is a parameterization for
depositional ice nucleation on pure AS from Baustian et al. [2010], as indicated by the hatched
line. Error bars in Scrit and temperature are reported as one standard deviation.
55
Figure 3.8. Optical microscope images of ice nucleating on the effloresced AS core of a C6/AS
particle that has undergone LLPS at 215 K. Ice forms around 20 s at the AS-organic interface. As
the ice grows beyond the dimensions of the organic shell, the shell spreads around it to reduce
the surface tension over the entire system, indicating that the shell is a flowing liquid. The size
bar corresponds to 10 µm.
56
interface (bottom right side of the inner part of particle). The initial ice crystal appears as a bright
spot and grows outwardly from that point; by 80 s, the ice has grown beyond the lateral
dimensions of the organic coating. As the ice grows, the organic coating spreads over it to reduce
the surface tension over the entire system. In the last frame, we see that the ice particle has now
grown beyond the size of the original ice nucleus and is still stretching out the organic coating
with it. This is more easily seen in Figure 3.9, where a line map of the ice particle after ice has
nucleated and grown is shown. As described previously, spectra are taken every 1 µm in the Y
direction; thus, every notch on the red line in the image of the particle corresponds to the location
of a spectrum. The spectra are shown as a function of distance, and their intensities are shown as
a color map, where warmer colors correlate to a higher intensity. From the line maps of the
ammonia and sulfate stretches, we can see that an ammonium sulfate particle exists between ~21
and 30 µm. Around that particle, between ~9 and 35 µm, there is a large water-ice signature at
3250 cm-1. Correlated with the ice stretch is the C-H stretch (2900 cm-1) that corresponds to the
presence of C6. Thus, not only can we visually see that the liquid organic coating is spreading
out as the ice grows, but we can verify that spectrally. Similar to our finding that liquid organic
coatings minimally affect the deliquescence and efflorescence behavior of AS, we conclude that
the water vapor must be diffusing through the liquid coating and ice is nucleating on the AS core.
Thus, for the C6/AS particles, we are observing immersion freezing of the organic solution in
contact with the AS cores. This explains why, even though these mixed particles are fully coated,
these particles are almost as efficient ice nuclei as pure, effloresced AS in the deposition
nucleation mode. To verify these findings with a more complex system with the same O:C ratio,
we conducted ice nucleation experiments on C6/C10 and C6/C10/AS mixtures. The results of
these experiments are shown in Figure 3.10. At temperatures ≥ 220 K, the C6/C10 and
57
Figure 3.9. Optical microscope image and corresponding Raman line map of C6/AS/H2O-ice at
215 K. Raman spectral maps, taken every 1 µm as indicated by the horizontal notches in the
microscopic image, are shown as a function of distance and blended using a smoothing function.
Raman intensities are shown as warm colors.
58
Figure 3.10. Scrit as a function of temperature for pure C6/C10 and C6/C10/AS. The thick and
thin solid lines refer to water and ice saturation respectively. The dashed line represents the Scrit
values for homogeneous nucleation of an aqueous droplet [Koop et al., 2000]. Also included is a
parameterization for deposition ice nucleation on pure AS from Baustian et al. [2010], as
indicated by the hatched line. The shaded regions split the experimental data into two viscosity
regimes based on flow. Green shaded region: C6/C10 observed to flow over growing ice crystal;
here we observed immersion freezing of the organic soution in contact with the AS core. Purple
shaded region: C6/C10 observed to be in low-flowing, semi-solid state; here we observed
depositional freezing on the C6/C10 shell. Error bars in Scrit and temperature are reported as one
standard deviation.
59
C6/C10/AS systems have similar flow behavior to pure C6 and C6/AS. Furthermore, their ice
nucleation behavior is qualitatively similar. Here, the pure organic system tends to be a poor IN;
however, it is important to note that the pure organic mixture nucleates ice below the Koop
homogeneous nucleation line and, therefore, the particles are likely acting as heterogeneous IN.
For the C6/C10/AS system, the particles were again seen to nucleate ice almost as efficiently as
pure AS in the depositional mode. Again, we find that at these temperatures we are observing
immersion mode nucleation by the AS core. Thus, water vapor diffuses through the liquid
organic shell, ice nucleates on the ammonium sulfate core, and the organic coating stretches out
with the ice as it grows (not shown). Not only was this confirmed visually, but video and line
map analysis similar to Figure 3.8 and 9 were also performed (not shown).
At 215 K, we found that ice nucleates differently for both C6/C10 and C6/C10/AS. This
is accompanied with an increase in ice nucleation efficiency of the pure C6/C10 system. Here,
we found that ice nucleates at the organic-air interface for both the pure organic and the organicsulfate system. This is shown in Figure 3.11 for select frames with the full nucleation movie
available in the supplementary DVD (Movie A.2). Here, the images clearly show that ice is
nucleating on the outside of the particle and not the inside. In addition, after ice nucleates on the
organic shell, the organic does not spread out around the ice, suggesting that the organic shell is
no longer a flowing liquid. This is verified by a line map taken of the ice particle (Figure 3.12).
Here we see there is a sulfate core between ~ 16 and 31 µm surrounded by an organic coating
(~12.5 to 37 µm); the ice, found between ~5.5 and 12.5 µm, however is not in contact with the
sulfate core, and only grows from the organic shell. Furthermore, we see that the ice stretch is
not correlated with the organic stretch. We believe that at these temperatures and relative
humidities, the organic mixture is no longer a liquid, but rather a highly viscous (semi-)solid or
60
Figure 3.11. Optical microscope images of ice nucleating on the shell of a C6/C10/AS particle
that has undergone LLPS at 215 K. Ice forms around 10 s at the organic-air interface. As the ice
grows, the organic shell does not spread around it, indicating that it may be in a highly vicious
(semi-)solid or glassy state. The size bar corresponds to 10 µm.
61
Figure 3.12. Optical microscope image and corresponding Raman line map of C6/C10/AS/H2Oice at 215 K. Raman spectral maps, taken every 1 µm as indicated by the horizontal notches in
the microscopic image, are shown as a function of distance and blended using a smoothing
function. Raman intensities are shown as warm colors.
62
glassy particle.
Despite the change in the heterogeneous nucleation site, the (semi-)solid or glassy
organic IN in this work nucleate ice depositionally nearly as efficiently as pure AS. Although we
are unable to probe this effect with our current experimental setup, we attribute this ice
nucleation efficacy to the incorporation of water molecules into the glassy matrix. Prior to
undergoing a glass transition, the organic will be in a liquid state that is in equilibrium with its
surrounding water vapor [Mikhailov et al., 2009]. Upon approaching the glass transition, the
diffusion of water in the organic matrix will drop, and water will become entrenched in the
viscous organic matrix [Zobrist et al., 2011]. These entrenched water molecules could facilitate
the formation of ice by providing a template similar to the crystal structure of ice [Pruppacher
and Klett, 1996].
Ice Nucleation on Solid Amorphous Organic Particles above the Glass Transition Line. The
experimental glass transition lines determined using differential scanning calorimetry by Zobrist
et al. [2008] for aqueous glasses of C6 and C10 are shown in Figure 3.13. The corresponding
shaded areas around each of these Tg curves are the associated errors calculated using the
parameterizations found in Zobrist et al. [2008]. Also shown is the predicted Tg curve for a 1:1
mixture (by mass) of C6 and C10 using a mixing rule that depends on the molar masses and
weight fractions of the individual polyols [Zobrist et al., 2008]. Since this predicted curve does
not have an associated error, the orange shaded region is simply bound by the pure C6 and C10
Tg curves. Plotted with the experimental and predicted Tg curves are the threshold ice nucleation
points for C6/C10 and C6/C10/AS at 210 and 215 K, where the organic particles/coatings were
visually and spectrally shown to behave like highly viscous (semi-)solids or glasses (Figure 3.11
63
Figure 3.13. The glass transition temperatures of C6, C10, and 1:1 C6/C10 shown as a function
of water activity. The pure C6 and pure C10 glass transition curves are experimental data from
Zobrist et al. [2008] using differential scanning calorimetry. The 1:1 C6/C10 glass transition
curve was calculated using a mixing rule that depends on weight percent and molar mass. Also
plotted are the conditions (temperature and RH) for depositional ice nucleation on C6/C10 and
C6/C10/AS, where the organic particle/coating was visually and spectrally shown to behave like
a highly viscous (semi-)solid or glass. For clarity, the ice melting curve (black dashed line) and
homogeneous freezing line (black solid line, [Koop et al., 2000]) are also plotted.
64
and Figure 3.12). As shown, these points are above the predicted glass transition line for a 1:1
mixture of C6 and C10.
To explain this behavior, we set out to approximate the viscosity of the C6/C10 at the
onset freezing conditions. The steep dependence of viscosity (η) on temperature can be described
by the Williams-Landel-Ferry equation [Debenedetti, 1996]:
log(η) = log(ηTg) ∙ [17.44(T – Tg)] / [ 51.6 + (T – Tg)],
(1)
where ηTg is the viscosity at the glass transition temperature, taken to be 1012 Pa s. For the ice
nucleation experiments at 215 K, ice nucleated on C6/C10 and C6/C10/AS at 70.7% RH. At this
RH, the predicted Tg for 1:1 C6/C10 is 185 K. Using Eq. (1), η for 1:1 C6/C10 is approximately
6.4 x 105 Pa s. Similarly, at 210 K, ice nucleated on C6/C10 and C6/C10/AS at 74.0% RH, which
correlates to a predicted Tg of 181 K and an η of approximately 3.7 x 105 Pa s. Thus, at 210 and
215K, pure C6/C10 and the C6/C10 coatings are considered amorphous semi-solids [Shiraiwa et
al., 2011].
Zobrist et al. [2011] have shown that the diffusion coefficients for a small molecule like
water diffusing through a glassy matrix at room temperature is ~10-10 cm2 s-1; however, the
diffusion constant was observed to decrease strongly with decreasing temperature. For example,
at 200 K, the diffusion coefficient for water through a glassy matrix drops 10 orders of
magnitude to 10-20 cm2 s-1. Similar temperature dependence could be expected for highly viscous
semi-solids above the glass transition. To the authors’ knowledge, no such measurements have
been made at temperatures relevant to this study. Shiraiwa et al. [2011] have measured the
diffusion of the small molecule ozone through an amorphous semi-solid matrix at room
temperature to be ~10-7 to 10-9 cm2 s-1. If a similar, strong temperature dependence followed for
65
amorphous semi-solids, the time for a water molecule to diffuse through a 1 µm semi-solid shell
may take much longer than that timescales of our ice nucleation experiments.
As mentioned before, the pure C6/C10 mixture nucleated ice below the Koop
homogeneous nucleation line from 220-230 K. Experiments similar to Figure 3.8 have confirmed
that these particles are able to flow and stretch with the growing ice particle (not shown).
Analysis of the ice nuclei were also conducted to confirm that solid impurities, capable of acting
as immerions mode nuclei were not present visually or spectally. Although it is beyond the scope
of this paper to ascertain the mechanism behind these results, the authors postulate that these
particles may have the flow behavior of a liquid, but are still sufficiently viscous to provide a
surface template for heterogeneous ice nucleation. To support this postulation, the viscosity of
the pure organic mixture at the temperature and relative humidity of the ice nucleation event has
been calculated using Equation 1. For the 220, 225, and 230 K experiments ice formation was
noted at approximately 79.8, 80.2, and 85.3% relative humidity with respect to water,
respectively; this corresponds to an estimated viscosity of 5.2x103, 1.5x103, and 1.2x102 Pa s,
respectively. These viscosities correlate to highly viscous liquids like ketchup [Koop et al.,
2011]. While these calculations provide only a rough estimation of these particles’ viscosity,
they suggest that the pure organic mixture at these temperatures and relative humidities might be
viscous enough to provide a surface for heterogeneous ice nucleation.
Thus, for phase-separated organic-sulfate particles studied here, we find that if an organic
coating is in its liquid state, then water vapor diffuses through the organic shell and nucleates on
the AS core. If the organic coating become sufficiently viscous, however, then the water vapor
can no longer diffuse through to the AS core, and ice nucleates on the organic shell. Two things
should be noted here. One, the AS core does not seem to affect the ice nucleation behavior of the
66
semi-solid organic; thus, the ice nucleation properties of these particles may depend almost
entirely on the properties of the organic species. Two, for mixed organic-sulfate particles with
liquid organic coating, the organic species minimally affects not only the deliquescence and
efflorescence, but also the ice nucleation behavior of AS
ATMOSPHERIC IMPLICATIONS
Recently, several studies have underlined the importance of LLPS and its effect on the
hygroscopic phase transitions of complex organic-sulfate aerosol. Song et al. [2012] have shown
that mixtures of three dicarboxlyic acids and AS will undergo LLPS as long as their average O:C
value was < 0.7. It was also shown that these phase-separated particles minimally affect the
deliquescence and efflorescence behavior of pure AS at 298 K, which is in agreement with
results from Bertram et al. [2011] taken at 273 and 290 K. This behavior has also been seen with
complex organic-sulfate particles at 298 K that were generated by condensing secondary organic
aerosol (SOA) produced by dark ozonolysis of α-pinene on AS seed particles [Smith et al.,
2011]. Here, the SOA was likely comprised of tens to hundreds of different organic molecules
with average O:C ratios of 0.39 and 0.44 as measured by the Aerodyne aerosol mass
spectrometer (AMS).
The deliquescence and efflorescence behavior of these phase-separated particles is in
stark contrast with uniphase organic-sulfate aerosol. For example, Marcolli et al. [2004] have
shown that particles consisting of five organic acids and AS that do not undergo LLPS, have a
DRH of 36.4%, and may inhibit efflorescence in these particles altogether. The authors
concluded that ambient organic-sulfate aerosol may exist predominantly in a liquid state. Similar
behavior was found for complex, chamber made SOA produced by the photo-oxidation of
67
isoprene (O:C ratios of 0.67 and 0.74) condensed onto AS seed particles [Smith et al., 2012].
Here, efflorescence was completely inhibited for organic volume fractions above 0.6, and the
DRH was as low as 40% for organic volume fractions approaching 0.9. These results are in good
agreement with the parameterization of Bertram et al. [2011], who have shown that phase
separation is generally inhibited for organics with an O:C ratio > 0.7, and that large organic-tosulfate ratios in uniphasic particles will depress both the DRH and ERH of organic-sulfate
particles.
Many recent studies of ambient organic aerosol, however, show an average O:C ratio of
the organic fraction as measured by the AMS < 0.7, even in heavily polluted areas like Mexico
City [Aiken et al., 2008]. Furthermore, factor analysis of 43 AMS data sets show that all semivolatile oxygenated organic aerosol (SV-OOA) had an average O:C ratio of 0.34 ± 0.14 and lowvolatility OOA (LV-OOA) had an average O:C ratio of 0.73 ± 0.14 [Ng et al., 2010]. Thus, all of
the SV-OOA and a part of the LV-OOA fall into the O:C range where LLPS is expected. Indeed,
You et al. [2012], have observed LLPS at 291 K in real-world, organic-sulfate particles collected
the Atlanta, GA region. Finally, modeling work from Jensen et al. [2010] have shown that
deliquesced ammonium sulfate particles that detrain from deep convective updrafts in the cold
tropical tropopause layer can undergo efflorescence and be dry 20-80% of their lifetime;
however, it is important to note that these results only hold if the organic coatings or mixtures do
not inhibit efflorescence. These two combined effects indicate that effloresced, phase-separated
particles can exist in the upper troposphere and could provide potential ice nuclei.
Our measurements show that phase separation in mixed organic-sulfate particles can
occur at temperatures as low as 245 K, and that liquid organic coatings minimally affect the ice
nucleation efficiency of effloresced ammonium sulfate cores down to 210 K. The heterogeneous
68
ice nucleation efficiency of solid, effloresced ammonium sulfate has already been established
[Abbatt et al., 2006; Baustian et al., 2010; Wise et al., 2010]. Upon cooling or drying,
atmospheric organics have been shown to undergo a glass transition [Koop et al., 2011], which
often acts as a surface for heterogeneous ice nucleation [Wilson et al., 2012]. In this work,
C6/C10 was observed to nucleate ice at Sice = 1.2 to 1.3 from 210 to 215 K. It should be noted,
however, that the particles used in this study are larger than potential ice nuclei expected to be
found in the upper troposphere [Froyd et al., 2009]; in fact, particles used in this experiment
have volumes a factor of hundreds to thousands larger than a typical accumulation mode particle.
This could induce a size effect, especially with regard to the diffusion of water to the particle
core. Since water diffusion times will be longer for thicker organic coatings/larger particles, the
particles used in this study could exhibit glassy behavior at warmer temperatures than smaller
particles typically found in the upper troposphere. Despite this size effect, these results are still in
accordance with previous work on ice nucleation on simple organic glasses using a different size
range of particles. For example, Murray et al. [2010] saw ice nucleate on glassy citric acid at Sice
= 1.23 for 210 K. In a follow-up study Wilson et al. [2012] saw ice nucleate on single component
aqueous organic glasses at Sice = 1.2 to 1.55 from 190.8 to 218.5 K. Finally, Baustian et al.
[Baustian et al., 2013] have shown that ice nucleates on glassy glucose, sucrose, and citric acid
between 210 and 235 K at Sice = 1.1 to 1.4. Glassy, chamber generated SOM from the oxidation
of naphthalene nucleated ice only slightly higher, from Sice = 1.36 to 1.52, at T < 230 K [Wang et
al., 2012b], but the authors state their minimum Sice values for this temperature range was 1.23.
Thus, the combined ambient O:C ratios, deliquescence, liquid-liquid phase separation,
and efflorescence behavior of organic-sulfate mixtures, even when the organic fraction is multicomponent, indicates that phase-separated particles could exist in the upper troposphere. Our
69
measurements show that phase-separated, effloresced organic-sulfate particles can be efficient
IN regardless of the phase state of the organic species, suggesting that these particle types are of
significance for atmospheric ice nucleation.
CONCLUSIONS
We have shown that particles consisting of mixtures of AS with C6 and C6/C10, both
having an O:C ratio of 0.5, will phase separate at temperatures from 245 to 260 K. At these
temperatures, it was also seen that these organic coatings minimally affect both the
deliquescence and efflorescence relative humidities of AS.
We have also shown that pure C6 is a poor ice nucleus, initiating the formation of ice
near or above the Koop homogeneous nucleation line. Pure C6/C10 behaved similarly from 220
to 235 K; however, at 215 K, this system became a more efficient ice nucleus. We attribute this
to the particle moving from a liquid to a highly viscous (semi-)solid or glassy state. Ice
nucleation on the particle exterior was confirmed both visually and spectrally using Raman line
maps.
For the mixed organic-sulfate particles, we saw that if that particles underwent phase
separation and efflorescence, and that if the coating was liquid, water vapor diffused through the
liquid organic shell and nucleated on the AS core; here, we were observing immersion freezing
of the organic shell by the AS core. If the organic coating becomes semi-solid or glassy,
however, then water vapor may no longer be able to diffuse through the organic on timescales
relevant to even the atmosphere; here, ice can depositionally freeze on the organic shell.
In conclusion, we find that phase-separated mixed organic-sulfate particles can be
excellent ice nuclei whether or not the organic coating is a liquid or a glass. If the coating is
70
liquid, then the AS nucleates ice in the immersion mode at similar Sice as pure AS in the
depositional mode. If, however, the coating becomes semi-solid or glassy, then it still may be
good ice nucleus, possibly due to the retention of water molecules in the organic matrix.
71
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CHAPTER 4: HETEROGENEOUS ICE NUCLEATION ON SIMULATED SECONDARY
ORGANIC AEROSOL
INTRODUCTION
Ice clouds are ubiquitous in the upper troposphere (UT) and affect climate by scattering
solar and terrestrial radiation and absorbing terrestrial radiation. Despite their ubiquity, ice
clouds are poorly constrained in climate models; for example, in their Fourth Assessment, the
International Panel on Climate Change did not include an aerosol indirect effect contribution
from ice clouds due to an insufficient understanding of ice nucleation by atmospheric aerosol
[Forster, 2007]. One reason for this poor understanding is that the chemical nature of ice nuclei
(IN) found in the UT is not well constrained. Further, the phases and morphologies of the IN
particles are entirely unknown.
Recently, Froyd et al. utilized a counter-flow virtual impactor coupled to a single particle
mass spectrometer to determine the chemical composition of UT background aerosol [Froyd et
al., 2009] and aerosol that nucleated into sub-visible cirrus particles [Froyd et al., 2010]. In both
cases, the dominant aerosol types were mixtures of organics and sulfates; surprisingly, the subvisible cirrus residues did not contain a large number of particles that are typically thought to be
excellent IN, such as mineral dust. While the ice nucleation properties of effloresced ammonium
sulfate particles are well known [Abbatt et al., 2006; Baustian et al., 2010; Wise et al., 2010], the
effect of adding organic species is uncertain.
Several laboratory studies have shown that coating efficient IN with organic species can
deactivate those IN. For example, Mohler et al. [2008] found that coating mineral dust with
secondary organic material from the dark ozonolysis of α-pinene required higher ice
76
supersaturations to nucleate ice compared to the bare dust particle. A recent study from our
group has discovered that the phase of the organic coating plays a critical role in determining the
ice nucleation ability of effloresced ammonium sulfate particles coated with an organic species
[Schill and Tolbert, 2013]. In that study, we found that if the organic coating is liquid, then the
organic material minimally affected the ice nucleation properties of ammonium sulfate; if the
coating became sufficiently viscous, however, then the organic coating dictated the ice
nucleation properties of the particle. This behavior was attributed to the fast and slow diffusion
of water through the liquid and viscous organic coatings, respectively. Several studies have
recently shown that soluble organic species can undergo a glass transition under atmospheric
conditions [Koop et al., 2011; Zobrist et al., 2008], and that those organic glasses can nucleate
ice heterogeneously [Baustian et al., 2013; Murray et al., 2010; Wilson et al., 2012]; however,
fewer studies have looked at the phase behavior and ice nucleation properties of complex
secondary organic aerosol. To the authors' knowledge, only one previous study has examined the
heterogeneous ice nucleation behavior of pure glassy secondary organic aerosol, which was
generated from the gas phase photooxidation of naphthalene in a Potential Aerosol Mass
chamber [Wang et al., 2012]. In a separate study, it has been shown that this reaction can
produce highly viscous semi-solid or glassy particles [Saukko et al., 2012].
In addition to the condensation of compounds formed via gas-phase oxidation, secondary
organic aerosol can be formed from aqueous reactions in cloud droplets and aqueous aerosol
(aqSOA) [Blando and Turpin, 2000]. In fact, recent modeling work has shown that this aqSOA
could contribute almost as much organic aerosol by mass as gas-phase oxidation of aerosol
precursors (gasSOA) [Ervens et al., 2011]. It has been shown that one pathway to aqSOA is via
dark reactions between small α–dicarbonyls and reduced amines in evaporating cloud droplets
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[De Haan et al., 2009; De Haan et al., 2011; Sedehi et al., 2013]. These reactions are often
accompanied by browning and oligomer formation, producing hundreds to thousands of distinct
peaks in their electrospray mass spectra; products include imines, imidazoles, and their
respective oligomeric products. These high molecular weight products are likely to promote
glass formation, as a literature compilation by Koop et al. [2011] found glass formation
temperature and molecular weight to be highly correlated.
In this work, we have studied the phase behavior and ice nucleation properties of
simulated evaporated cloud droplets containing methylglyoxal and methylamine. We find that
these simulated evaporated cloud droplets are in a semi-solid to glassy phase state, as indicated
by their behavior when exposed to mechanical pressure as well as their flow behavior.
Furthermore, we find that they are poor IN, in contrast to previous ice nucleation studies on
simple mixtures of glassy organics. In addition, we explored the effect of adding ammonium
sulfate to the simulated cloud droplets. We find that adding ammonium sulfate lowers the
viscosity of the resulting aqSOA, and that it changes not only the mode of nucleation, but also
increases the overall ice nucleation efficacy of the particle.
EXPERIMENTAL
Raman Microscope. The experimental setup has been described in detail previously [Baustian
et al., 2010; Schill and Tolbert, 2013]. Briefly, a Nicolet Almega XR Dispersive Raman
spectrometer has been coupled to an Olympus BX51 research grade microscope with 10x, 20x,
50x, and 100x capabilities. In addition, the Raman microscope has been outfitted with a Linkam
THMS 600 environmental cell that can control the temperature (T) of the sample from -191 to
600 °C. The relative humidity inside the environmental cell is controlled by mixing dry and wet
78
flows of N2; the wet flow of N2 is produced by bubbling dry N2 through a fritted glass bubbler
immersed in high-purity water. The absolute water pressure inside of the cell is measured by a
Buck Research CR-1A chilled mirror hygrometer in line with the cell, which has an accuracy of
0.15 K. The relative humidity inside of the cell is determined by taking the ratio of the absolute
water pressure as determined by the dew point hygrometer to the equilibrium vapor pressure over
water as parameterized by Murphy and Koop [2005]. A Gast diaphragm pump is connected to
the back of the hygrometer, ensuring that the flow through the system is maintained at 1 L min-1.
Raman spectra and spectral maps were acquired using a frequency doubled Nd:YAG
laser operating at 532 nm. All spectra were obtained using either 50x or 100x objectives,
corresponding to diffraction limited spot sizes of 1.3 and 1.1 µm and depths of field of 11 and 8
µm, respectively. Spectra were collected from 150-4000 cm-1, with a typical spectral resolution
of 2-4 cm-1. Raman line and two-dimensional maps were collected by obtaining spectra every 1
µm in the Y and XY directions, respectively.
Sample Preparation. Simulated cloud droplets were prepared by first mixing a solution of 8
mM methylglyoxal and 8 mM methylamine. While such a mixture turns yellow over a period of
hours, the solution was always used immediately. The solutions were aspirated into a Meinhard
TR-50 glass concentric nebulizer, and the nebulized spray was directed onto a hydrophobically
treated fused silica disk. The nebulized droplets were allowed to coagulate into supermicron
droplets and the silica disk was immediately transferred to the environmental cell and exposed to
~0% RH at 298 K. Upon drying, the concentration of the reactants increases about three orders
of magnitude to 5-10 M [De Haan et al., 2011], driving aqueous reactions and leaving aqSOA
particles ~1-40 µm in lateral diameter.
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To study the effect of adding ammonium sulfate to the simulated cloud droplets, a
solution of 8 mM methylglyoxal, 8 mM methylamine, and 8 mM ammonium sulfate was
nebulized under the exact same conditions as above. These droplets were also immediately
transferred to the environmental cell at ~0% relative humidity and 298 K.
Impact-Flow Experiments. To conduct impact-flow experiments, aqSOA particles were first
dried in the environmental cell at 298 K and ~0% RH for at least 10 min. The cell was then
opened and the particles were sandwiched by adding an additional fused silica disc. To minimize
influence from the laboratory RH, the particles were protected by a blanket of dry N2 during this
process. Mechanical pressure was applied to the outside of both of the discs; the additional disc
was then removed and the original disc was placed back inside the environmental cell. In
concurrence with Murray et al. [2012], the mechanical pressure exerted on aqSOA will flatten
and splatter liquid particles and shatter semi-solid or glassy particles.
In order to ensure that particles in impact-flow experiments did not experience high
relative humidities upon opening the cell and sandwiching the particles, impact experiments
were conducted on effloresced LiCl particles of a similar size distribution. At 298 K, LiCl has a
deliquescence relative humidity of 11.3 ± 0.3% [Greenspan, 1977]; thus, if the particles flatten
or splatter, then the particles experienced an RH ≥ 11.3%. The results of these LiCl impact
experiment can be seen in Figure A.1. As shown, the effloresced LiCl particles started out
roughly spherical and upon impact were shattered. Since the particles shattered, they could not
have undergone deliquescence, indicating that the particles experienced an RH < 11.3% upon
opening the cell and sandwiching the particles between two discs.
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If the aqSOA particles shattered, flow experiments were then conducted at various
temperatures by raising the relative humidity over the shattered particles. In each experiment, 1-3
shattered particles were observed under 50x magnification. The relative humidity was raised by
keeping a constant dew point and steadily decreasing the temperature. Specifically, after the dew
point remained stable for at least 10 min, T was reduced at 10 K min-1 until RH ≈ 60%. The
sample was then cooled at a rate of 0.1 K min-1, corresponding to an RH ramp rate of 0.6%
min-1, until the crack(s) in a particle started to wane and/or the edges of the shattered particle
began to round. This RH was defined as the Start of Flow RH (RHflow). The T was further
dropped until RH ≈ 95%, by which time the particles in this study had fully liquefied and
resumed a spherical morphology.
For aqSOA + ammonium sulfate studies, impact experiments could not be conducted
because the particles contained crystalline material; here it is not possible with our current setup
to determine whether the particles shattered under mechanical pressured due to the crystalline
material, the amorphous (semi-)solid organic, or both, regardless of the phase of the organic
material. For these particles, the deliquescence RH (DRH) of the crystalline material was
determined instead. For deliquescence experiments, the particle was also allowed to sit at 298 K
and 0% RH for at least 10 min to ensure that the particle was nominally dry. Deliquescence
experiments were then conducted in the exact same manner as the flow experiments, except the
visual cue was now the prompt dissolution of the crystalline material. For both flow and
deliquescence experiments, at least 3 experiments were performed on 3 separate sample discs.
Ice Nucleation Experiments. For ice nucleation experiments, deposited particles were allowed
to sit at 298 K and 0% RH for at least 10 min. The ice saturation ratio (S ice = PH2O/VPice) over
each particle was then increased by keeping the dew point constant and steadily decreasing T. In
81
this study, after the dew point remained stable for at least 10 min, T was decreased from 298 K at
a rate of 10 K min-1 until Sice ~ 0.9. The sample was then cooled at a rate of 0.1 K min-1,
corresponding to an Sice ramp rate of 0.01 K min-1, until the onset of ice was observed. For each
ice particle formed, a Raman line map of that particle was taken to determine whether the
organic portion of the particle had spread out over the ice and, therefore, was in a flowing state.
After this initial Raman line map was taken, the ice was sublimed by turning off the flow of wet
N2, and an additional line map of the particle that nucleated ice was taken.
RESULTS AND DISCUSSION
A 50x image of methylglyoxal + methylamine aqSOA can be seen in Figure 4.1a. As
shown, the particles are spherical and show no internal structure, suggesting that they are in an
amorphous state. This is expected as previous work by De Haan et al. [2011] has shown that dark
reactions in simulated evaporated cloud droplets containing methylglyoxal and methylamine
form products with varied functional moieties, including imines and imidazoles, and contain
hundreds of distinct peaks in their electrospray mass spectra; thus, aqSOA produced from these
reactions are unlikely to crystallize. From Figure 4.1a, however, we are unable to discern if they
are liquid or (semi-)solid. Figure 4.1b shows the same particles, but after mechanical pressure
was applied. Here, the particles have shattered after impact, suggesting that they are in a highly
viscous, likely solid (glassy) state. Without dynamic viscosity measurements, however, we
cannot discern highly viscous semi-solids from glasses. Thus, we will refer to these particles as
amorphous (semi-)solids. In a separate experiment, the shattered particles were allowed to sit at
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Figure 4.1. 50x image of methylglyoxal + methylamine aqSOA particles (a) freshly dried and
(b) immediately after impact. As shown, the amorphous particles shatter under mechanical
pressure, indicating that they are in a semi-solid or glassy state. The size bar in the bottom right
hand corner corresponds to 20 µm.
83
298 K and 0% RH for 12 hours. In this time period, these particles did not exhibit any visual
flow behavior. In a similar experiment, Renbaum-Wolff et al. [2013] found that gasSOA from
the photoxoidation of α-pinene could be shattered by a needle under comparable conditions;
using an advanced multiphysics software package, they surmised that if the edges of a simulated
shattered particle did not flow more than 0.5 µm after 8 hours, it had a viscosity of ≥ 108 Pa s,
correlating to a particle that was in the semi-solid to glassy range.
A typical image series of an impact-flow experiment can be seen in Figure 4.2 and the
full video record can be seen in the supplementary DVD (Movie A.3). Through the course of this
particular experiment, the RH increases from 51 to 93% as the temperature drops from 253.3 to
246.6 K. As shown, there is no apparent visual change in the particle from Figure 4.2a to 2b,
even though the RH is increasing. At a set RH, however, the edges of the shattered particle start
to round (Figure 4.2c), which we have defined as RHflow. As the RH is increased beyond RHflow,
the particle continues to liquefy (Figure 4.2d) and eventually flows back together into a sphere to
reduce the surface tension of the system (Figure 4.2e). This process occurs because water has a
lower viscosity than the aqSOA and, therefore, acts as a plasticizer. It should be noted that full
liquefactions will depend on the RH ramp rate and the particle size. While our ramp rates are
relevant to the updraft rate of an air parcel in the UT [Karcher and Strom, 2003], our particles
are much larger than particles typically found in the UT [Froyd et al., 2010; Froyd et al., 2009].
To mitigate this size effect, we have used RHflow, much like Wang et al. [2012] and Baustian et
al. [2013] used onset water uptake, instead of full liquefaction to indicate a change in flow
behavior under our experimental conditions.
The start of flow, RHflow, for several methylglyoxal + methylamine experiments can be
seen in Figure 4.3. As shown, there is a clear temperature effect, where particles at colder
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Figure 4.2. A typical image series of a flow experiment on a shattered methylglyoxal +
methylamine aqSOA particle. In this particular experiment, the RH increases from 51 to 93% as
the temperature drops from 253.3 to 246.6 K. Initially, as the RH around the shattered particle (a)
increases, there is no apparent change in in the particle morphology (b). At a set RH, however,
the cracks within the particle start to wane and the edges of the particle start to round (c). We
have defined this point as RHflow. As the RH is increased beyond RHflow, the particle continues to
liquefy (d), eventually attaining a low enough viscosity to form back into near spherical as to
reduce the surface tension of the system (e). The size bar in the bottom right hand corner
corresponds to 20 µm.
85
Figure 4.3. The Start of Flow RH and onset ice saturation ratios for depositional nucleation on
methylglyoxal + methylamine aqSOA as a function of temperature. The solid line indicates water
saturation (100% RH) and the dashed gray lines correspond to 90, 80, 70, and 60% RH. The
dashed black line corresponds to the homogeneous ice nucleation limit [Koop et al., 2000]. For
clarity, two regions of different behavior have been shaded. In the green shaded region, the
particles liquefy before they can nucleate ice. In the purple shaded region, the particles nucleate
ice depositionally before they liquefy. Error bars are reported as one standard deviation.
86
temperatures require higher RH for the edges of the particle to start flowing. A similar trend was
shown for the water uptake of glassy SOA made from the gas-phase photooxidation of
naphthalene [Wang et al., 2012]. At 225 K, we could no longer measure RHflow, because at this
temperature the semi-solid/glassy particles would nucleate ice depositionally first. Thus, at T ≤
225 K, impact-flow experiments were no longer performed and instead ice nucleation
experiments on un-shattered particles were performed. An example image of ice depositionally
nucleating on an aqSOA particle at 220 K can be seen in Figure 4.4a. To verify that the aqSOA
particles were in a non-flowing state, we conducted Raman line maps of ice particles similar to
those found in Schill and Tolbert [2013] (not shown). Here, the organic Raman signals (C-H
stretching vibrations, ~2900 cm-1) were not observed on the ice crystal that formed. This shows
that the aqSOA did not spread out over the growing ice surface and provides further evidence
that the particle is in a non-flowing, (semi-)solid state at these onset conditions. These conditions
can also be found in Figure 4.3. As shown, these aqSOA particles require ice supersaturations of
~30-50%, depending on temperature, to nucleate ice.
To study the effect of adding ammonium sulfate to aqSOA, we also performed
deliquescence and ice nucleation studies on simulated evaporated cloud droplets that contained 8
mM each of methylglyoxal, methylamine, and ammonium sulfate (aqSOA + AS). As mentioned
above, we were unable to conduct impact-flow experiments on these particles because a
crystalline phase was present, and so particle shattering would provide no additional information
on the phase of the organic material. Therefore, for these particles, we conducted deliquescence
experiments on the crystalline material instead. Interestingly, the presence of crystalline material
suggests that the particles were not sufficiently viscous to inhibit the efflorescence; we attribute
this to the plasticizing effect of the inorganic sulfate, which will be discussed later in the
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Figure 4.4. 50x images of (a) ice depositionally nucleating on a methylglyoxal + methylamine
aqSOA particle, and (b) the immersion freezing of aqSOA by AS islands. Both images were
taken at 220 K. The size bar in the bottom left-hand corner corresponds to 20 µm.
88
manuscript. It is important to note that small α–dicarbonyls have also been shown to react with
ammonium ions. This may also contribute to the change in viscosity; however, these reactions
produce similar light absorbing and oligomeric molecules as methylglyoxal + methylamine
aqSOA [Noziere et al., 2009; Sareen et al., 2010; Yu et al., 2011]. A typical image series of a
deliquescence experiment on aqSOA + AS can be seen in Figure 4.5 and the full video can be
found in the supplemental DVD (Movie A.4). Through the course of this particular experiment,
the RH increases from 49 to 88% as the temperature drops from 254.1 to 247.3 K. At low RH, it
appears that the particle is in a core-shell configuration (Figure 4.5a). We have also taken a 2D
Raman map of another representative particle under similar conditions (Figure A.2). The Raman
map also suggests that these particles appear to consist of an ammonium sulfate core with a very
thin organic shell. As the RH increases, however, several visual clues unveil the true morphology
of these particles. First, the organic shell takes up water, revealing that the ammonium sulfate
core is not one particle, but many small particles (Figure 4.5b) embedded in an organic matrix.
We will refer to these particles as AS islands from now on. As the organic continues to take up
water, the viscosity of the matrix decreases, which allows the AS islands to diffuse throughout
the particle (Figure 4.5c). Eventually, the AS islands undergo prompt deliquescence (Figure
4.5d). Above this deliquescence point, the particle is visually a homogeneous aqueous solution of
aqSOA and ammonium sulfate (Figure 4.5e). Thus, for these mixed particles, the phase
progression of the particle upon humidification can be summarized as: compact AS islands in a
glassy shell, AS islands in a liquid organic matrix, and then fully deliquesced particles. These
deliquescence relative humidities (DRH) are shown in Figure 4.6. As shown, the DRH at all of
the temperatures explored is 78 ± 2%; thus, the liquid aqSOA does not seem to affect the DRH
89
Figure 4.5. A typical image series of a deliquescence experiment on methylglyoxal +
methylamine aqSOA + AS. In this particular experiment, the RH increases from 49 to 88% as the
temperature drops from 254.1 to 247.3 K. Initially, the particle looks to be in an effloresced AS
core-organic shell morphology (a). As the RH is increased around the particle, the glassy shell
takes up water and reveals that the AS core is not one consolidated crystal, but many AS islands
(b). As the organic matrix liquefies, the AS islands are free to diffuse throughout the particle (c).
At a set RH, the AS islands promptly deliquesce (d), turning into a homogeneous aqueous
organic-sulfate particle (e). The size bar in the lower right hand corner corresponds to 20 µm.
90
of ammonium sulfate.
At 230 K, we could no longer measure the deliquescence relative humidity of the AS
islands because they nucleated ice first. Since the islands are immersed in an aqueous organic
matrix, these particles are nucleating ice in the immersion mode. An example image of the
immersion freezing of the aqSOA matrix by AS islands at 220 K can be seen in Figure 4.4b. The
onset conditions for ice nucleation by the AS islands immersed in an aqSOA matrix are also
shown in Figure 4.6. It is important to note that in all cases, the mode of nucleation was
immersion freezing by AS islands and not depositional freezing by the glassy organic shell. This
was also verified by taking video of the sublimation process (Movie A.5). Here as the ice
sublimes, we see that the AS islands are contained to the original dimensions of the particle;
however, the organic matrix has now spread out with the ice, indicating that it was in a liquid,
flowing state at the time of ice nucleation. Upon ice sublimation, however, the organic matrix
vitrifies upon drying and converts from a liquid back into a semi-solid or glass, leaving an
outline of the ice particle (Movie A.5). As shown in Figure 4.6, it can be seen that the
supersaturation required for ice nucleation of the aqSOA + AS are lower than for depositional
freezing on the pure aqSOA glassy particles. There are two important things to note here: one,
adding AS lowers the RH at which the semi-solid or glassy aqSOA shell starts to flow. As seen
from Figures 4.3 and 4.4, from 215-235 K, the pure aqSOA can act as a surface for depositional
nucleation and be in a non-flowing state at 85-90% RH; by simply adding AS before drying, the
aqSOA had turned into an aqueous organic matrix by 75 to 80% RH, and could have undergone
a humidity induced phase transition at much lower RH. We attribute this to the plasticizing effect
of residual inorganic sulfate in the aqSOA matrix; this effect has clearly been seen in the past
91
Figure 4.6. The deliquescence RH of AS islands in a liquid methylglyoxal + methylamine
aqSOA matrix and the onset ice saturation ratios for immersion mode nucleation of
methylglyoxal + methylamine aqSOA by AS island as a function of temperature. For
comparison, a fit for depositional ice nucleation on pure methylglyoxal + methylamine aqSOA
data shown in Figure 4.3 has been added. The solid line indicates water saturation (100% RH)
and the dashed gray lines correspond to 90, 80, 70, and 60% RH. The dashed black line
corresponds to the homogeneous ice nucleation limit. For clarity, two regions of different
behavior have been shaded. In the green shaded region, the AS islands deliquesce before they
can nucleate ice. In the purple shaded region, the AS islands nucleate ice in the immersion mode
before they can fully deliquesce. Error bars are reported as one standard deviation.
92
with both ammonium sulfate and sulfuric acid [Baustian et al., 2013; Saukko et al., 2012; Zobrist
et al., 2008]. The second thing to note is that by adding the plasticizer AS to aqSOA, not only
does the mode of nucleation change from depositional to immersion, but a result of that is that
the particles become more efficient ice nuclei as well.
ATMOSPHERIC IMPLICATIONS
Previously, several studies have looked at the ice nucleation efficiency of single
component organic glasses or simple glassy organic mixtures. The first study of heterogeneous
ice nucleation on an organic glass was conducted by Murray et al. [2010] In that work, the
authors found that glassy citric acid required low ice supersaturations, Sice = 1.23, to nucleate ice
in the depositional mode. In many of the subsequent studies, single component or simple
mixtures of glassy organics were also found to be efficient ice nuclei in the depositional mode
[Baustian et al., 2013; Wilson et al., 2012]. As a result, in one study, it was concluded that
organic glasses could compete with ice nucleation on mineral dust particles in mid-latitude cirrus
clouds [Wilson et al., 2012].
To the author's knowledge, the first comprehensive study to probe ice nucleation on
complex organics glasses was the study by Wang et al. [2012]. In that work, they found that
secondary organic aerosol made from the chamber oxidation of naphthalene could nucleate ice
heterogeneously, implying that they are semi-solid or glassy. Furthermore, they were able to
show that their onset water uptake points coincided with a parameterization for the glass
transition line for secondary organic aerosol developed in Koop et al. [2011] In contrast to
previous studies on single-component glasses, these complex organic glasses required higher ice
supersaturations to nucleate ice. In another previous study, Mohler et al. [2008] studied ice
93
nucleation on gasSOA generated from the dark ozonolysis of α-pinene. There, they probed pure
gasSOA and saw that it required supersaturations over the Koop homogeneous freezing line to
nucleate ice. Unfortunately, the phase of these gasSOA particles was not determined in this
particular study. However, several previous works have reported that SOA from the dark
ozonolysis of α-pinene can be glassy, according to particle evaporation kinetics [Cappa and
Wilson, 2011; Vaden et al., 2011] as well as particle flow behavior [Renbaum-Wolff et al., 2013].
Thus, in the present manuscript, we assume that the gasSOA in the work by Mohler et al. [2008]
was in a semi-solid or glassy state. Therefore, from the current data, we can generally split the
ice nucleation data on amorphous organic (semi-)solids into two coarse groups with different ice
nucleation properties: "simple sugar/acid" amorphous organic (semi-)solids made up of either
pure sugars/organic acids or simple mixtures of sugars/organic acids that are generally efficient
IN, and "SOA-like" amorphous (semi-)solids made from the gas-phase oxidation or aqueousphase reactions of SOA precursors, which are generally poor IN. This is better seen in Figure
4.7a. Here, we have color coded the amorphous organic (semi-)solids according to these
definitions. Simple sugars/organic acids, which consist of six components or less, are colorcoded red. In contrast, SOA-like amorphous (semi-)solids, which could contain hundreds to
thousands of individual components, are color-coded blue. By color coding according to these
definitions, it can be seen that simple sugar/acid amorphous organic (semi-)solids require lower
onset ice saturation ratios to nucleate ice than their SOA-like counterparts. Although there are
only a limited number of studies of ice nucleation on SOA-like systems
94
Figure 4.7. The onset ice saturation ratios for heterogeneous ice nucleation on single and
complex amorphous organic (semi-)solids (a) and heterogeneous ice nucleation on
methylglyoxal + methylamine aqSOA and immersion freezing of aqSOA by AS islands (b). For
comparison, a fit for immersion freezing of the liquid organic 1,2,6-hexanetriol by AS has been
added. The solid and dashed black lines correspond to water saturation and the Koop
homogeneous freezing line [Koop et al., 2000], respectively. As shown, the ice nucleation
behavior of amorphous organic (semi-)solids can be split up into two coarse factions: simple
sugar/acid amorphous organic (semi-)solids (red markers) tend to be efficient IN, while SOAlike amorphous (semi-)solids (blue markers) tend to require higher supersaturations nucleate ice.
Our results for methylglyoxal + methylamine aqSOA is in agreement with this general trend,
while adding AS to the aqSOA tends to change the mode of nucleation and increase the ice
nucleation efficacy of the particles. Error bars in present data are reported as one standard
deviation.
95
sampled, Figure 4.7a suggests that real amorphous organic (semi-)solids found in the atmosphere
could, in general, be poor IN.
In the present study, we have determined the ice nucleation efficiency of one type of
aqSOA, generated from the dark reactions of small aldehydes and amines. Our data is plotted
over the previous work in Figure 4.7b. In concurrence with previous experiments on gasSOA,
these (semi-)solid aqSOA particles can nucleate ice in the depositional freezing mode, but are
poor IN. We hypothesize the different ice nucleation behavior of simple sugar/acid and SOA-like
amorphous organic (semi-)solids may be due to their differences in surface chemistry. The
sugars/organic acids shown in Figure 4.7a are molecules with predominantly alcohol moiety;
given that these particles are amorphous, there is a high likelihood that there are considerable OH groups at the particle surface. In contrast, the predicted oxygenated products expected to be
found in naphthalene gasSOA tend to have more carbonyl moieties than alcohols [Saukko et al.,
2012]. Similarly, the major products expected from the dark reaction of methylglyoxal +
methylamine in evaporating droplets under the pH conditions in our experiments are imine
oligomers [Sedehi et al., 2013], which have more carbonyl moieties than alcohol. Previously, it
was found that increasing the surface hydrophilicity would increase the ice nucleation efficiency
of soot [Gorbunov et al., 2001] and crystalline organic particles [Schill and Tolbert, 2012]. In the
study by Wang et al. [2012], the O:C ratio of the naphthalene gasSOA was determined by the
Aerodyne aerosol mass spectrometer. Interestingly, Wang et al. saw no significant difference in
ice nucleation efficiency between his low, medium, and high O:C ratio samples, which had
values of 0.27, 0.54, and 1.0, respectively. Although there are no explicit mechanisms for the
reaction products from the photooxidation of naphthalene in a Potential Aerosol Mass chamber,
the results from Saukko et al. [2012] suggest that the expected reactions oxygenate naphthalene
96
via the formation of carbonyls and carboxylic acids. Given the evidence above, we believe that
not just surface hydrophilicity, but surface -OH density may be a large driving force in the ice
nucleation efficiency of amorphous organic aerosol. It should be noted that we did not measure
the surface –OH density, so these interpretations are speculative. Further, several other
parameters vary between these two broadly defined categories that may also be affecting the ice
nucleation efficiency.
Finally, our results also show that by adding AS to aqSOA, we were able to lower the
viscosity of the aqSOA matrix due to the plasticizing effect of residual AS in the aqSOA matrix.
This allowed the AS to effloresce upon drying, causing AS islands to embed in the aqSOA
organic matrix. Upon liquefaction of the aqSOA matrix by humidification of the dried particles,
these AS islands could then potentially serve as immersion IN and increase the ice nucleation
efficacy of these particles in comparison to depositional ice nucleation on pure aqSOA (Figure
4.7b). The efficiency of freezing aqueous solutions of organics by AS in the immersion mode is
in agreement with a previous study from our group examining the immersion freezing of aqueous
1,2,6-hexanetriol by effloresced AS over a similar temperature range [Schill and Tolbert, 2013].
At colder temperatures, however, there is a small deviation between the two data sets. Here, ice
nucleation on AS islands immersed in methylglyoxal + methylamine aqSOA require slightly
higher supersaturations than AS particles coated with 1,2,6-hexanetriol. Identifying the source of
this deviation is beyond the scope of this paper, but the authors postulate that the higher
superaturations required for the AS islands to freeze the aqSOA matrix could be due to higher
viscosity of the matrix, and hence slower water diffusion in the aqSOA matrix relative to 1,2,6hexanetriol.
97
The plasticizing effect of AS on glassy organic aerosol has been noted in previous
publications [Baustian et al., 2013; Saukko et al., 2012; Zobrist et al., 2008]. Thus, although
SOA-like glasses may be a poor IN, if a sufficient amount of AS is present such that AS
efflorescence is not inhibited [Bodsworth et al., 2010], glassy organic-sulfate particles can
become efficient IN since the ice nucleation efficiency of AS in the immersion mode is
minimally affected by the presence of liquid aqSOA. Finally, changing the mode of ice
nucleation also changes the ice crystal habit of the ice particle (Figure 4.4), which could affect
the light scattering properties of ice crystals that nucleated from mixed organic-sulfate particles
found in the upper troposphere.
98
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CHAPTER 5: HETEROGENEOUS ICE NUCLEATION ON SIMULATED SEA-SPRAY
AEROSOL USING RAMAN MICROSCOPY
INTRODUCTION
Cirrus clouds are composed almost entirely of ice and are present over 50% of the time in
the tropics [Wang et al., 1996]; up to half of cirrus clouds in tropical regions are associated with
anvil cirrus, which form at high altitudes in the moist outflows of deep convective clouds
[Massie et al., 2002]. Ice clouds can affect climate by scattering incoming solar radiation and
scattering and absorbing outgoing terrestrial radiation. The balance of these radiative effects,
however, is dependent on the microphysical properties of the ice particles [Jensen et al., 1994],
which are affected by the presence of ice freezing nuclei [Carrio et al., 2007]. Thus,
understanding the sources of ice freezing nuclei in deep convective clouds is integral to
unraveling the effects of anvil cirrus on climate.
Previously, it was thought that homogeneous ice nucleation would dominate cirrus
formation; however, modeling [Jensen et al., 2012; Jensen et al., 2010] and field studies [Cziczo
et al., 2013; Strom et al., 2003] have indicated that heterogeneous pathways may be the
dominant mechanism for cirrus cloud formation, especially in the Northern Hemisphere. For
example, in a compilation of data from four aircraft measurement campaigns in regions of high
cirrus abundance, heterogeneous ice nucleation accounted for 94% of the cirrus cloud
encounters, based on analysis of ice residuals [Cziczo et al., 2013]. In two of the four campaigns,
sea salt in ice residues was largely enhanced above sea salt in both background upper
tropospheric aerosol and interstitial aerosol. Because it was assumed that sea-salt particles would
be deliquesced under cirrus conditions, it was previously concluded that these residuals were the
102
result of homogeneous freezing [Cziczo et al., 2004]. Two separate studies have re-examined this
assumption, and found that crystalline NaCl and NaCl·2H2O particles can depositionally
nucleate ice prior to deliquescence [Wagner and Mohler, 2013; Wise et al., 2012]. To the
authors’ knowledge, this is the only proposed mechanism for heterogeneous ice nucleation on
sea-salt particles under cirrus conditions. One limitation of this mechanism is that natural
saltwater contains additional inorganic ions that may cause sea-salt particles to take up water
under cirrus conditions and form brine layers on their surface. For such internally mixed liquidsolid particles, the surface properties of the sea-salt aerosol could differ greatly from pure NaCl
particles [Finlayson-Pitts and Hemminger, 2000]; however the ability of liquid-solid sea-salt
particles to nucleate ice remains unexplored.
Sea-spray particles also contain organic species that are often enriched in the aerosol
phase compared to natural seawater [O'Dowd et al., 2004]. Several studies have shown that
carbohydrate-like organic species may form a large portion of the organic mass in sea spray
aerosol. For example, laboratory studies using a 33-m wave breaking channel have shown that
200-nm to 1-µm sea-spray aerosol are dominated by a particle type that contains sea salt and
organic carbon [Prather et al., 2013]. Further, solid-state NMR studies on aerosol generated
from Atlantic seawater suggest that submicron sea-spray particles contained up to 73% organics
by mass during times of high biological activity [Facchini et al., 2008]. Additionally, infrared
spectroscopy and positive matrix factorization studies on aerosol collected in the North Atlantic
and Arctic have determined that an ocean-derived factor accounted for 68 and 37% of the
organic mass of submicron aerosol, respectively [Russell et al., 2010]. Moreover, 88% of this of
this ocean-derived factor was due to an organic hydroxyl signature whose calculated infrared
spectrum was similar to reference carbohydrates. Using scanning transmission x-ray microscopy
103
near edge absorption fine structure, a majority of aerosol collected during the same campaign
were identified as cuboids with thin, homogenous coatings of organic hydroxyls and potassium.
This type of morphology suggests that NaCl crystallized out of a homogenous solution
containing dissolved carbohydrate-like organics and additional inorganic ions. Finally, despite
having lower mass fractions of organics in supermicron particles, Raman microscope images and
spectra suggest that supermicron particles may also exist as cuboids with thin, organic coatings
that can affect ice nucleation [Ault et al., 2013]. Although the presence of dissolved sugars in
sea-spray particles is well known, no studies on the ice nucleation behavior of mixed seasalt/sugar particles have been reported.
In the present study, we have examined the ice nucleation properties of effloresced
synthetic sea-salt particles. To emulate the inorganic ions of saltwater, the commercially
available aquarium salt Instant Ocean® was used. This salt mixture attempts to replicate the
concentration of every major and minor inorganic ion found in natural seawater. Our results
indicate that sea-salt particles will be in an internally mixed liquid-solid state at ice saturation
from 215 to 235 K. Despite having a liquid layer surrounding the crystalline core, these particles
can still act as efficient heterogeneous mode ice nuclei at lower temperatures through immersion
nucleation. To simulate the effect of adding dissolved carbohydrate-like organic material to seasalt particles, we mixed Instant Ocean® with sucrose 1:1 by mass. For the 1:1 Instant
Ocean®:sucrose mixtures, we find that the particles effloresce into crystalline sea-salt particles
with an organic coating. This organic coating was found to be glassy under lower temperature
conditions, and acts as an efficient heterogeneous ice nucleus in agreement with previous ice
nucleation studies on sugar-like organic glasses [Baustian et al., 2013; Murray et al., 2010;
Wilson et al., 2012].
104
EXPERIMENTAL
Raman Microscope. The experimental apparatus has been described previously in detail
[Baustian et al., 2010; Schill and Tolbert, 2013], Briefly a Thermo Nicolet Almega XR Raman
spectrometer has been coupled to an Olympus BX-51 microscope with 10x, 20x, 50x and 100x
capabilities, which correspond to numerical apertures of 0.25, 0.40, 0.50, and 0.60, respectively.
The Raman microscope has also been outfitted with a Linkam THMS600 environmental cell. A
temperature stage inside the cell can be controlled from -196° to 600 °C, with an accuracy of
±0.1 K. Water partial pressure inside the cell is controlled by mixing flows of dry and humidified
nitrogen. A Buck research CR-A1 dew point hygrometer in line with the cell measures the frost
point with an accuracy of ±0.1 K. A Gast diaphragm pump at the exit of the hygrometer ensures
that the flow through the cell is at least 1 L min-1.
Raman spectra were obtained using a 532 nm frequency-doubled Nd:YAG laser as the
excitation source. Spectra were taken from 200 to 4000 cm-1 with a typical resolution of 2-4
cm-1. Single spectra typically consisted of 256 co-added scans. Points in line maps typically
consisted of 16 co-added scans. In all cases, the single-scan collection time was 1 s. Single
spectra and line maps were taken with 50x and 100x long-range objectives, which focus the laser
to a spot size of approximately 1.3 and 1.1 µm, respectively [Everall, 2010]. Unless otherwise
noted, Raman spectra were taken at the center of each particle and Raman line maps consisted of
Raman spectra taken every micron in the Y direction.
Materials. Sea-salt solutions were prepared by adding the synthetic sea-salt (SSS) mixture
Instant Ocean® to high purity water. Instant Ocean® is a commercially available product that is
used as an aquarium salt and seeks to contain every major, minor, and trace element necessary
105
for aquatic life. Furthermore, Instant Ocean® has previously been used as a proxy for sea-salt
particles in heterogeneous oxidation [Dilbeck and Finlayson-Pitts, 2013] and low temperature
deliquescence experiments [Koop et al., 2000a]. To simulate mixed sea-salt dissolved organic
carbon (DOC) particles found in marine environments, SSS was mixed with the disaccharide
sucrose [Mallinckrodt, AR® (ACS)] in a 1:1 ratio by mass in high purity water. The total weight
percent of all solutions was 0.2%. Sucrose was chosen as a proxy for DOC, as it has been
previously shown that DOC can be up to 80% carbohydrate-like organics [Aluwihare et al.,
1997]. Further, sucrose has been used in the past as a proxy for atmospheric organic glasses
[Zobrist et al., 2008], including in experiments regarding water uptake and evaporation [Bones et
al., 2012] and heterogeneous ice nucleation [Baustian et al., 2013; Wilson et al., 2012].
Particle Generation. SSS and SSS:sucrose particles were generated by nebulizing droplets from
0.2 wt% solutions of SSS and 1:1 SSS:sucrose, respectively. Solutions were self-aspirated into a
Meinhard TR-50 glass concentric nebulizer. The nebulized spray was directed at a
hydrophobically treated fused silica disk, where the droplets were allowed to settle and coagulate
into supermicron droplets. This droplet-laden disk was then transferred to the environmental cell,
where the temperature and relative humidity (RH) were 25 °C and approximately 0%,
respectively. The particles were exposed to these conditions for at least ten minutes to ensure that
the particles were nominally dry. The lateral diameters of the resulting dry particles ranged from
1-20 µm.
Deliquescence and Ice Nucleation Experiments. Each deliquescence and ice nucleation
experiment began at 25 °C and approximately 0% RH. During the experiment, the RH inside the
environmental cell was controlled by keeping a constant frost point and lowering the temperature
in a controlled manner. Specifically, after the frost point was steady for at least 10 minutes, the
106
temperature was lowered at a rate of 10 K min-1 until the ice saturation ratio (Sice = Pwater/VPice)
was approximately 0.9. At 215 to 235 K, the RH with respect to water at Sice = 0.9 ranged from
53 to 63%. The cell was then cooled at a rate of 0.1 K min-1 until deliquescence or ice nucleation
was observed visually and confirmed spectrally.
In this work, deliquescence is defined as the RH when all crystalline material inside the
particle dissolves. This is an important distinction for sea-salt particles, which begin to take up
water at RHs much lower than the full deliquescence RH (DRH). Deliquescence observations
were conducted using the 50x objective for 10-20 particles at a time. Experiments were
conducted on various sized particles at various positions on the fused-silica disc to minimize
systematic errors. Additionally, several experiments were re-conducted at the coldest
temperatures using a cooling rate of 0.01 K min-1, to ensure that solvation kinetics were not
artificially inflating reported DRH.
For ice nucleation experiments, initial observation of ice was monitored by scanning the
entire disc using the 10x objective. Great care was taken to ensure that the first nucleation event
was observed; however, it cannot be discounted that other ice particles had formed outside of the
field of view after the observation of this event. If more than one particle had nucleated ice
within the field of view, that value was discarded and the experiment repeated. After the first ice
particle was noted, the 50x objective was used to verify the existence of ice both visually and
spectrally. The presence of ice was confirmed in the Raman spectra by the emergence of a broad
band centered near 3250 cm-1 (-OH stretch of water-ice). Only after confirmation of ice, Sice was
set to approximately 1.0 and a line map of the entire particle was taken. The ice was then
sublimed by turning off the flow of the humidified nitrogen and an image of the dry, bare
107
nucleus was recorded along with an additional Raman spectrum. In all cases, ice had formed on a
particles rather than the fused-silica disc.
Water Uptake Experiments. During each deliquescence/ice nucleation experiment, it was noted
that pure SSS particles had taken up water and individual particles were in an internally mixed
liquid-solid state. To quantify the transition from solid to liquid-solid, we conducted separate
water uptake experiments on effloresced SSS particles along the deliquescence/ice nucleation
experimental trajectories used in this study. As in deliquescence/ice nucleation experiments, each
water uptake experiment began at 25 °C and approximately 0% RH, and the RH inside the
environmental cell was controlled by keeping a constant frost point and lowering the
temperature. In contrast to the deliquescence/ice nucleation experiments, however, the
temperature was lowered in 0.5 K steps. At each step, the temperature was held for at least five
min; at the end of this five min, optical images and Raman spectra were taken. An example of a
typical water uptake experiment can be found in Figure 5.1. At approximately 0% RH, the
Raman spectra contains hydrate (3400 cm-1) and sulfate (1010 cm-1) peaks, likely due to MgSO4,
CaSO4, and their respective hydrates (Figure 5.1a) [Ault et al., 2013]. A 50x gray-scaled image
of a particle at approximately 0% RH is shown in Figure 5.1b. The irregular, cuboidal shape and
bright opacity due to light scattering suggest that the particle is crystalline. The degree of
brightness, however, is difficult to assess using the gray-scaled image. Therefore, a binary
contrast enhanced image (grayscale cutoff value = 120) found to the right of each gray-scaled
image is included to qualitatively assess the degree of white light scattering. As the RH
surrounding the sea-salt particle is increased up to 15%, there are minimal changes in both the
Raman spectra (Figure 5.1a) and the binary image (Figure 5.1c); at 17% RH, however, the
particle visually darkens as indicated by an increase in black pixels in the binary contrast image
108
Figure 5.1. Raman spectra (a) and 50x gray-scaled and binary contrast enhanced images (b-e)
from a typical water uptake experiment along the 215 K experimental trajectory. As shown, the
particle undergoes minimal change both visually and spectrally from 0% RH to 15% RH. At
17% RH, however, the particle visually darkens. Further, a small peak at 981 cm-1 (dotted
vertical line) appears and the intensity of the –OH stretch (3200-3600 cm-1) increases. These
changes correspond to the appearance of unbound, dissolved sulfate [Fung and Tang, 2000] and
the uptake of water, respectively. For comparison, a fully deliquesced SSS particle from a
separate experiment at 235 K and its Raman spectra are reported (f). A separate particle is
needed because SSS particles nucleate ice prior to deliquescence at 215 K. The dry lateral
diameter of the particle (b-e) is approximately 15 µm.
109
(Figure 5.1d). Additionally, a small peak centered at 981 cm-1 appears and the shoulders of the
hydrate peak (3200-3600 cm-1) increase in the Raman spectra (Figure 5.1a). The small peak
centered at 981 cm-1 corresponds to unbound or dissolved sulfate [Fung and Tang, 2000] and the
broad peak in the -OH region indicates uptake of water. By 25% RH, the sharp peak at 981 cm -1
and the broad peak from 3200-3600 cm-1 have both increased in intensity and the particle has
darkened considerably (Figure 5.1e). For reference, a fully deliquesced SSS particle from a
separate experiment has also been provided (Figure 5.1f). Thus, at 17% RH, it appears that the
cuboidal particle has taken up a small layer of water, dampening light scattering, and producing
shifts in the Raman spectra. We define the RH at the onset of these changes as the water uptake
RH, 17% in this case.
Quality Assurance/Quality Control. For each deliquescence, ice nucleation, and water uptake
experiment at least three experiments were conducted at each temperature. Further at least two
solutions and two separate fused silica discs were used at each temperature. Finally, each
experiment was conducted on a freshly sprayed disc. Error bars are reported as one standard
deviation.
RESULTS AND DISCUSSION
The DRH and heterogeneous ice nucleation onsets for effloresced SSS are shown in
Figure 5.2. At temperatures warmer than ~230 K, SSS particles fully deliquesce before they
nucleate ice. The average DRH from 230 to 235 K was 80.8±0.3%. These points are similar to
the DRH values of pure NaCl from 236 to 240 K (78±2%) [Wise et al., 2012], suggesting that the
final crystalline inclusions to deliquesce are NaCl. This is agreement with the study by Koop et
al. [Koop et al., 2000a], who have shown that the DRH of SSS from 239 to 273 K is not
110
Figure 5.2. The full DRH and onset ice saturation ratios for heterogeneous ice nucleation on
effloresced SSS particles as a function of temperature. The solid line indicates water saturation
(100% RH), and the dashed gray lines correspond to 90, 80, 70, and 60% RH. The dashed black
line corresponds to the homogeneous ice nucleation limit [Koop et al., 2000b]. Also shown are
ice nucleation data on effloresced NaCl and NaCl·2H2O [Wise et al., 2012]. For clarity, two
regions of different behavior have been shaded. In the green shaded region, the particles
deliquesce before they can nucleate ice. In the purple shaded region, the particles nucleate ice
heterogeneously before they fully deliquesce. Error bars are reported as one standard deviation.
111
significantly different from pure NaCl. At temperatures below ~225 K, the SSS particles
nucleate ice before they fully deliquesce. Also shown in Figure 5.2 are previous ice nucleation
results from the same setup on effloresced NaCl and NaCl·2H2O [Wise et al., 2012]. It can be
seen that the heterogeneous ice nucleation behavior of SSS differs from effloresced NaCl
particles. Both effloresced NaCl and NaCl·2H2O nucleate ice before they deliquesce above 230
K, but the SSS particles deliquesce before they nucleate ice. Additionally, below 230 K, both
particle types nucleate ice heterogeneously, but sea-salt particles require higher Sice values to
nucleate ice than pure NaCl particles. Thus, the presence of additional inorganic ions seems to
affect the hygroscopic and surface properties of sea-salt particles compared to pure NaCl
particles.
To investigate this further, we conducted water uptake experiments along each
experimental trajectory (Figure 5.3), and collected optical microscope images and Raman spectra
of SSS particles through the course of a typical ice nucleation experiment (Figure 5.4). As shown
in Figure 5.3, as the frost point is held constant and the temperature is decreased, the SSS
particles take up water well below the ice saturation line for all experimental trajectories
explored in this work. Further, there seems to be little temperature dependence in the water
uptake RH for pure SSS particles; across all temperatures studied, the average water uptake RH
was 17±1%. This is corroborated by the findings in Figure 5.4. As shown, the SSS particle
begins the experiment as a cuboid with a sulfate peak at 1010 cm-1, consistent with effloresced
SSS particles (Figure 5.4a). By 37% RH, the particle is above the water uptake RH and has
visually darkened, the sulfate band has shifted to 981 cm-1, and the area under the –OH peak has
increased (Figure 5.4b). At this point, the particle is in an internally mixed liquid-solid phase and
can only nucleate ice in the immersion mode. At the Sice ~ 1, the particle has picked up enough
112
Figure 5.3. Water uptake relative humidities for pure, effloresced SSS particles from 225 to 255
K. Also plotted are the deliquescence/ice nucleation results from Figure 5.2. The solid black line
corresponds to Sice = 1 and the dotted gray lines correspond to deliquescence/ice nucleation
experimental trajectories for constant frost points. As shown, the water uptake RH for all
temperatures explored are lower than the ice saturation curve. Thus, the particles are in a mixed
solid/liquid phase at the start of the deliquescence/ice nucleation experiments from 215 to 235 K.
113
Figure 5.4. 50x images of an SSS particle dry (a), during a typical ice experiment, (b), at Sice ~ 1
(c), and after it has nucleated ice at 215 K (d). As shown, SSS particles take up an appreciable
amount of water before the start of an ice experiment, and, therefore, must be nucleating ice in
the immersion mode. In the lower panel, Raman spectra taken at the center of the particle are
shown.
114
water that the outline of the particle is now spherical, although there are still crystalline
inclusions inside available as immersion mode ice nuclei (Figure 5.4c). These inclusions are
better visualized in the binary contrast image (threshold = 75) provided (Figure 5.4c, inset).
Further, this morphological change is accompanied by a large increase in the area under the -OH
stretch. In the final frame (Figure 5.4d), the particle has nucleated ice in the immersion mode; the
crystalline inclusion that acted as the ice nucleus can be seen in the center of the particle. This is
shown more clearly in the Raman line map (Figure 5.5). Here, the hydrate and sulfate stretches
are correlated with the dimensions of the ice nucleus. The water-ice stretch, however, extends
beyond the dimensions of the hydrate and sulfate stretches.
To investigate the effect of sugar-like molecules on sea-salt particles, we mixed Instant
Ocean® with equal parts sucrose, a disaccharide similar in composition to the sugar-like
molecules found in the ocean. The DRH and heterogeneous ice nucleation onsets for these
particles from 215 to 235 K are shown in Figure 5.6. At temperatures above ~225 K, these mixed
particles fully deliquesce before they nucleate ice. Further, in comparing of Figures 5.2 and 5.6,
it can be seen that the addition of sucrose to SSS slightly decreases their DRH (76±2%). This
behavior has been noted previously for mixed inorganic-organic particles [Brooks et al., 2002].
At temperatures below ~220 K, however, the SSS:sucrose particles nucleate ice before
they fully deliquesce. Also shown in Figure 5.6 are the Sice values for the onset of heterogeneous
ice nucleation on pure, glassy sucrose from 215 to 235 K from Baustian et al. [2013]. As shown,
the onset Sice values for the SSS:sucrose particles are similar to the pure amorphous sucrose
particles, suggesting that a glassy sucrose coating could be dictating the ice nucleation properties
of SSS:sucrose particles below ~220 K. To investigate this further, we have collected optical
images and Raman spectra through the course of a typical ice nucleation experiment on a
115
Figure 5.5. 100x line map of the SSS particle that has nucleated ice in Figure 5.4d. As shown the
edges of the ice particle exhibit only water-ice stretches, but the immersion ice nucleus in the
center of the particle is correlated with the hydrate and sulfate peaks.
116
Figure 5.6. The full deliquescence RHs and onset ice saturation ratios for heterogeneous ice
nucleation on effloresced SSS:sucrose particles as a function of temperature. The solid line
indicates water saturation (100% RH), and the dashed gray lines correspond to 90, 80, 70, and
60% RH. The dashed black line corresponds to the homogeneous ice nucleation limit [Koop et
al., 2000b]. Also shown are ice nucleation onsets for pure, amorphous sucrose [Baustian et al.,
2013]. For clarity, two regions of different behavior have been shaded. In the green shaded
region, the particles deliquesce before they can nucleate ice. In the purple shaded region, the
particles nucleate ice heterogeneously before they fully deliquesce. Due to the kinetic effects
associated with humidity-induced glass transitions, these shaded regions are relevant for the
trajectories, humidification rate, and particle sizes used in this study. Error bars are reported as
one standard deviation.
117
SSS:sucrose particle (Figure 5.7). As shown, when the particle is at Sice = 0.00, the particle is a
slightly irregular cuboid with a dark coating. This is consistent with an effloresced particle in an
amorphous organic matrix [Ault et al., 2013; Russell et al., 2010]. The absence of a sulfate peak
at 981 cm-1 indicates that the aforementioned bound sulfate species are present in these mixed
sea-salt organic particles. Although sucrose is a hygroscopic species [Zobrist et al., 2008], the
particle does not visually take up water at Sice = 0.53 (Figure 5.7b) or near ice saturation at Sice =
0.97 (Figure 5.7c). We suggest that this is because the organic matrix is in a highly viscous semisolid or glassy state under these temperature and RH conditions [Zobrist et al., 2011]. This is in
concurrence with the Raman spectral data, which verify that, within the limitations of the
spectrometer, the particle does not take up water. This is in stark contrast with the results on pure
SSS (Figure 5.4). In Figure 5.7d, the mixed sea-salt/organic particle appears to have nucleated
ice in the depositional mode. We cannot, however, discount the possibility that a thin, nm-scale
liquid layer has formed on the surface of the organic matrix [Berkemeier et al., 2014]. Here, a
thin, but fully-encompassing aqueous organic layer may form on the surface of the glassy matrix
if the RH surpasses the partial deliquescence glass transition RH. Under these conditions, the
glassy matrix is in quasi-equilibrium with the surrounding water vapor. Thus, immersion mode
freezing of a nm-scale layer of water by the sub-surface highly-viscous organic matrix cannot be
discounted; however, due to the limitations of our instrument, we will continue to refer to this as
depositional freezing. Regardless of the freezing mode, however, these results suggest that the
bulk of the organic matrix is viscous enough to serve as a heterogeneous IN under these
conditions. To confirm this, we have taken a line map of the particle in Figure 5.7d (Figure 5.8).
In previous studies, we have shown that low viscosity, liquid organics will coat growing ice
particles to reduce the surface tension over the entire system [Schill and Tolbert, 2013]. In
118
Figure 5.7. 50x images of an SSS:sucrose particle dry (a), during a typical ice experiment (b), at
Sice ~ 1 (c), and after it has nucleated ice at 215 K. As shown, SSS:sucrose does not uptake a
quantifiable amount of water prior to ice nucleation, and, therefore, could be nucleating ice in
the depositional mode or in immersion mode with trace amounts of water. In the lower panel, the
Raman spectra taken at the center of the particle are shown.
119
Figure 5.8. 100x line map of the SSS:sucrose particle that has nucleated ice in Figure 5.6d. As
shown the ice particle has formed on the outside of the particle and the organic stretch is not
spatially correlated with the ice stretch.
120
Figure 5.8, the ice particle has formed on the outside of the particle and the organic stretch is not
spatially correlated with the ice stretch, indicating that the organic did not spread out over the
surface of the growing ice particle. Thus, the organic coating was in a non-flowing, likely glassy
state and, therefore, was available as a surface for heterogeneous ice nucleation.
Finally, the authors note that at 225 and 230 K pure amorphous sucrose nucleates ice
depositionally at RH values similar to that observed for full deliquescence of mixed SSS:sucrose
particles. Thus, the inorganic ions in sea salt must act a plasticizing agent and lower the glass
transition temperature and/or RH compared to the pure glass. This behavior has been noted
before for organic glasses containing inorganic sulfuric acid [Saukko et al., 2012], sulfate
[Baustian et al., 2010; Gregory P. Schill et al., 2014], and NaCl [Bones et al., 2012].
ATMOSPHERIC IMPLICATIONS
In a compilation of flight data from four different aircraft campaigns spanning almost 10
years and covering a range of geographic regions and seasons, it was shown that sea-salt
particles were enhanced in heterogeneous ice residuals compared to background free
tropospheric aerosol. In two of the flight missions, Cirrus Regional Study of Tropical Anvils and
Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) and Tropical Composition,
Cloud, and Climate Coupling (TC4), sea-salt particles were found to be 19 and 31% of all ice
residuals, respectively; these numbers are greatly enhanced from the abundance of sea-salt
particles found in near cloud, free troposphere air (<1%), pointing to a selective heterogeneous
ice nucleation mechanism. It is important to note that these cirrus clouds were associated with
deep convection, and the in-cloud and near-cloud particle populations may be different; previous
121
studies, however, have shown that even in-cloud interstitial aerosol is far depleted in sea-salt
particles compared to ice residuals [Cziczo et al., 2004].
Wise et al. [2012] proposed that the ice nuclei could be effloresced NaCl particles, and
also found that NaCl·2H2O particles are particularly efficient ice nuclei, with ice nucleating near
Sice = 1.0. Follow up work using the Aerosol Interaction and Dynamics in the Atmosphere
(AIDA) chamber and infrared spectroscopy confirmed that NaCl·2H2O particles can form below
227.1 K and are efficient depositional ice nuclei [Wagner and Mohler, 2013]. The salt particles
observed in the AIDA chamber study had slightly higher Sice values (1.15-1.25), but are still
considered to have essentially the same ice nucleating ability as mineral dust under these
conditions [Wagner and Mohler, 2013]. It should be noted that that the onset Sice values found in
these two laboratory experiments are not directly comparable due to differences in particle size,
mass, and available surface area [Hoose and Moehler, 2012].
Although pure NaCl is an efficient ice nucleus, sea-salt aerosol contains additional
deliquescent inorganic salts that alter the chemical composition and can impact hygroscopic
properties. Here, we have shown that dry SSS particles form internally mixed liquid-solid phases
as the RH is increased under cirrus conditions. Consequently, these internally mixed liquid-solid
SSS particles have different ice nucleation mechanisms and efficiencies than pure NaCl. We find
that SSS particles deliquesce prior to nucleating ice above 230 K; however these particles
undergo efficient immersion ice freezing from 215 to 225 K with Sice = 1.08-1.29. In addition,
we have also shown SSS:sucrose particles may have a glassy organic coating readily available
for depositional ice nucleation below 220 K at Sice = 1.12-1.16. Above 225 K, however, these
particles deliquesce prior to ice nucleation. Thus, we suggest that sea-salt and mixed sea-
122
salt/disaccharide particles can influence ice cloud formation in both the depositional and
immersion mode below 225 K.
The anvil cirrus encounters during the heterogeneous freezing cases during CRYSTALFACE were collected from 11.3 to 13.9 km; the campaign-average temperatures for these
altitudes was approximately 204 to 224 K [L Wang et al., 2006]. Further, ice residuals were
sample during TC4 from convective anvil cirrus 8-13 km [Cziczo et al., 2013]. This corresponds
to
campaign-mean
temperatures
as
cold
as
215
K
[Selkirk
et
al.,
2010].
These temperatures overlap with the regions of heterogeneous ice nucleation for the sea-salt and
mixed sea-salt/organic particles in this study. Finally, the convective inflows during CRYSTALFACE and TC4 were primarily marine environments. Modelling studies have shown that anvil
cirrus may be influenced most by boundary layer aerosol [Carrio et al., 2007]. Thus, we suggest
that the large enhancement of sea salt in ice residuals could be due to the convective inflow of
boundary layer sea-salt particles that nucleate ice in the immersion mode, or mixed seasalt/organic particles nucleate that ice in the depositional mode if they have thin, glassy organic
coatings.
The remaining two aircraft campaigns, Costa Rice AURA Validation Experiment
(CRAVE) and Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) did not exhibit
large enhancement of sea-salt particles in ice residuals [Cziczo et al., 2013]. These campaigns
sampled anvil cirrus at similar altitudes, but had different convective inflow regions. In
MACPEX, the convective inflows of all flights were exclusively continental. CRAVE, however,
presents an interesting case. Anvil cirrus sampled during CRAVE had convective inflow regions
that were an even mixture of continental, marine, and coastal environments. Although the
campaign-averaged ice residuals from the CRAVE campaign do not show large enhancements in
123
sea salt, individual flights were recorded to be largely enhanced in sea salt [Froyd et al., 2010].
For example, in a dust impacted, tropical anvil cirrus encounter sampled from 12-14 km, sea-salt
particles were the dominant ice residual even over mineral dust. Unfortunately, the specifics of
this particular flight were not reported. Therefore, the temperature is not known. This work has
shown that heterogeneous ice nucleation on sea-spray aerosol may provide a mechanism for low
temperature anvil cirrus formation. Consistent with this, data from CRYSTAL-FACE and TC4,
as well as select flights from the CRAVE, suggests sea-salt is enhanced in ice residuals from
low-temperature anvil cirrus whose convective inflows are from primarily marine regions.
124
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127
CHAPTER 6: SUMMARY AND CONCLUSION
Studies of the ice nucleation properties of laboratory generated organic-sulfate and
organic-sea-salt particles were conducted in an effort to better understand impacts of these
particles on cirrus cloud formation and, therefore. The primary analysis techniques used in these
studies were a Knudsen-effusion cell coupled to IR spectroscopy and mass spectrometry and
Raman microscopy coupled to an environmental cell. The results from these studies are
summarized below.
In Chapter 2, the heterogeneous ice nucleation efficiency of a series of thin, C3-C6
monocarboxylic acid films between 180 and 200 K was investigated using a Knudsen cell flow
reactor. At each temperature, the critical ice saturation ratio for depositional nucleation as well as
the effective contact angle was found to be strongly dependent on the chemical nature of the
film. For the organic acids used in this study, increasing the O:C ratio lowered the
supersaturation required for the onset of heterogeneous ice nucleation and decreased the effective
angle of contact. This was deemed a result of an increase in surface hydrophilicity, which allows
the film to better adsorb a metastable, ice-like layer of water that serves as a template for the new
phase of ice. These ice nucleation results are in excellent agreement with ice nucleation on
laboratory generated α-pinene secondary organic aerosol as well as on predominantly organic
particles collected in Mexico City.
In Chapter 3, Raman microscopy coupled with an environmental cell was used to study
the low temperature deliquescence, efflorescence, and liquid-liquid phase separation behavior of
2:1 mixtures of organic polyols (1,2,6-hexanetriol, and 1:1 1,2,6-hexanetriol + 2,2,6,6tetrakis(hydroxymethyl)cyclohexanol) and ammonium sulfate from 240-265 K. Further, the ice
128
nucleation efficiency of these organic-sulfate systems after liquid-liquid phase separation and
efflorescence was investigated from 210-235 K. Raman mapping and volume-geometry analysis
indicated that these particles contain solid ammonium sulfate cores fully engulfed in organic
shells. For the ice nucleation experiments, we found that if the organic coatings are liquid, water
vapor diffuses through the shell and ice nucleates on the ammonium sulfate core. In this case, the
coatings minimally affect the ice nucleation efficiency of ammonium sulfate. In contrast, if the
coatings become semi-solid or glassy, ice instead nucleates on the organic shell. Consistent with
recent findings that glasses can be efficient ice nuclei, the phase-separated particles are nearly as
efficient at ice nucleation as pure crystalline ammonium sulfate.
Chapter 4 explored the phase behavior and the ice nucleation properties of secondary
organic aerosol made from aqueous processing (aqSOA). AqSOA was made from the dark
reactions of methylglyoxal with methylamine in simulated evaporated cloud droplets. The
resulting particles were probed from 215-250 K using Raman spectroscopy coupled to an
environmental cell. We find these particles are in a semi-solid or glassy state based upon their
behavior when exposed to mechanical pressure as well as their flow behavior. Further, we find
that these aqSOA are poor depositional ice nuclei, in contrast to previous studies on simple
mixtures of glassy organics. Additionally, we have studied the effect of ammonium sulfate on the
phase, morphology, and ice nucleation behavior of the aqSOA. We find that the plasticizing
effect of ammonium sulfate lowers the viscosity of the aqSOA, allowing the ammonium sulfate
to effloresce within the aqSOA organic matrix. Upon humidification, the aqSOA liquefies before
it can depositionally nucleate ice and the effloresced ammonium sulfate can act as an immersion
mode ice nucleus. This change in the mode of nucleation is accompanied by an increase in the
overall ice nucleation efficiency of the aqSOA particles.
129
Finally, in Chapter 5, the deliquescence and heterogeneous ice nucleation behavior of
simulated sea-spray aerosol was probed from 215 to 235 K using Raman microscopy coupled to
an environmental cell. Water uptake prior to deliquescence was also probed on sea-salt particles
along deliquescence/ice nucleation experimental trajectories. Synthetic sea-salt particles were
generated from solutions of aquarium salt, which emulates all major and minor ions that are
found in saltwater. Our results indicate that, under cirrus cloud conditions, sea-salt particles will
be in an internally mixed liquid-solid phase, with a brine layer surrounding a crystalline core.
Above 230 K, the core of the internally mixed liquid-solid particle fully deliquesces prior to ice
nucleation. Below 225 K, however, the crystalline core can act as efficient immersion-mode ice
nuclei prior to full liquefaction. To simulate the presence of carbohydrate-like organics in sea
spray aerosol, sea-salt particles were mixed with the disaccharide sucrose 1:1 by mass. At low
relative humidity, the sea-salt particles effloresced into an amorphous organic-salt matrix. Above
225 K, sea-salt/sucrose particles fully deliquesce prior to ice nucleation; however, at lower
temperatures, the organic-salt matrix was found to be glassy and could act as an efficient
depositional ice nucleus. This work suggests the possible role of sea-salt aerosol in lowtemperature ice nucleation. Consistent with this, data from aircraft campaigns suggests sea-salt
aerosol is enhanced in ice residuals from low-temperature anvil cirrus whose convective inflow
regions are primarily marine environments.
In conclusion, common organic-inorganic particles may be very efficient heterogeneous
ice nuclei in the upper troposphere. In particular, particles containing an effloresced ammonium
sulfate core embedded in a liquid organic matrix are nearly as efficient ice as bare ammonium
sulfate crystals, regardless of the organic composition. Additionally, we have shown that pure
organic aerosol or organic coatings may be effective ice nuclei. Among these efficient organic
130
ice nuclei are crystalline organics with high O:C ratios. Further, we have also shown that some
glassy particles may be efficient ice nuclei, especially sugars or sugar-like molecules like
polyols. In contrast, secondary organic aerosol may be a poor ice nucleus, which might explain
why organic-sulfate particles are diminished in lower cirrus ice residuals compared to
background free tropospheric aerosol. Indeed, other than sea-salt particles containing dissolved
organic carbon, organic glasses found in the atmosphere may be poor IN.
131
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CHAPTER 8: APPENDIX
Supporting Figures
Figure A.1. 50x images of an effloresced LiCl particle before (a) and after (b) impact. The size
bar in the bottom left hand corner corresponds to 20 µm.
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Figure A.2. A 50x image (a), 100x Raman 2D maps (b), and a 100x Raman spectrum taken from
the center (c) of a representative aqSOA + AS particle at 298 K and 0% RH. The dry lateral
diameter of the particle is approximately 6.5 microns. The main features of the spectrum in panel
(c) are a sharp peak at 972 cm-1 (due to the symmetric stretching mode of SO42- from AS), a
broad band centered at 2900 cm-1 (due to the C-H stretching from the aqSOA). In panel (b), the
sulfate-containing region is shown as a color map where warmer colors refer to a higher intensity
of the SO42- peak (972 cm-1). The organic-containing region is shown as a grey-scaled color map
where warmer colors correlated to a higher intensity of the C-H peak (2900 cm-1). From the
overlaid map in panel (b), the particle appears to be in a core-shell configuration with an AS core
surrounded by a thin aqSOA shell, similar to the optical image found in panel (a). The shell,
however, is thinner than our spectral resolution and, therefore, the particle may be homogenous.
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Supporting Movies:
Movie A.1. A movie, sped up 32x, of ice nucleating on a mixed 1,2,6-hexanetriol-ammonium
sulfate particle (org:sulf = 2:1) that had previously undergone liquid-liquid phase separation and
efflorescence. The movie was recorded using 50x magnification at a temperature of 215 K. Ice
nucleates inside of the coated particle at the ammonium sulfate-organic interface (bottom right
side of the inner part of particle). The initial ice crystal appears as a bright spot and grows
outwardly from that point. As the ice grows beyond the dimensions of the organic shell, the shell
spreads around it to reduce the surface tension over the entire system, indicating that the shell is
a flowing liquid.
Movie A.2. A movie, sped up 32x, of ice nucleating on a mixed 1,2,6-hexanetriol/2,2,6,6tetrakis(hydroxymethyl)cyclohexanol-ammonium sulfate particle (org:sulf = 2:1) that had
previously undergone liquid-liquid phase separation and efflorescence. The movie was recorded
using 50x magnification at a temperature of 215 K. Ice nucleates on the outside of the coated
particle at the organic-air interface (bottom right side particle). Here, as the ice grows, the
organic does not spread around the ice to reduce the surface tension over the entire system,
indicating that the organic shell is no longer a flowing liquid.
Movie A.3. A movie, recorded under 50x magnification and sped up 30x, of a typical impactflow experiment on methylglyoxal + methylamine aqSOA. In this particular experiment, the RH
increases from 51 to 93% as the temperature drops from 253.3 to 246.6 K. Initially, as the RH
around the shattered particle increases, there is no apparent change in in the particle morphology.
At a set RH, however, the cracks within the particle start to wane and the edges of the particle
start to round. We have defined this point as RHflow. As the RH is increased beyond RHflow, the
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particle continues to liquefy, eventually attaining a low enough viscosity to form back into near
spherical as to reduce the surface tension of the system. The dry lateral diameter of the particle is
approximately 20 microns µm.
Movie A.4. A movie, recorded under 50x magnification and sped up 30x, of a typical
deliquescence experiment on methylglyoxal + methylamine aqSOA. In this particular
experiment, the RH increases from 49 to 88% as the temperature drops from 254.1 to 247.3 K.
Initially, the particle looks to be in an effloresced AS core-organic shell morphology. As the RH
is increased around the particle, the glassy shell takes up water and reveals that the AS core is
not one consolidated crystal, but many AS islands. As the organic matrix liquefies, the AS
islands are free to diffuse throughout the particle. At a set RH, the AS islands promptly
deliquesce, turning into a homogeneous aqueous organic-sulfate particle. The dry lateral
diameter of the particle is approximately 20 µm.
Movie A.5. A movie, taken under 50x magnification and sped up 10x, of ice subliming a
methylglyoxal + methylamine aqSOA + AS particle that had previously nucleated ice in the
immersion mode. As shown, the organic matrix has spread out with the ice, indicating that it was
in a liquid, flowing state at the time of ice nucleation. Upon ice sublimation, the organic matrix
vitrifies upon drying and converts from a liquid back into a semi-solid or glass, leaving an
outline of the ice particle.
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