LETTERS
PUBLISHED ONLINE: 19 APRIL 2009 | DOI: 10.1038/NGEO499
Inadvertent climate modification due to
anthropogenic lead
Daniel J. Cziczo1,2 *, Olaf Stetzer2 , Annette Worringen3 , Martin Ebert3 , Stephan Weinbruch3 ,
Michael Kamphus4 , Stephane J. Gallavardin2,4 , Joachim Curtius4,5 , Stephan Borrmann4,6 ,
Karl D. Froyd7 , Stephan Mertes8 , Ottmar Möhler9 and Ulrike Lohmann2
Aerosol particles can interact with water vapour in the
atmosphere, facilitating the condensation of water and the
formation of clouds. At temperatures below 273 K, a fraction
of atmospheric particles act as sites for ice-crystal formation.
Atmospheric ice crystals—which are incorporated into clouds
that cover more than a third of the globe1 —are thought
to initiate most of the terrestrial precipitation2 . Before
the switch to unleaded fuel last century, the atmosphere
contained substantial quantities of particulate lead; whether
this influenced ice-crystal formation is not clear. Here,
we combine field observations of ice-crystal residues with
laboratory measurements of artificial clouds, to show that
anthropogenic lead-containing particles are among the most
efficient ice-forming substances commonly found in the
atmosphere3 . Using a global climate model, we estimate that
up to 0.8 W m−2 more long-wave radiation is emitted when
100% of ice-forming particles contain lead, compared with
when no particles contain lead. We suggest that post-industrial
emissions of particulate lead may have offset a proportion of
the warming attributed to greenhouse gases.
It is highly certain that the anthropogenic addition of greenhouse
gases to the atmosphere has caused global warming4 . Conversely,
the addition of small aerosol particles has caused regional ‘solar
dimming’ phenomena5 that counteract some greenhouse-gas
warming in a so-called direct effect. Aerosol particles can also
interact with atmospheric water vapour by acting as sites of
condensation and can thereby lead to the formation of clouds.
Clouds, in turn, affect the global radiative balance by reflecting
solar energy or trapping terrestrial radiation6 ; this is termed an
aerosol ‘indirect effect’. Net warming or cooling is dependent
on cloud properties such as altitude and thickness. Depending
on the ambient temperature and saturation, warm liquid water
clouds, cold ice clouds or intermediate-temperature mixed-phase
clouds can form. Owing to the typically high altitude and
often remote location, the nucleation of ice is the less wellunderstood process. Despite this uncertainty, cirrus ice clouds
can influence the Earth’s radiative budget owing to their large
global coverage7 . For example, it has been shown8 that indirect
aerosol perturbations to ice clouds can cause radiative changes
of the same magnitude as the direct radiative impact of all
anthropogenic particles.
Ice can nucleate homogeneously from the aqueous droplets
commonly found in the atmosphere, but this requires a supercooling of ∼40 K below the equilibrium freezing temperature9,10 .
Atmospheric ice formation at higher temperatures requires the
presence of a special solid particle that acts as an ice nucleus10 .
Owing to the presence of a surface that enhances the stability of an
ice embryo, this process is known as heterogeneous ice nucleation.
An ice nucleus can cause ice nucleation by several modes, for
example from within a liquid water droplet or by acting as a site for
the direct deposition of water vapour. Not all solid aerosol particles
enhance ice nucleation, but several materials have been shown to
act as ice nuclei. These include, but are not limited to, mineral dust,
anthropogenic metal oxides, pollen and bacteria9 . Each aerosol type
shows a characteristic saturation and temperature, which varies
depending on the mode by which freezing occurs. This freezing
onset point is dependent on the surface area of the particle and
some researchers have theorized that there are specific ‘active sites’
that stabilize an ice embryo10 . Larger particles and materials that
inherently contain more active sites thus exhibit a higher nucleation
temperature11 . The exact nature of these sites remains unknown,
but features such as surface defects are one candidate. Another atmospherically important property of ice nuclei is their abundance.
For example, silver iodide and some biological materials are known
to be effective ice nuclei, but it is uncertain if their abundance is large
enough to induce a global radiative impact12 .
Here, we have determined the nature of ice nuclei using three
complementary methods: (1) creating artificial clouds by exposing
ambient aerosol to controlled supersaturation and temperature,
thereby mimicking atmospheric cloud formation, (2) examining
the residue of ice crystals from naturally occurring clouds
and (3) creating artificial clouds on laboratory-prepared aerosol
particles (Supplementary Information contains a full discussion
of the methods and data statistics in Supplementary Table S1).
For all methods, mass spectrometry was used as the analytical
technique and the percentage represents a number fraction of
analysed particles. Using the first method, we find that mineral
dust is the most common atmospheric aerosol type that acts as an
1 Atmospheric
Sciences and Global Change Division, Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, Washington 99354, USA,
for Atmospheric and Climate Science, ETH Zurich, Universitätstrasse 16, CH-8092 Zürich, Switzerland, 3 Institute for Applied Geosciences,
Technical University Darmstadt, Schnittspahnstraße 9, D-64287 Darmstadt, Germany, 4 Institute for Atmospheric Physics, Johannes Gutenberg-University
of Mainz, Joh.-Joachim-Becher-Weg 21, D-55099 Mainz, Germany, 5 Institute for Atmospheric and Environmental Sciences, Goethe-University Frankfurt
am Main, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany, 6 Particle Chemistry Department, Max Planck-Institute for Chemistry, D-55128 Mainz,
Germany, 7 Chemical Sciences Division, National Oceanic and Atmospheric Administration, 325 Broadway Ave., Boulder, Colorado 80305, USA,
8 Leibniz-Institute for Tropospheric Research, D-04318 Leipzig, Germany, 9 Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe,
Postfach 3640, D-76021 Karlsruhe, Germany. *e-mail: [email protected].
2 Institute
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NATURE GEOSCIENCE DOI: 10.1038/NGEO499
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a
a
Detector signal (normalized)
1
0.1
Mg+
Ca+
Si+
C+
Zn+
Pb+
0.01
I+
0.001
0
50
100
150
200
Ion mass/charge
1
Detector signal (normalized)
b
Al+
50
Al+
b
K+
100
150
Ion mass/charge
200
Fe+
Na+
0.1
1 µm
0.01
BaO+
Pb+
0.001
0.0001
0
50
100
150
Ion mass/charge
200
Figure 1 | Positive ion mass spectra of single ice-crystal residual
particles. a, Spectrum obtained at the Storm Peak Laboratory (3,200 m
above sea level) in the Colorado Rocky Mountains when ambient aerosol
was exposed to cirrus cloud conditions within a CFDC. b, Spectrum
obtained at the JRS (3,600 m above sea level) in the Swiss Alps from the
residue of an ice crystal from a mixed-phase cloud. c, Spectrum obtained
during experiments when Arizona Test Dust was exposed to cirrus cloud
conditions within the AIDA chamber.
effective ice nucleus, representing 49% of all analysed particles13 .
Figure 1a is the mass spectrum of one exemplary particle. The single
component found with the highest frequency was lead, present
in 32% of ice nuclei (an example of the isotopic identification of
lead is given in Supplementary Fig. S1). The second method, the
separation of ice crystals from naturally occurring clouds, yielded
similar results for ice residue. The distinction between ice nuclei
2
Pb inclusions
Pb+
1
Detector signal (normalized)
Fe+
C+
0.1
0
c
1 µm
K+
Figure 2 | Secondary electron images of ice-crystal residual particles
from mixed-phase clouds at the JRS. a, Lead inclusions (black dots),
visible from an overlay of a scanning electron image and lead mapping by
energy dispersive X-ray microanalysis of this residual silicate particle. b, A
carbon-, chlorine- and lead-containing residual particle where lead is in the
form of organic lead and/or lead halides.
and ice residue is that the latter contains the ice nuclei and any
gas and particle-phase material that adhered to the ice crystal
during its atmospheric lifetime. As an example, Fig. 1b shows that
the most common class of ice residue was again mineral dust,
observed for 67% of the residue. The most common component
was lead, observed in 42% of this residue. The frequency of mineral
dust found using the first and second methods is in agreement
with past studies, including identification of the central aerosol
particle embedded within snowflakes9 . The correlation between
lead and ice nucleus concentration was observed in a previous study
of bulk ice nucleus properties14 , although this was attributed to
both also being correlated to particle size. The authors suggested
studies at the single-particle level to verify this14 as have been
carried out in this study.
The results using the first two methods motivated the third set
of studies, the formation of artificial clouds on collected samples of
mineral dust prepared in a laboratory environment. This allowed
control of conditions with greater precision than was possible in
a field setting; previous studies on idealized mineral dust samples
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LETTERS
a
180
Kaolinite
Kaolinite + 1% PbSO4
PbSO4
1.5
Ice water path (g m¬2)
170
160
RHi (%)
150
140
1.0
ECHAM5¬Pb0
ECHAM5¬Pb1
ECHAM5¬Pb10
ECHAM5¬Pb100
0.5
0
130
80° S 60° S 40° S 20° S
120
0°
20° N 40° N 60° N 80° N
110
100
220
b
230
240
Temperature (K)
250
260
Figure 3 | Temperature and relative humidity required for the onset of ice
nucleation using a cloud chamber. Conditions required for ice nucleation
by kaolinite mineral dust and kaolinite doped with lead sulphate are shown.
Below ∼240 K, the lead-doped kaolinite particles, which mimic
atmospheric aerosol that coagulated with lead, induce ice nucleation at a
lower saturation for a given temperature than undoped kaolinite. Pure lead
sulphate is, in contrast, an inefficient nucleus. The solid line represents the
saturation vapour pressure of liquid water.
have been used to determine specific ice nucleation threshold
conditions15 . For the collected samples of mineral dust, lead was
present in 8% of the aerosol particles, but in the subset that caused
ice nucleation, lead was found in 44%. Figure 1c is an example
of a lead-containing particle that nucleated ice under controlled
laboratory conditions.
By analysis of Antarctic ice cores, Boutron and Patterson16
showed that lead from mineral dust and volcanic emissions
were the main atmospheric sources during pre-industrial times.
Anthropogenic activities now dominate, accounting for >99% in
the mid-1980s. At that time, most of the lead introduced to the
atmosphere was from tetraethyl lead (TEL) added to automotive
petrol. With regulation of TEL, total lead has dropped significantly,
with a 20-fold decrease reported in the continental United States
in the two decades since 1980 (ref. 17). TEL in light aviation
fuel is now thought to be the dominant new source, although
the uncertain and increasing emissions from coal combustion
and smelting could be equivalent or greater3,17 . In a study of
atmospheric particles from 50 nm to 2 µm diameter, Murphy et al.3
showed that lead was detectable in ∼5–10%. The size dependence
of the lead concentration and the mixing state supported the
coagulation of small (<50 nm) primary lead particles with preexisting atmospheric aerosol as the source of most of the particles3 .
Since the mid-1940s it has been known that artificial ice nuclei
could be produced18 . Two materials found to nucleate ice as high as
a few degrees below 273 K were silver and lead iodide19 , although
the high toxicity of the latter limited its use. Lead oxides and
mixtures with ammonium iodide were subsequently found to be
similar, if not better, ice nuclei than silver and lead iodide (ref. 20).
Baklanov et al. showed that pure lead-containing materials were not
required for ice nucleation; instead, lead need only be present as a
surface inclusion on an inert core so long as its size remains at or
above that of an active site, assumed to be ∼30–50 nm in diameter21 .
Figure 2 contains electron microscope images of lead-containing
ice residuals from the Jungfraujoch Research Station (JRS). Using
energy-dispersive X-ray microanalysis, lead was identified in ∼15%
of the ice residuals. Surface inclusions, detected on 80% of the
lead-containing particles, are denoted as black dots. Chemical
Outgoing long¬wave radiation (W m¬2)
Latitude
1
0
¬1
¬2
80° S 60° S 40° S 20° S
0°
20° N 40° N 60° N 80° N
Latitude
Figure 4 | Atmospheric properties with and without the influence of
anthropogenic lead. a,b, Ice water path (a) and outgoing long-wave
radiation (b) difference between the present-day and pre-industrial times.
The four sensitivity conditions are 0, 1, 10 and 100% of mineral dust
containing lead inclusions.
analysis shows lead bound in several forms including lead sulphide,
lead halides, organic lead and lead oxide. Individual inclusions are
of the order of 10 nm diameter, consistent with coagulation of
small primary lead particles onto pre-existing atmospheric aerosol
as it passes through lead source regions3 . This is the method
recommended by Baklanov et al. for maximizing the ice nucleating
potential of an inert aerosol, unintentionally carried out, by adding
surface inclusions of ice nucleus material21 . We therefore contend
that anthropogenic emissions of lead to the atmosphere have had
the effect of ‘supercharging’ pre-existing particles, making them
highly efficient ice nuclei. Furthermore, we note that the altitude
at which light aircraft fly is above the planetary boundary layer
and directly places the emitted lead at an altitude where ice and
mixed-phase clouds form.
The ice nucleation potential of atmospheric aerosol that
coagulated with small primary lead particles has not previously
been investigated. Previous research has instead focused on the
production of artificial ice nuclei for weather modification19–21 .
The form of atmospheric lead may vary, depending on the specific
emission, with lead sulphate and oxide being two common forms3 .
The former has not been investigated as an ice nucleus. Laboratory
studies were carried out by atomization of the precipitate of lead
sulphate crystals in a water slurry containing kaolinite mineral
dust. The results are illustrated in Fig. 3. Doping was at an
atmospherically relevant level of 1% lead sulphate to kaolinite by
mass3 and this was compared with undoped particles. The presence
of lead decreased the saturation required for the nucleation of ice
when compared with pure kaolinite by ∼10–20% relative humidity
with respect to water ice at temperatures below ∼240 K.
To estimate the climatic effect of lead-containing ice nuclei,
global climate model simulations were carried out with homogeneous and heterogeneous nucleation in cirrus clouds with
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NATURE GEOSCIENCE DOI: 10.1038/NGEO499
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temperatures below 238 K (refs 22, 23). We assume that 0, 1, 10 or
100% of the internally mixed mineral dust particles in a present-day
climate simulation contain lead and that lead-containing particles
initiate freezing at 110% relative humidity with respect to ice
(RHi). The first case is a modern world with no lead emissions,
the second and third are estimates for the present day and the
fourth may be similar to conditions if TEL was unregulated. The
remaining immersed dust particles initiate freezing at 130% RHi
and all other supercooled solution droplets freeze homogeneously.
As shown in Fig. 4, the change in the ice water path, the vertically
integrated ice water content in mixed-phase and cirrus clouds,
since pre-industrial times is largest in the mid-latitudes of the
Northern Hemisphere. The increase is similar in the four sensitivity
simulations despite the fact that the increase in lead-containing
mineral dust particles causes more particles to initiative freezing at
lower RHi. This increase in lead-containing particles corresponds
to warmer temperatures and lower cloud altitudes, and thus, more
emitted long-wave radiation. As compared with the case where
mineral dust particles contain no lead in the present day, up to
0.8 W m−2 more long-wave radiation is emitted when 100% of the
mineral dust aerosols contain lead.
Methods
Three types of experiment were undertaken to determine the nature of ice nuclei
and ice residue. First, ice crystals were formed from ambient aerosol within a
litre-sized continuous flow diffusion chamber24 (CFDC). Also known as ‘cloud
chambers’, CFDCs are used to mimic the atmospheric temperature and relative
humidity at which clouds form to facilitate experiments in a controlled laboratory
or field setting. Ice crystals formed within the CFDC were isolated for analysis
using counterflow virtual impaction25 . Second, crystals from naturally occurring
mixed-phase clouds at the Jungfraujoch Research Station (JRS) were separated for
analysis using an ice counterflow virtual impaction26 . Third, experiments were
conducted within the Aerosol Interactions and Dynamics in the Atmosphere
(AIDA) cloud chamber, an 84 m3 actively cooled vessel27 , as well as a CFDC on
collected samples of mineral dust. Subsequent experiments were conducted on
collected samples doped with lead. Size and compositional analysis of ice nuclei
and ambient aerosol was carried out using single-particle mass spectrometry28,29
and electron microscopy coupled to energy-dispersive X-ray microanalysis30 .
Supplementary Information contains a full discussion of the methods. Data
statistics are given in Supplementary Table S1.
Received 4 December 2008; accepted 19 March 2009;
published online 19 April 2009
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15. Möhler, O. et al. Efficiency of the deposition mode ice nucleation on mineral
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16. Boutron, C. F. & Patterson, C. C. Lead concentration changes in Antarctic ice
during the Wisconsin/Holocene transition. Nature 323, 222–225 (1986).
17. Environmental Protection Agency. Latest findings on National Air Quality,
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at <http://www.epa.gov/air/airtrends/aqtrnd02/2002_airtrends_final.pdf>.
18. Vonnegut, B. The nucleation of ice formation by silver iodide. J. Appl. Phys. 18,
593–595 (1947).
19. Schaefer, V. J. Silver and lead iodides as ice-crystal nuclei. J. Meteorol. 11,
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20. Reischel, M. T. Lead nuclei from reactions involving lead, ammonia, and
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21. Baklanov, A. M. et al. The influence of lead iodide aerosol dispersity on its
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23. Stier, P. et al. The aerosol-climate model ECHAM5-HAM. Atmos. Chem. Phys.
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Acknowledgements
We thank P. J. DeMott, D. M. Murphy and D. S. Thomson for their assistance with the
measurements. We also acknowledge the effort of all of the participants of the INSPECT
and CLACE field studies, the support of the High Altitude Research Foundation
Gornergrat and Jungfraujoch and the experimental group at AIDA. This research
was supported by the Atmospheric Composition Change the European Network for
Excellence, ETH Zurich, the German Research Foundation and Pacific Northwest
National Laboratory directed research funding.
Author contributions
D.J.C., single-particle mass spectrometry, data analysis and paper writing; O.S., ice
nucleation experiments and data analysis; A.W., M.E. and S.W., sample acquisition,
electron microscopy and data interpretation and analysis; M.K., S.J.G., J.C. and
S.B., mass spectrometer development, sample acquisition for single-particle mass
spectrometry and data analysis; S.M., ice crystal sample acquisition and data analysis;
O.M. and K.D.F., conducted AIDA experiments and data analysis; U.L., GCM
programming and data analysis.
Additional information
Supplementary information accompanies this paper on www.nature.com/naturegeoscience.
Reprints and permissions information is available online at http://npg.nature.com/
reprintsandpermissions. Correspondence and requests for materials should be
addressed to D.J.C.
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SUPPLEMENTARY INFORMATION
doi: 10.1038/ngeo499
Supplementary Information
Methods
Three types of experiments were undertaken to determine the nature of ice nuclei and ice
residue. First, ice crystals were formed from ambient aerosol within a liter-sized
continuous flow diffusion chamber (CFDC)1. Ice crystals were then separated for analysis
using a counterflow virtual impactor (CVI)2. Second, ice crystals in naturally occurring
mixed-phase clouds were separated for analysis using an ‘Ice-CVI’ specifically designed
for mixed-phase conditions3. Third, experiments were conducted within the aerosol
interactions and dynamics in the atmosphere (AIDA) cloud chamber4 on collected
samples of mineral dust. Experiments were also performed on collected samples of
mineral dust doped with lead with a CFDC. Analysis was in all cases performed using
single particle mass spectrometry (SPMS)5,6. Electron microscopy (EM) coupled to
energy dispersive X-ray microanalysis7 was also used for ambient ice residue
characterization.
CFDC
A diffusion chamber functions by generating an environment with a defined temperature
below the melting point of water and a supersaturated relative humidity with respect to
ice. These conditions are produced in the space between two ice coated walls held at
different temperatures and mimic those at which atmospheric mixed-phase and ice clouds
form. Diffusion leads to a linear gradient in temperature and absolute humidity between
the walls but, because of the exponential relation between temperature and saturation
vapor pressure, a supersaturation is achieved close to the center. A sample aerosol flow
layered between two particle-free sheath flows constrains the conditions to which the
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supplementary information
doi: 10.1038/ngeo499
sample is exposed. CFDC volumes were a few liters with a residence time on the order of
10-30 seconds1.
CVI
A conventional CVI inertially separates particles by directing a dry, particle-free, flow of
gas in a direction opposite that of the sample. The rate of this ‘counterflow’ can be
adjusted to create a specific inertial cutpoint2. A conventional CVI was used for ice
crystal separation during droplet-free conditions in the CFDC and AIDA experiments8.
One limitation is that a conventional CVI can not be used to distinguish frozen and liquid
hydrometeors of the same inertia when sampling in mixed-phase conditions. A custom
built Ice-CVI3 was used in order to overcome this limitation for field studies. A vertically
oriented inlet horn in combination with a virtual impactor (VI) was used to remove
precipitating ice particles larger than 20 µm diameter. The upper limit of 20 µm assured
an aspiration efficiency of ~1 for smaller ice particles. Downstream of the VI a preimpactor (PI) removed supercooled drops by contact freezing on cold impaction plates.
Ice particles bounced and passed the impaction plates. A conventional CVI was located
downstream of the PI to reject interstitial particles smaller than 5 µm. Ice particles in the
5 - 20 µm diameter range were thus passed by the Ice-CVI. As with a conventional CVI
these small ice crystals were injected into a particle-free and dry carrier gas which led to
evaporation and allowed for the analysis of the residual material.
AIDA
The ice nucleation properties of laboratory-prepared aerosol, including re-dispersed
mineral dust samples, can be investigated at simulated conditions of natural clouds in the
AIDA chamber at the Forschungszentrum Karlsruhe4,5. Experiments typically begin at ice
2
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supplementary information
saturated conditions in the 84 m3 actively-cooled cloud chamber due to a thin ice coating
on the chamber walls. The formation of a cloud is induced by expansion cooling via
vacuum pumping which mimics an updrafting atmospheric air parcel. Droplets and ice
particles are detected by optical particle counters, in situ scattering and depolarization
measurements, and Fourier transform infrared spectroscopy. Clouds typically exist within
the AIDA chamber for several minutes.
SPMS
The single particle mass spectrometers used in this study draw aerosol into the instrument
through an aerodynamic inlet which imparts a size dependent velocity. Light scattering as
particles pass through two visible lasers beams set a known distance apart indicates the
presence of a sample and, when combined with the size dependant velocity, provides the
aerodynamic diameter. A near ultraviolet laser is triggered by the light scattering event to
desorb the aerosol material and ionize it. The chemical composition can then be inferred
from ions detected using a time of flight mass spectrometer. Analysis is performed in situ
and in real time on a particle by particle basis. A recent review of SPMS was conducted
by Murphy6.
The lower size limit of a SPMS is set by the detection limit of scattered light; this is
normally between 150-250 nm aerodynamic diameter. The upper limit is set by the
aerodynamic properties of the inlet and this is normally between 2 and 3 �m diameter6. It
is noteworthy that previous studies have shown that atmospheric ice nuclei are found
within this range9,10.
Without extensive laboratory testing particle matrix effects and the disparate ionization
efficiencies of common aerosol materials single particle mass spectrometers produce
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supplementary information
doi: 10.1038/ngeo499
qualitative, not quantitative, data11. Isotopic abundance can be used to identify specific
elements, as is shown in Figure S1 for lead in an ice residual from a naturally occurring
mixed-phase cloud. Gallavardin et al. showed that specific mineralogical information was
difficult to ascertain on the single particle level, however, and we therefore use the broad
classifications ‘mineral dust’ and ‘lead containing’ in this study12. A list of the number of
spectra analyzed for each study is given in Table S1.
EM
Size, morphology, and elemental composition of individual ice residue and interstitial
particles (i.e., those which did not form droplets or ice) were analyzed during the field
studies using environmental scanning EM and transmission electron microscopy (TEM),
each combined with energy dispersive X-ray microanalysis7. Samples from the Ice-CVI
were impacted onto EM grids which were analyzed at the Institute for Applied
Geosciences at the Technical University Darmstadt. Lead inclusions of <100 nm diameter
were studied with High Resolution-TEM.
Field Sites
Storm Peak Laboratory (SPL)
Field experiments were conducted on ambient aerosol at SPL located at 3200 meters
above sea level (m.s.l.) in the Colorado Rocky Mountains. The high altitude and absence
of local sources results in access to free tropospheric conditions, typically during
overnight periods when subsidence of the planetary boundary layer occurs13. Experiments
at SPL were performed during the fall of 2001 and spring of 2004 by exposing free
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tropospheric aerosol to cirrus cloud formation conditions with a CFDC. Ice crystals were
then separated with a CVI and analyzed using SPMS14.
Jungfraujoch Research Station (JRS)
The JRS is located at 3600 m.s.l. in the Swiss Alps in a region with minimal local particle
sources. Mixed-phase clouds occur with a frequency of 40% during late winter and early
spring at this site15. Ice crystals were separated when mixed-phase clouds were present
using the Ice-CVI. Residue was analyzed with SPMS and samples were collected for offline EM during the late winter and early spring of 2006 and 2007.
Laboratory Particle Preparation
Laboratory studies at the AIDA chamber were conducted on Arizona Test Dust (Powder
Technology, Inc.). The mineral samples were dispersed into a particle-free flow of dry
synthetic air with a small scale powder disperser (TSI, Inc.). Large dust particles were
removed in an inertial impactor with a cutoff diameter of 1 �m diameter before being
added to the aerosol chamber.
Laboratory CFDC experiments were performed on kaolinite clay (Fluka, Inc.). The
kaolinite was dispersed in distilled deionized water and aerosolized with a custom build
atomizer. Aerosol both in an un-doped state and associated with 1% lead sulfate by mass
was produced and size selected at 200 nm diameter with a differential mobility analyzer
(TSI, Inc.). The internal mixing state of mineral dust and lead was ascertained with
SPMS.
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doi: 10.1038/ngeo499
Global Climate Model Simulations
For the global climate simulations the ECHAM5 general circulation model was used.
ECHAM5 includes a two-moment cloud microphysics scheme16 coupled to the
ECHAM5-HAM two-moment aerosol scheme which predicts the aerosol mixing state in
addition to the aerosol mass and number concentrations17. The size-distribution is
represented by a superposition of log-normal modes including the major global aerosol
compounds sulfate, black carbon, organic carbon, sea salt and mineral dust. In order to
avoid changes in meteorology, both the pre-industrial and present-day simulations were
nudged to the ECMWF ERA40 reanalysis data for the year 2000 after an initial spin-up
of 3 months17. The aerosol emissions of the pre-industrial simulations are representative
of the year 175017. The simulations were conducted in T42 horizontal resolution (~2.8º x
2.8º) with 19 vertical layers.
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doi: 10.1038/ngeo499
Supplementary Figure 1
0.25
intensity (a.u.)
0.20
0.15
0.10
0.05
0.00
200
205
210
215
mass/charge
Supplementary Figure 1: Identification of lead from isotopic abundances. This
exemplary ice residual, shown in arbitrary units (a.u.) of signal intensity versus positive
ion mass to charge, is from a mixed-phase cloud at the Jungfraujoch Research Station.
The isotopic abundance of lead at mass 204, 206, 207, and 208 is 1.4, 24.1, 22.1, and
52.4 %, respectively. This pattern, discernable using SPMS, was used to identify lead.
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Supplementary Table 1
Experiment
1
2
3
Cloud Source Location Ice Nuclei Ice Residue
Aerosol
SPMS Spectra Mineral Dust
CFDC
SPL
X
Ambient
~2500 total
49%
(cloud chamber)
~250 IN
Natural clouds
JRS
X
Ambient
~1750 total
67%
(mixed-phase)
~500 residue
AIDA
AIDA
X
Resuspended
~2400
100%
(cloud chamber)
mineral dust
Lead
32%
42%
44%
Supplementary Table 1: Source, location, ice crystal and aerosol origin, and spectral
statistics for the 3 experimental studies. Experiments 1 and 3 utilized ice chambers while
2 was performed from within naturally occurring clouds. Experiments 1 and 2 were
conducted on ambient aerosol while 3 was performed on resuspended mineral dust. In
total ~3150 ice-forming aerosols were analyzed of which 42% contained lead. Note that
lead is present at the 5-10% level in ambient aerosol18.
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