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THE CONTRIBUTION OF BLACK CARBON TO ICE NUCLEATING PARTICLE
CONCENTRATIONS FROM PRESCRIBED BURNS AND WILDFIRES
G. P. SCHILL1, P. J. DEMOTT1, K. J. SUSKI1, E. W. EMERSON2, E. J. T. LEVIN1, T. C. HILL1, J. K.
KODROS1, D. K. FARMER2, J. R. PIERCE1, AND S. M. KREIDENWEIS1
1
Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO
2
Department of Chemistry, Colorado State University, Fort Collins, CO
Keywords: Ice Nucleation, Black Carbon, Immersion, Mixed-Phase Clouds, CFDC, SP2.
INTRODUCTION
Black carbon (BC) has been implicated as a possible ice-nucleating particle (INP) due to its
abundance in the upper troposphere (Schwarz et al., 2010). The role of BC as an INP, however, remains
unclear. For example, Cozic et al. (2008) report that soot particles were enhanced in the ice phase
compared to background aerosol at the high Alpine research station Jungfraujoch; however, future studies
at the same site did not see a similar enhancement of BC in ice crystals (Kamphus et al., 2010).
Additionally, several aircraft studies have shown that fresh and aged biomass-burning particles influence
cloud glaciation (Stith et al., 2011; Twohy et al., 2010), but the specific material responsible for ice
nucleation was not definitively determined.
Laboratory results on carbonaceous combustion particles also show conflicting results, especially
for laboratory-generated BC particles. Several studies have reported no observable ice nucleation on soot
particles within their instrument’s detection limit from -15 to -40 °C (Friedman et al., 2011; Koehler et al.,
2009), and suggest that soot particles do not play a significant role in heterogeneous ice formation in
mixed-phase clouds. These negative results are in disagreement with a number of previous studies
(DeMott, 1990; Diehl and Mitra, 1998), which observed ice nucleation on soot particles at temperatures
above -30 °C. These positive results have led to one review article to suggest that BC could potentially
rival the importance of mineral dust in mixed-phase clouds below -15 °C (Murray et al., 2012).
In a similar fashion, current global climate models that incorporate theoretical formulations of
heterogeneous ice nucleation often use only the positive laboratory results of ice nucleation on BC and,
subsequently, often identify BC as the second-most-abundant INP type after mineral dust (Hoose et al.,
2010; Savre and Ekman, 2015). Previous studies have shown that BC from diesel exhaust does not
contribute significantly to INP concentrations relevant to mixed-phase clouds (Schill et al., 2016). In
contrast, real-world biomass burning events have been shown to be a source of INP (Mccluskey et al.,
2014; Prenni et al., 2012) and that BC can contribute to INP concentrations from laboratory burns (Levin
et al., 2016). The direct contribution of BC from real-world biomass burning to INP concentrations,
however, has not been well defined.
In this paper, we present results from field studies that use a newly developed technique to
determine the direct contribution of BC to INP concentrations. Data sets were collected from prescribed
burns and from wildfires of opportunity in the Western United States.
METHODS
INP concentrations from biomass-burning events were measured using the Colorado State
University continuous-flow diffusion chamber (CSU-CFDC) (Rogers et al., 2001; DeMott et al., 2015), a
well characterized thermal gradient diffusion chamber for real-time ice nucleation activity measurements
of flowing aerosols.
Additionally, BC measurements were taken with the Single Particle Soot Photometer (SP2). The
SP2 uses optical detectors to measure the interaction of individual aerosol particles with a high-intensity
1064 Nd:YAG laser. Those particles that absorb light at 1064 nm, which is primarily refractory BC (rBC)
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in the atmosphere, are heated to their vaporization temperature and emit thermal radiation, a process
termed laser-induced incandescence. Since the SP2 selectively vaporizes rBC in an aerosol stream, the
SP2 can be used as an rBC-specific pre-filter for the CSU-CFDC. Thus, by modifying the SP2 to allow the
CSU-CFDC to sample from its exhaust, and by toggling the SP2 laser on and off, the direct contribution of
BC to real-world INP concentrations can be determined (Figure 1) (Levin et al., 2014).
The SP2-CFDC was taken into the field to sample from real-world biomass burning events. All
measurements were conducted in the CSU Mobile Laboratory (Brown Specialty Vehicles, Lawrence, KS).
Prescribed burns were sampled at the Konza Biological Research Station (KBPS, Manhattan, KS), a
tallgrass prairie site jointly maintained by the Nature Conservancy and Kansas State University.
Furthermore, several wildfires, including the Beaver Creek (Walden, CO), Cliff Creek (Bondurant, WY),
and Pioneer (Lowman, ID) wildfires, were sampled in the Western United States.
These results and previous findings of the contribution of BC to INP concentrations in diesel
exhaust from Schill et al. (2016) will be input into the Goddard Earth Observing System chemicaltransport model (GEOS-Chem). Here, new parameterizations will be made to apply over the model
aerosol field outputs in order to determine the potential INP concentrations. Both the potential INP
concentration available at the surface as well as the number of INP active as a function of zonally
averaged latitude and altitude will be shown. These results will be compared to parameterizations from the
previously mentioned positive studies that are often used in models.
Figure 1. Schematic representation of the CSU-CFDC (a) and the SP2-CFDC (b) techniques.
CONCLUSIONS
Preliminary results of the contribution of rBC to the number concentration of INP (NINP) and
particles greater than 500 nm in optical diameter (N500) are shown in Figure 2. All data sets are normalized
to total particle concentrations from a condensation particle counter in order to account for variability in
smoke plume concentrations. As shown, rBC often contributes to approximately 20% of N500 from
wildfires, but does not contribute more than 10% to NINP across all burns. Additional ice nucleation
studies, using an offline analysis on filter samples collected during the same prescribed burns and
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wildfires, are ongoing. Such techniques are able to determine the contribution of both biological and
organic species to INP concentrations by pre-treatment with heat and hydrogen peroxide, respectively
(Hill et al., 2016). Comprehensive data, including offline freezing analysis, new parameterizations, and
GEOS-Chem outputs will be shown in the conference presentation.
Figure 2. The contribution of rBC to INP concentrations (NINP) and particles with an optical diameter
greater than 500 nm (N500).
ACKNOWLEDGEMENTS
This material is based upon work supported by the National Science Foundation Division of Atmospheric
and Geospace Sciences under award AGS1433517. Funding for PJD, KJS, EJTL, TCH, SMK, and other
logistical support was provided by NASA Earth Science Division award NNX12AH17G. Funding for
EWE was provided by NOAA award NA14OAR4310148.
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