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Effects of sulfuric acid and ammonium sulfate coatings on the ice nucleation properties of kaolinite.. Eastwood, Michael L.; Cremel, Sebastien; Wheeler, Michael; Murray, Benjamin J.; Girard, Eric; Bertram, Allan K. 2009

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Effects of sulfuric acid and ammonium sulfate coatings on the icenucleation properties of kaolinite particlesMichael L. Eastwood,1Sebastien Cremel,1Michael Wheeler,1Benjamin J. Murray,1,3Eric Girard,2and Allan K. Bertram1Received 12 September 2008; revised 2 November 2008; accepted 11 November 2008; published 28 January 2009.[1] The onset conditions for ice nucleation on H2SO4coated, (NH4)2SO4coated, and uncoated kaolinite particlesat temperatures ranging from 233 to 246 K were studied.We define the onset conditions as the relative humidity andtemperature at which the first ice nucleation event wasobserved. Uncoated particles were excellent ice nuclei; theonset relative humidity with respect to ice (RHi) was below110% at all temperatures studied, consistent with previousmeasurements. H2SO4coatings, however, drastically alteredthe ice nucleating ability of kaolinite particles, increasingthe RHirequired for ice nucleation by approximately 30%,similar to the recent measurements by Mo¨hler et al. [2008b].(NH4)2SO4coated particles were poor ice nuclei at 245 K,but effective ice nuclei at 236 K. The differences betweenH2SO4and (NH4)2SO4coatings may be explained by thedeliquescence and efflorescence properties of (NH4)2SO4.These results support the idea that emissions of SO2andNH3may influence the ice nucleating properties of mineraldust particles. Citation: Eastwood, M. L., S. Cremel, M.Wheeler, B. J. Murray, E. Girard, and A. K. Bertram (2009),Effects of sulfuric acid and ammonium sulfate coatings on the icenucleation properties of kaolinite particles, Geophys. Res. Lett.,36, L02811, doi:10.1029/2008GL035997.1. Introduction[2] Ice nucleation can occur in the atmosphere by eitherhomogeneous nucleation or heterogeneous nucleation. Het-erogeneous nucleation typically involves solid substrates,which are often called ice nuclei. These ice nuclei have thepotential to modify climate by changing the formationconditions and properties of ice and mixed-phase clouds.[3] Mineral dust particles are abundant in the atmosphere,and both laboratory and field measurements have shownthat mineral dust particles are effective ice nuclei [see, e.g.,Cziczo et al., 2004; DeMott et al., 2003; Pruppacher andKlett, 1997; Twohy and Poellot, 2005]. During their lifetimein the atmosphere, mineral dust particles can become coatedwith inorganic and organic material [Usher et al., 2003].There have only been a few studies that have directlycompared the ice nucleation properties of both coated anduncoated mineral dust particles at temperatures relevant forthe troposphere in order to isolate the effect of coatings[Archuleta et al., 2005; Knopf and Koop, 2006; Mo¨hler etal., 2008a, 2008b; Salam et al., 2007]. A few other studieshave measured the freezing properties of aqueous inorganicor organic solution droplets containing mineral dust par-ticles [Ettner et al., 2004; Hung et al., 2003; Zobrist et al.,2008; Zuberi et al., 2002].[4] In the following, we investigate the onset conditionsfor ice nucleation on H2SO4coated, (NH4)2SO4coated, anduncoated kaolinite particles at temperatures ranging from233 to 246 K, a temperature range relevant for the lowertroposphere. Here we define the onset conditions as therelative humidity and temperature at which the first icenucleation event was observed. Kaolinite represents asignificant component of mineral dust, comprising approx-imately 5–10% of aerosolized mineral dust [Glaccum andProspero, 1980]. This research should be useful whentrying to determine if anthropogenic emissions of SO2andNH3affect climate by influencing natural ice nuclei such asmineral dust.2. Experiment2.1. Ice Nucleation Measurements[5] The apparatus used in these studies has been describedin detail previously [Dymarska et al., 2006; Parsons et al.,2004]. It consists of an optical microscope coupled to a flowcell in which the relative humidity could be accuratelycontrolled. Mineral dust particles (either coated or uncoated)were deposited on the bottom surface of the flow cell; therelative humidity with respect to ice (RHi) inside the cellwas increased, and the conditions under which ice crystalsfirst formed were determined with a reflected light micro-scope (this is defined as the onset RHiand temperature).The RHiover the particles was controlled by continuouslyflowing a mixture of dry and humidified He through theflow cell. The bottom surface of the flow cell, whichsupported the particles, consisted of a glass cover slidetreated with dichlorodimethylsilane to make a hydrophobiclayer, which reduced the probability of ice nucleationdirectly on the surface.[6] Typical experimental RHitrajectories used in our icenucleation experiments are illustrated in Figure 1. At thebeginning of the experiments, the particles were exposed toa flow of dry He gas (RHi< 1%) at room temperature. Next,the temperature of the cell was rapidly lowered and the RHiwas set to approximately 95%. The nucleation experimentswere then conducted by steadily decreasing the temperatureand thus increasing the RHi.TheRHiramp rate wasapproximate 1% minC01. We also carried out experimentsusing a ramp rate of approximately 0.5% minC01. No differ-ences in results for the two ramp rates were obtained,GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L02811, doi:10.1029/2008GL035997, 2009ClickHereforFullArticle1Department of Chemistry, University of British Columbia, BritishColumbia, Canada.2Department of Earth and Atmospheric Sciences, University of Quebecat Montreal, Montreal, Quebec, Canada.3Now at School of Chemistry, University of Leeds, Leeds, UK.Copyright 2009 by the American Geophysical Union.0094-8276/09/2008GL035997$05.00L02811 1of5suggesting the aqueous coatings were in equilibrium withthe gas-phase water vapor.[7] We also carried out growth rate calculations ofaqueous solution droplets using the equations presentedby Pruppacher and Klett [1997] to further confirm thatthe aqueous coatings were in equilibrium with the gas-phasewater vapor in our experiments. These calculations showthat for aqueous coatings 5 mm in thickness, the coatingsshould be very close to equilibrium with the gas-phasewater vapor under our experimental conditions. The uncer-tainty in our measurements due to non-equilibrium condi-tions is at most 3% RHi.2.2. Sample Preparation and Thickness of the Coatings[8] The uncoated kaolinite particles were prepared byfirst mixing kaolinite in high-purity water (composition was1% kaolinite by mass) to create a suspension. The suspen-sion was then placed in an ultrasonic bath for 10 minutes.To deposit the particles on the glass slide, the suspensionwas passed through a nebulizer using high-purity nitrogenas a carrier gas. The flow from the nebulizer was directed ata hydrophobic glass slide and droplets containing theparticles were deposited on the surface of the slide uponimpaction. Water then evaporated, leaving behind the kao-linite particles. The coated kaolinite particles were preparedby mixing the kaolinite and coating material in high-puritywater (composition was 1% kaolinite and 0.2% coatingmaterial by mass). This suspension was then placed in theultrasonic bath prior to nebulization. Coated particles pro-duced by this method had an average weight fraction ofH2SO4or (NH4)2SO4of 0.167 under dry conditions,[9] The thicknesses of the coatings in our experimentswere estimated based on the compositions of the startingsuspensions and assuming a spherical core shell model (i.e.a kaolinite core surrounded by a uniform H2SO4or(NH4)2SO4coating). According to our calculations, underdry conditions (<1% RHi) a kaolinite core with a diameterof 15 mm will have a 0.7 mm thick coating, and a 5 mm corewill have a coating of 0.2 mm. A coating of 0.2 mmrepresents at least a few hundred sulphate layers coveringthe surface of the particle.[10] To further characterize the thickness of the coatingswe monitored the change in particle size as the relativehumidity with respect to water (RHw) was increased from<1% to 95%. From the change in size we estimate the totalamount of water adsorbed when cycling between <1% and95% RHwusing the thermodynamic model of Clegg et al.[1998]. From this we then estimated the amount of H2SO4in each particle and thus the thickness of the H2SO4coatingunder dry conditions. Measurements made for 15 individualparticles yielded an average weight fraction for the coatingof 0.12 ± 0.07 under dry conditions. The uncertainty in thisvalue derives from the uncertainty in the relative humiditymeasurements.2.3. Particle Number, Particle Size and Total SurfaceArea[11] In our experiments a typical sample held between100 and 1000 individual particles. As mentioned above wedefine the onset conditions as the RHiand temperature atwhich the first ice nucleation event occurred. Hence, ourresults correspond to when 1 to 0.1% of the kaoliniteparticles nucleated ice. The total surface area of mineraldust deposited in any particular experiment ranged from 1C210C04to 2 C2 10C03cm2. Over this narrow range, the onsetresults did not depend on the surface area. To illustrate thispoint, in the auxiliary material we present onset data forcoated and uncoated kaolinite particles as a function ofsurface area.1Since the results did not depend on surfacearea, for each temperature and each type of particle wecombine all the data together and calculate an average andstandard deviation for the onset RHi(see Figure 2). For eachtemperature and each type of particle, at least six measure-ments were carried out. Based on an analysis of all theparticles studied and assuming a normal distribution ofFigure 1. Typical experimental trajectories. The dot-dashed lines represent experimental conditions (RHiandtemperature) at which the H2SO4coatings are 10, 20 and 30wt % H2SO4.Figure 2. Summary of ice nucleation results for coatedand uncoated kaolinite particles. The error bars represent95% confidence intervals based on at least six measure-ments per data point.1Auxiliary materials are available in the HTML. doi:10.1029/2008GL035997.L02811 EASTWOOD ET AL.: FREEZING OF COATED KAOLINITE PARTICLES L028112of5particle sizes, the mean diameters for the uncoated particles,H2SO4-coated particles and (NH4)2SO4-coated particleswere 7.7 mm, 7.8 mm and 6.9 mm with standard deviationsof 5.3 mm, 5.7 mm and 5.0 mm, respectively.3. Results and Discussion3.1. Onset Results for Uncoated Kaolinite[12] The onset results for uncoated kaolinite particles areshown in Figure 2. The data show that only a smallsupersaturation (<110% RHi) is required for an ice nucle-ation event to occur in these experiments. These results,which correspond to deposition freezing [Pruppacher andKlett, 1997], are in excellent agreement with results weobtained earlier for uncoated kaolinite particles [Dymarskaet al., 2006; Eastwood et al., 2008]. The method of samplepreparation in the current experiments differs from themethod in our previous studies: in the previous studies weused a dry dispersion technique to generate the aerosolparticles. In the current study we generated the uncoatedparticles using the same technique used to generate thecoated particles – nebulizing an aqueous suspension – inorder to have the same experimental conditions in both theuncoated and coated work. For a detailed discussion onprevious measurements of the ice nucleation properties ofuncoated kaolinite particles see Eastwood et al. [2008].3.2. Onset Results for Kaolinite Particles CoatedWith H2SO4[13] Onset results for H2SO4coatings are also shown inFigure 2. These results correspond to either immersionfreezing or condensation freezing depending on the temper-ature [Pruppacher and Klett, 1997]. In contrast to theuncoated results, the H2SO4coated particles required muchhigher supersaturations before ice nucleation occurred. Thedata show that H2SO4coatings drastically altered the icenucleating ability of kaolinite particles, increasing the RHirequired to initiate ice nucleation by approximately 30%.[14] Before discussing a possible explanation for the shiftin ice nucleating ability of H2SO4coated particles, we firstdiscuss the composition of the H2SO4coatings during theice nucleation experiments with the aid of Figure 1. Thedot-dashed lines in Figure 1 show the experimental con-ditions at which the H2SO4coatings are 10, 20, and 30 wt%H2SO4(the remainder being H2O), calculated using themodel of Clegg et al. [1998]. Figure 1 illustrates that foralmost all of the experimental conditions, the H2SO4coat-ings are concentrated aqueous solutions (>10 wt % H2SO4)and highly acidic (the pH of a 10 wt % H2SO4solution isless than 0). This may help explain our freezing results(see below).[15] Kaolinite is a clay mineral whose unit cell is bestdescribed as layers of aluminosilicate sheets held togetherby hydrogen bonds. The ‘‘top’’ surface of these sheets is agibbsite-like surface with exposed hydroxyl groups. The‘‘bottom’’ surface of these sheets is a siloxane surface withexposed oxygen atoms [Stumm, 1992]. The Point of ZeroCharge (defined as the pH at which the net surface charge iszero) for the gibbsite-like surface is approximately 6, whilethe Point of Zero Charge for the edge surfaces is approx-imately 7–8 [Stumm, 1992]. As mentioned above, the pH ofthe H2SO4coating was below 0 under most experimentalconditions. As a result, the kaolinite particles should exhibithighly protonated, positively charged surfaces with a protonsurface density C241.2 C2 1014cmC02[Stumm, 1992]. Underthese conditions sulfate anions can be strongly adsorbed tometal oxide surfaces due to electrostatic attractions or thesulfate anions can adsorb by ligand substitution [Karltun,1997; Peak et al., 1999; Stumm, 1992; Zhang and Peak,2007]. Strongly adsorbed sulfate anions will change thechemical and physical properties of solid mineral interface.Hence it is not surprising that we observed higher onset RHivalues for ice nucleation in our experiments with H2SO4coatings.[16] Knopf and Koop [2006] studied Arizona Test Dust(ATD) particles with and without H2SO4coatings, but theydid not observe significant differences due to the coatings.A direct comparison with these previous results is difficult,however, since ATD is a complex mixture of minerals.Archuleta et al. [2005] studied metal oxides particles, withand without H2SO4coatings. At 213 K, they observed anincrease in RHiafter coating for 200 nm aluminum oxideand amorphous aluminum silicate particles, but a decreasein RHifor 200 nm iron oxide particles. In contrast, at 228 Kthe change in the RHidue to the coatings on 200 nmparticles was less than the uncertainty in the measurements.Differences between our measurements and the measure-ments by Archuleta et al. [2005] include the mineralsstudied and the particle size. Also, the coatings used byArchuleta et al. [2005] were between 2.9 and 7.1 layersthick, which are much smaller than our coating thicknesses.Finally Mo¨hler et al. [2008b] studied illite particles coatedwithsulfuricacid(H2SO4massfractionabout30%)at210K.They observed a significant reduction in the ice nucleationefficiency of the particles after coating. Specifically, anincrease in RHinecessary for ice nucleation of approxi-mately 35–40% was observed. Our results are similar tothese previous results.3.3. Onset Results for (NH4)2SO4Coatings[17] The onset results for (NH4)2SO4coated particles arealso shown in Figure 2. At 240 and 245 K, the onset RHivalues are significantly higher than the uncoated case.However, at 236 K, the coated particles are just as efficientat nucleating ice as the uncoated kaolinite particles. Clearly,the (NH4)2SO4coatings are influencing the ice nucleatingability of the kaolinite particles much differently than theH2SO4coatings. A possible explanation for this differencemay be related to the phase of the coatings. The H2SO4coatings will remain liquid during our experiments, whereasthe (NH4)2SO4coatings can undergo deliquescence andefflorescence. This can complicate the situation since icenucleation can occur directly on crystalline (NH4)2SO4[Abbatt et al., 2006; Shilling et al., 2006; Zuberi et al.,2002].[18] Figure 3 illustrates the possible phase behavior of an(NH4)2SO4coating during a typical freezing experiment.Our experiments first start at a very low RHi(<1%). Underthese conditions, the (NH4)2SO4coatings are expected to becrystalline, based on previously reported values for theefflorescence relative humidity of (NH4)2SO4droplets con-taining kaolinite cores [Pant et al., 2006]. The RHiis thenincreased and, above 100% RHi, ice can nucleate on thecrystalline (NH4)2SO4coating. In this case, ice nucleationoccurs by deposition freezing [Pruppacher and Klett,L02811 EASTWOOD ET AL.: FREEZING OF COATED KAOLINITE PARTICLES L028113of51997]. If ice nucleation does not occur, the solid (NH4)2SO4coating can take up water at the deliquescence relativehumidity. Above this relative humidity, the (NH4)2SO4coating is a liquid and ice nucleation can occur by immer-sion freezing [Pruppacher and Klett, 1997].[19] Based on the discussion above, our data suggest thatcrystalline (NH4)2SO4coatings are effective ice nuclei atapproximately 236 K. This is consistent with previouslaboratory data that show that ice nucleation can occur onsupermicron (NH4)2SO4particles at RHi-values less than110% and temperatures less than 225 K [Abbatt et al., 2006;Shilling et al., 2006]. This is illustrated in the auxiliarymaterial where we compare our (NH4)2SO4data with theseprevious measurements.[20] At 245 K, the onset results for (NH4)2SO4aresignificantly higher than the uncoated case. This suggeststhat aqueous (NH4)2SO4coatings also increase the RHivalues required for ice nucleation, although not as muchas aqueous H2SO4coatings. This may also be becausesulfate anions are adsorbed to the mineral surface whencoated with aqueous (NH4)2SO4. Martin et al. [2001]showed using spectroscopy that sulfate anions chemisorbedto a range of metal oxide surfaces even in ammoniumsulfate solutions. Therefore it seems likely that the sulfateanions are also interacting with the kaolinite surface in our(NH4)2SO4experiments.4. Conclusions and Atmospheric Implications[21] Our initial results support the idea that anthropogenicemissions of SO2and NH3may influence the ice nucleatingproperties of mineral dust particles by increasing the relativehumidity required for ice nucleation. This shift in icenucleation conditions may influence the frequency andproperties of ice clouds. Calculations using a cloud parcelmodel are needed to explore this possibility further. Onearea where these results may be especially important is theArctic region where a large fraction of the aerosol particles(including insoluble aerosols such as mineral dust) can becoated with acidic sulfate [Bigg, 1980].[22] We have reported initial results for coated anduncoated kaolinite particles. Additional experiments as afunction of particle size, surface area, and coating thicknessare also needed. Measurements of the fractions of ice-activemineral particles as a function of temperature and humiditywould also be beneficial.[23] Acknowledgments. We thank D. A. Knopf for many helpfuldiscussions. This research was supported by CFCAS and NSERC.ReferencesAbbatt, J. P. D., S. Benz, D. J. Cziczo, Z. Kanji, U. Lohmann, and O.Mohler (2006), Solid ammonium sulfate aerosols as ice nuclei: A path-way for cirrus cloud formation, Science, 313, 1770–1773.Archuleta, C. M., P. J. Demott, and S. M. Kreidenweis (2005), Ice nuclea-tion by surrogates for atmospheric mineral dust and mineral dust/sulfateparticles at cirrus temperatures, Atmos. Chem. Phys., 5, 2617–2634.Bigg, E. K. (1980), Comparison of aerosol at four baseline atmosphericmonitoring stations, J. Appl. Meteorol., 19, 521–533.Clegg, S. L., P. Brimblecombe, and A. S. Wexler (1998), Thermodynamicmodel of the system H+-NH4+-SO42C0-NO3C0-H2O at tropospheric tempera-tures, J. Phys. Chem. A, 102, 2137–2154.Cziczo, D. J., D. M. Murphy, P. K. Hudson, and D. S. Thomson (2004),Single particle measurements of the chemical composition of cirrus iceresidue during CRYSTAL-FACE, J. Geophys. Res., 109, D04201,doi:10.1029/2003JD004032.DeMott, P. J., D. J. Cziczo, A. J. Prenni, D. M. Murphy, S. M. Kreidenweis,D. S. Thomson, R. Borys, and D. C. Rogers (2003), Measurements of theconcentration and composition of nuclei for cirrus formation, Proc. Natl.Acad. Sci. U.S.A., 100, 14,655–14,660.Dymarska, M., B. J. Murray, L. M. Sun, M. L. Eastwood, D. A. Knopf, andA. K. Bertram (2006), Deposition ice nucleation on soot at temperaturesrelevant for the lower troposphere, J. Geophys. Res., 111,D04204,doi:10.1029/2005JD006627.Eastwood, M. L., S. Cremel, C. Gehrke, E. Girard, and A. K. Bertram(2008), Ice nucleation on mineral dust particles: Onset conditions,nucleation rates and contact angles, J. Geophys. Res., 113, D22203,doi:10.1029/2008JD010639.Ettner, M., S. K. Mitra, and S. Borrmann (2004), Heterogeneous freezing ofsingle sulfuric acid solution droplets: Laboratory experiments utilizing anacoustic levitator, Atmos. Chem. Phys., 4, 1925–1932.Glaccum, R. A., and J. M. Prospero (1980), Saharan aerosols over thetropical North Atlantic: Mineralogy, Mar. Geol., 37, 295–321.Hung, H. M., A. Malinowski, and S. T. Martin (2003), Kinetics of hetero-geneous ice nucleation on the surfaces of mineral dust cores inserted intoaqueous ammonium sulfate particles, J. Phys. Chem. A, 107, 1296–1306.Karltun, E. (1997), Modelling SO42C0surface complexation on variablecharge minerals: I. H+and SO42C0exchange under different solution con-ditions, Eur. J. Soil Sci., 48, 483–491.Knopf, D. A., and T. Koop (2006), Heterogeneous nucleation of ice onsurrogates of mineral dust, J. Geophys. Res., 111, D12201, doi:10.1029/2005JD006894.Martin, S. T., J. Schlenker, J. H. Chelf, and O. W. Duckworth (2001),Structure-activity relationships of mineral dusts as heterogeneous nucleifor ammonium sulfate crystallization from supersaturated aqueous solu-tions, Environ. Sci. Technol., 35, 1624–1629.Mo¨hler, O., S. Benz, H. Saathoff, M. Schnaiter, R. Wagner, J. Schneider,S. Walter, V. Ebert, and S. Wagner (2008a), The effect of organic coatingon the heterogeneous ice nucleation efficiency of mineral dust aerosols,Environ. Res. Lett., 3, 025007, doi:10.1088/1748-9326/3/2/025007.Mo¨hler, O., J. Schneider, S. Walter, A. J. Heymsfield, C. Schmitt, and Z. J.Ulanowski (2008b), How coating layers influence the deposition modeice nucleation on mineral particles, paper presented at 15th InternationalConference on Clouds and Precipitation, Int. Comm. on Clouds andPrecip., Cancun, Mexico. (Available at http://convention-center.net/iccp2008/abstracts/Program_on_line/Poster_11/Moehler_poster_extended.pdf)Pant, A., M. T. Parsons, and A. K. Bertram (2006), Crystallization ofaqueous ammonium sulfate particles internally mixed with soot and kao-linite: Crystallization relative humidities and nucleation rates, J. Phys.Chem. A, 110, 8701–8709.Parsons, M. T., J. Mak, S. R. Lipetz, and A. K. Bertram (2004), Deliques-cence of malonic, succinic, glutaric, and adipic acid particles, J. Geophys.Res., 109, D06212, doi:10.1029/2003JD004075.Peak, D., R. G. Ford, and D. L. Sparks (1999), An in situ ATR-FTIRinvestigation of sulfate bonding mechanisms on goethite, J. Colloid Inter-face Sci., 218, 289–299.Figure 3. Schematic of the possible phase behavior of an(NH4)2SO4coating during a typical freezing experiment.L02811 EASTWOOD ET AL.: FREEZING OF COATED KAOLINITE PARTICLES L028114of5Pruppacher, H. R., and J. D. Klett (1997), Microphysics of Clouds andPrecipitation, Kluwer Acad., Dordrecht, Netherlands.Salam, A., U. Lohmann, and G. Lesins (2007), Ice nucleation of ammoniagas exposed montmorillonite mineral dust particles, Atmos. Chem. Phys.,7, 3923–3931.Shilling, J. E., T. J. Fortin, and M. A. Tolbert (2006), Depositional icenucleation on crystalline organic and inorganic solids, J. Geophys.Res., 111, D12204, doi:10.1029/2005JD006664.Stumm, W. (1992), Chemistry of the Solid-Water Interface, Willey-Interscience,New York.Twohy, C. H., and M. R. Poellot (2005), Chemical characteristics of iceresidual nuclei in anvil cirrus clouds: Evidence for homogeneous andheterogeneous ice formation, Atmos. Chem. Phys., 5, 2289–2297.Usher, C. R., A. E. Michel, and V. H. Grassian (2003), Reactions onmineral dust, Chem. Rev., 103, 4883–4939.Zhang, G. Y., and D. Peak (2007), Studies of Cd (II)-sulfate interactions atthe goethite-water interface by ATR-FTIR spectroscopy, Geochim.Cosmochim. Acta, 71, 2158–2169.Zobrist, B., C. Marcolli, T. Peter, and T. Koop (2008), Heterogeneous icenucleation in aqueous solutions: The role of water activity, J. Phys.Chem. A, 112, 3965–3975.Zuberi, B., A. K. Bertram, C. A. Cassa, L. T. Molina, and M. J. Molina(2002), Heterogeneous nucleation of ice in (NH4)2SO4-H2Oparticleswith mineral dust immersions, Geophys. Res. Lett., 29(10), 1504,doi:10.1029/2001GL014289.C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0A. K. Bertram, S. Cremel, M. L. Eastwood, and M. Wheeler, Departmentof Chemistry, University of British Columbia, BC V6T 1Z1, Canada.(bertram@chem.ubc.ca)E. Girard, Department of Earth and Atmospheric Sciences, University ofQuebec at Montreal, Montreal, QC H3C 3P8, Canada.B. J. Murray, School of Chemistry, University of Leeds, Leeds LS2 9JT,UK.L02811 EASTWOOD ET AL.: FREEZING OF COATED KAOLINITE PARTICLES L028115of5

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