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Deposition ice nucleation on soot at temperatures relevant for the lower troposphere. Dymarska, Magdalena; Murray, Benjamin J.; Sun, Lumin; Eastwood, Michael L.; Knopf, Daniel A.; Bertram, Allan K. 2006

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Deposition ice nucleation on soot at temperatures relevant for thelower troposphereMagdalena Dymarska,1Benjamin J. Murray,1Limin Sun,1Michael L. Eastwood,1Daniel A. Knopf,1and Allan K. Bertram1Received 26 August 2005; revised 7 November 2005; accepted 1 December 2005; published 25 February 2006.[1] The ice nucleating efficiency of many important atmospheric particles remains poorlyunderstood. Here we investigate the ice nucleation properties of a range of soot typesincluding soot that has been treated with atmospherically relevant amounts of ozone. Wefocus on deposition nucleation below water saturation and at temperatures ranging from243 to 258 K. For our experimental conditions, ice nucleation never occurred attemperatures above 248 K and below water saturation. Below 248 K, ice occasionallyformed in our experiments with no indication of the formation of water droplets prior toice formation. However, even at these temperatures the relative humidity with respect toice (RHi) was close to water saturation when ice nucleation was observed, suggestingwater nucleation may have occurred first followed by ice nucleation during thecondensation process. We also performed a complimentary set of experiments where weheld soot particles at 248 K and RHi= 124 ± 4%, which is just below water saturation, fora period of 8 hours. From these measurements we calculated an upper limit of theheterogeneous ice nucleation rate coefficient of 0.1 cmC02sC01. If the number of sootparticles is 1.5 C2 105LC01in the atmosphere (which corresponds to urban-influenced ruralareas), then the number of ice particles produced below water saturation at theseconditions is at most 0.1 particles LC01on the basis of our upper limit. We conclude fromour studies that deposition nucleation of ice on most types of soot particles is notimportant in the Earth’s troposphere above 243 K and below water saturation.Citation: Dymarska, M., B. J. Murray, L. Sun, M. L. Eastwood, D. A. Knopf, and A. K. Bertram (2006), Deposition ice nucleation onsoot at temperatures relevant for the lower troposphere, J. Geophys. Res., 111, D04204, doi:10.1029/2005JD006627.1. Introduction[2] Clouds play an important role in climate by scatteringand absorbing radiation. Currently, clouds and their inter-action with aerosol particles provide some of the greatestuncertainties in predictions of climate change [Intergovern-mental Panel on Climate Change, 2001]. This is, in largepart, because the properties of clouds and their formationprocesses are poorly understood, particularly the propertiesand formation processes of mixed phase clouds and iceclouds [Penner et al., 2001].[3] Ice particles can form in the atmosphere when icehomogeneously nucleates in aqueous particles or ice het-erogeneously nucleates on solid particles. Heterogeneousfreezing can occur in four different modes: depositionnucleation, condensation freezing, immersion freezing,and contact freezing [Vali, 1985]. Deposition nucleationoccurs when vapor absorbs onto a solid surface and istransformed into ice. Condensation freezing refers to thesequence of events whereby cloud condensation initiatesfreezing of the condensate. Immersion freezing occurs whenice nucleates on a solid particle immersed in a liquiddroplet, and contact freezing occurs when a solid particlecollides with a liquid droplet, resulting in ice nucleation[Pruppacher and Klett, 1997; Vali, 1985]. In practice, thedistinction between condensation freezing and immersionfreezing is not always clear. In order to understand the rolethat mixed phase and ice clouds play in the atmosphere,these formation processes must be understood and quanti-fied. At temperatures between 273 and 238 K ice formationnecessarily occurs by heterogeneous freezing. This temper-ature range is more relevant for the lower troposphere. Aspointed out by Vali [1996] in a review on ice nucleation, theorigin of ice in lower-tropospheric clouds is not resolved,and it remains a question of great importance and in need ofnew efforts. This paper focuses on ice nucleation in thedeposition mode at temperatures relevant for the lowertroposphere.[4] Soot particles are ubiquitous in the Earth’s tropo-sphere, [Finlayson-Pitts and Pitts, 2000; Heintzenberg,1989; Seinfeld and Pandis, 1998] and ice core measure-ments suggest that their concentrations in the atmospherehave increased from preindustrialization to modern times[Lavanchy et al., 1999]. If soot particles are effective icenuclei (IN), they have the potential to significantly impactJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D04204, doi:10.1029/2005JD006627, 20061Department of Chemistry, University of British Columbia, Vancouver,British Columbia, Canada.Copyright 2006 by the American Geophysical Union.0148-0227/06/2005JD006627$09.00D04204 1of9the Earth’s climate indirectly by changing the properties andlifecycle of mixed phase and ice clouds on a global scale[DeMott, 2002; DeMott et al., 1997; Gierens, 2003; Jensenand Toon, 1997; Lohmann, 2002; Lohmann and Feichter,2005]. In the lower troposphere, an increase in soot particlesmay lead to more frequent glaciation of supercooledclouds and increase the amount of precipitation via theice phase. Further, this may reduce the cloud cover in thelower troposphere and result in increased absorption ofsolar radiation [Lohmann, 2002; Lohmann and Feichter,2005]. However, this aerosol indirect effect on climate isuncertain, in part, because the conditions at which icenucleates on soot particles in the atmosphere are notknown.[5] At present there have only been a limited number oflaboratory studies on the ice nucleating ability of sootparticles at temperatures above 238 K [DeMott, 1990; Diehland Mitra, 1998; Gorbunov et al., 2001]. Types of sootparticles investigated in the previous studies include par-ticles produced from the combustion of acetylene [DeMott,1990], kerosene [Diehl and Mitra,1998],benzeneandtoluene [Gorbunov et al., 2001], as well as soot producedby thermal decomposition of benzene [Gorbunov et al.,2001]. The previous measurements suggest that soot par-ticles are potentially important ice nuclei in the atmosphere.However, more work is still needed to understand icenucleation on soot over this temperature range (above238 K). For example, the ice nucleating ability as a functionof relative humidity below water saturation needs to beinvestigated at these temperatures, since the previous stud-ies mainly focused on ice nucleation at or slightly aboveliquid water saturation.[6] In the following we investigate ice nucleation onsoot particles focusing on deposition nucleation belowwater saturation and at temperatures ranging from 243 Kto 258 K. Experiments were done as a function of bothtemperature and relative humidity. For these studies weused several different types of soot particles and carbonblack particles with a range of physical and chemicalproperties. We also carried out experiments to determineif the oxidization of soot particles increases their abilityto act as ice nuclei. It has previously been speculated thatoxidation of soot by ozone in the atmosphere willincrease the ice nucleation ability of soot particles[Gorbunov et al., 2001]. As a test of this hypothesis,we exposed Lamp Black 101, a commercial carbon black,to ozone for extended periods of time, and then tested itsice nucleation ability in the deposition mode. Finally, forcomparison purposes, we investigated the ice nucleatingability of kaolinite using similar experimental parametersas the experiments involving soot. Kaolinite is believed tobe a significant component of mineral dust in the atmo-sphere [Glaccum and Prospero, 1980; Pye, 1987].2. Methods[7] The different soots and carbon blacks used in thesestudies are listed in Table 1. Three different samples of n-hexane soot were provided by Dwight M. Smith, Universityof Denver. The first sample was produced by burning n-hexane under ambient conditions in an open vessel, result-ing in a diffusion flame. The second and third samples weregenerated using an apparatus designed for producing pre-mixed flames with variable oxidant to fuel ratios. It has beenshown that there is a linear relationship between the state ofsoot surface oxidation and the air to fuel ratio [Chughtai etal., 2002]. The International Steering Committee for BlackCarbon Reference Materials has recommended using n-hexane soot as a model for soot in the atmosphere becausea large amount of soot characteristics and reactivity dataalready exists in the scientific literature on this type of sootand because of the option to vary the n-hexane sootproperties by varying the combustion conditions (http://www.du.edu/C24dwismith/bcsteer.html). Properties of n-hex-ane soot have been documented by Smith and coworkers[Akhter et al., 1985; Chughtai et al., 2002]. Lamp Black101, Degussa FW2 (which is a channel type black), andPrintex 40 (which is a furnace type black) are commercialcarbon blacks. Degussa FW2 is posttreated with NO2andhas been used in the past in laboratory heterogeneouschemistry studies [see for example Choi and Leu, 1998;Disselkamp et al., 2000; Tabor et al., 1994]. Lamp Black101 is essentially nonvolatile at 1223 K and has been usedin the past for ice nucleation studies [DeMott et al., 1999].Neither Lamp Black 101 nor Printex 40 are posttreated.Relevant properties of these soot particles and carbon blacksare summarized in Table 1. The kaolinite particles used inour experiments were purchased from Fluka Chemika(purum; natural grade).[8] The apparatus used in these studies consisted of anoptical microscope coupled to a flow cell in which therelative humidity could be accurately controlled. The flowcell, which is shown in Figure 1, was similar to the one usedpreviously to measure deliquescence and crystallization ofsupermicron organic and mixed organic-inorganic particles[Pant et al., 2004; Parsons et al., 2004a, 2004b]. In thecurrent experiments soot or kaolinite particles were depos-ited on the bottom surface of the flow cell; the relativehumidity with respect to ice (RHi) inside the cell wasincreased, and the conditions under which water dropletsor ice crystals formed were determined with a reflected lightmicroscope (Zeiss Axiotech 100) equipped with a 10xTable 1. Physical Characteristics of the Soot Types Investigated in This StudyType of Soot Volatiles,a% BET-Surface Area,bm2gC01C% H% O% N%N-hexane soot: diffusion flame ND 89 ± 2 87 to 95 1.6 to 1.2 11 to 6 NDN-hexane soot: air/fuel = 0.53 ND 100 ± 2 ND ND ND NDN-hexane soot: air/fuel = 2.4 ND 156 ± 11 ND ND ND NDLamp Black 101 1 20 98.5 0.4 0.4 0.1Degussa FW2 (Channel Black) 17 460 88 1.1 9.9 0.7Printex 40 (Furnace Black) 0.9 90 ND ND ND NDaVolatiles were determined by heating a sample in a muffle furnace for 7 min at 950C176C.bBET (Brunauer, Emmett, and Teller) surface area was calculated from the N2absorption isotherms recorded at 77 K.D04204 DYMARSKA ET AL.: ICE NUCLEATION ON SOOT2of9D04204objective lens. The RHiover the particles was controlled bycontinuously flowing a mixture of dry and humidified Hethrough the flow cell.[9] The bottom surface of the flow cell, which supportedthe particles, consisted of a glass cover slide treated withdichlorodimethylsilane (DCMS) to make a hydrophobiclayer, which reduced the probability of ice nucleationdirectly on the surface. Prior to the treatment with DCMSthe glass slide was thoroughly cleaned in a pirahna solution(3:1 mixture by volume of sulfuric acid and hydrogenperoxide), rinsed in high-purity water (distilled water furtherpurified with a Millipore system) and methanol (HPLCgrade), and any remaining contaminant particles removedwith a dry ice cleaning system (Sno Gun-IIk, Va-TranSystems). The treatment with DCMS involved placing theslides in an airtight chamber together with 2–3 droplets ofDCMS solution (Fluka, 5% DCMS in heptane). The slideswere not in direct contact with the droplets, rather theDCMS coated the glass slides via vapor deposition.[10] All samples were prepared and the flow cell con-structed within a filtered air lamina flow hood. This greatlyreduced the possibility of contamination of the samples byambient atmospheric and laboratory particles. Soot or kao-linite particles were deposited on a hydrophobic glass slide(the bottom surface of the flow cell) using the followingtechnique. The dry soot or kaolinite particulates were placedin a glass vessel immersed in an ultrasonic bath. A flow ofN2(99.999%) was passed through the glass vessel, andvibrations from the ultrasonic bath caused the dry particlesto be suspended in the flow of N2. This flow was thendirected at the hydrophobic glass slide, and the soot orkaolinite particles were deposited on the slide by impaction.Soot agglomerates or kaolinite particles deposited on thesubstrate were always less than 40 mm in diameter. Theoptical resolution limit of our microscope was C241 mm. Atypical sample held between 200 to 800 individual particles,a majority of which were between 1 and 20 mm in diameter.Shown in Figure 2 is an image of a typical soot samplerecorded prior to a deposition freezing experiment.[11] The flow cell was located on a cooling stage. Thetemperature of the cooling stage and hence the flow cell wasregulated with a refrigerating circulator (Thermo NeslabRTE-740). An insulating spacer, made from polychlorotri-fluoroethylene (PCTFE), was placed between the hydro-phobic glass slide and the flow cell body. This ensured thatthe coldest portion of the flow cell was the glass substrate(by C2410 K), thus preventing unwanted ice nucleation inother parts of the cell. All seals within the cell were madewith Viton O-rings.[12] A flow of humidified gas was introduced to one sideof the cell and exited on the other where its frost point wasmeasured with a frost point hygrometer (General Eastern).From the frost point measurements, the water vapor pres-sure (PH2O) was calculated using the parameterization ofMarti and Mauersberger [1993] which is in excellentagreement with the more rigorous equations from Murphyand Koop [2005] over our frost point range. A flow ofhumidified gas was generated by passing a flow of He(99.999%) over a reservoir of ultrapure water (distilledwater was further purified using a millipore system). Thedesired PH2Owas adjusted by altering the temperature of thewater reservoir and diluting the humidified flow with asecond flow of dry He. A continuous and constant flow ofbetween 1900 to 2100 cm3minC01(at 273.15 K and 1 atm)was maintained throughout the course of the experiment.The He gas used in these experiments was first passedthrough a trap containing molecular sieves (1/1600pellets,Type T4A) at 77 K and then through a 0.02 mm filter(Anodisc 25).[13] A Pt-100 resistance temperature detector (RTD) fromOmega was embedded within the aluminum base to mea-sure the temperature of the bottom surface of the cell. TheRTD was calibrated against the dew point or ice frost pointwithin the cell, as done previously [Middlebrook et al.,1993; Parsons et al., 2004b]. To do this water droplets orice crystals (depending on the temperature) were allowed tonucleate and grow on the bottom substrate, by slowlydecreasing the temperature of the cell. Then the celltemperature was slowly (rate = 0.1 K minC01) ramped upuntil the water droplets or ice crystals were observed toshrink (depending on whether water or ice particles werepresent). Next the cell temperature was slowly decreasedagain until the water droplets/ice crystals started to grow.From these observations we determined the temperature atwhich water droplets or ice crystals were in equilibriumFigure 1. Flow cell and location of microscope objective(Al = aluminum and PCTFE = polychlorotrifluoroethylene).Figure 2. Image of a typical soot sample recorded prior toa deposition freezing experiment.D04204 DYMARSKA ET AL.: ICE NUCLEATION ON SOOT3of9D04204with the gas-phase water vapor. At this point the tempera-ture of the bottom surface of the cell was equal to the dewpoint or ice frost point of the vapor (again depending onwhether water or ice particles were present), which wasdetermined independently from the hygrometer measure-ments. (The hygrometer measurements gave the ice frostpoint, but this could be converted into a dew point using thesaturation vapor pressure of water from Koop et al. [2000]and the saturation vapor pressure of ice from Marti andMauersberger [1993].) The size of the water droplets or icecrystals was accurately determined from the digitallyrecorded images. By ramping the temperature of the cellup and down, the offset for the RTD was accuratelydetermined for each experiment.[14] The RHiwithin the cell was calculated with thefollowing equation:RHi¼PH2OPiceTcellðÞð1Þwhere Pice(Tcell) is the saturation vapor pressure of ice at thetemperature of the bottom surface of the flow cell. Pice(Tcell)was calculated using the parameterization of Marti andMauersberger [1993], and PH2Owas calculated as discussedabove.[15] In most nucleation experiments, the RHiwas rampedfrom below 100% to water saturation by decreasing thetemperature of the cell at 0.1 K minC01, and maintaining aconstant PH2Oinside the cell. Typical experimental RHitrajectories are illustrated in Figure 3 for four differentinitial temperatures of 258 K, 253 K, 248 K, and 243 K.For the remainder of the document these experiments willbe referred to as RHiramp experiments. Images of the sootor clay particles were recorded digitally every 10 s orC240.017 K, while simultaneously PH2Oand the cell temper-ature were recorded. From the images we determined theRHiat which water droplets or ice particles first formed inour experiments (i.e., the onset of water or ice nucleation).[16] As mentioned above we also examined the icenucleating properties of Lamp Black 101 after controlledexposes to O3. Lamp Black 101 particles were deposited ona hydrophobic glass cover slide and exposed to O3within aflow tube in which [O3] was measured with a downstreamchemical ionization mass spectrometer (CIMS). The flowtube and CIMS instrument have been described by Knopf etal. [2005]. In these experiments, the pressure of N2in theflow tube was 2–4 Torr and O3was generated by photolysisof O2at 254 nm. The following ozone exposures (PO3t)were used: 1.6 C2 10C03, 7.6 C2 10C03, 13.0 C2 10C03, 25.1 C210C03, 35.6 C2 10C03, 94.8 C2 10C03atm s. This is equivalent toexposing the soot to 80 ppb of O3at atmospheric pressurefor 0.2, 1.1, 1.9, 3.6, 5.1, and 13.7 days respectively. An O3concentration of 80 ppb at atmospheric pressure corre-sponds to relatively polluted conditions [Finlayson-Pittsand Pitts, 2000].[17] We also carried out nucleation experiments with longobservation times and at constant RHiin order to constrain,as much as possible, the heterogeneous nucleation ratecoefficient of ice on soot in the deposition mode (see belowfor a further discussion). For the remainder of the documentwe will refer to these experiments as constant RHiexperi-ments. In these experiments we employed n-hexane soot(air/fuel ratio = 2.4). As mentioned above, the InternationalSteering Committee for Black Carbon Reference Materialshas recommended using n-hexane in atmospheric studies. Inthe constant RHiexperiments, the temperature of theparticles was held at C24248 K, while the relative humiditywas held at 124 ± 4% RHi, which is just below watersaturation. The particles were held at these conditions for anextended period of time (approximately 8 hours) and weremonitored to determine if ice nucleated during this longobservation time.3. Results and Discussion3.1. RHiRamp Experiments[18] As mentioned above, PH2Owas held constant whilethe temperature of the cell was reduced in order to increasethe RHiwithin the flow cell. The temperature was decreaseduntil either water droplets or ice particles were observed.The RHiat which we observed either water droplets or iceparticles are illustrated in Figure 4 for the blank hydropho-bic glass slide as well as the different soot samples. The datapoints correspond to when we first observed either water orice (in other words either the onset of water or ice forma-tion). The error bars associated with the data points werecalculated from the uncertainties of PH2Oand Pice(Tcell),which resulted from the uncertainty in the dew pointmeasurements and the calibration of temperature of thebottom surface of the flow cell. The dashed lines inFigure 4 represent water saturation (i.e., relative humiditywith respect to water is 100%). The open symbols indicatethat water droplets were first observed, and the solidsymbols indicate that only ice particles were observed withno indication of the formation of water droplets prior to iceformation. From this information we make conclusions onFigure 3. Typical experimental trajectories of RHi, wheretemperature was reduced at a rate of 0.1 K minC01, while thewater partial pressure was constant. The trajectories werecalculated using the saturation vapor pressure of water fromKoop et al. [2000] and the saturation vapor pressure of icefrom Marti and Mauersberger [1993].D04204 DYMARSKA ET AL.: ICE NUCLEATION ON SOOT4of9D04204the ice nucleating ability of soot in the deposition modebelow water saturation only (see below). In the experimentswhere water droplets were first observed (open symbols),ice nucleation would occasionally occur at a later time. Thisoccurred with both the blank as well as with soot particles.However, we cannot determine from our results if theformation of ice after the formation of liquid droplets wasdue to the soot particles or the substrate. Hence we do notdraw conclusions on the ice nucleation ability of soot in thecondensation or immersion mode from these experiments.Also note that we do not draw conclusions on the CCNability of soot from these experiments.[19] The results in Figure 4 show that at 248 K and above,water droplets, rather than ice, always appeared first in ourexperiments. This occurred at water saturation, as expected.From this we can conclude that ice nucleation neveroccurred at temperatures above 248 K and below watersaturation for our experimental conditions (observation timeand soot particle concentrations). If ice nucleation did occur,ice particles would rapidly grow and prevent the formationof water droplets at water saturation by depleting the watervapor. As shown in Table 1, we investigated soots with arange of volatilities and carbon-to-oxygen ratios. Since weobserved the same results for all the different soots inves-tigated, below water saturation and above 248 K, our resultssuggest that the volatility and carbon-to-oxygen ratio doesnot significantly influence ice formation in this regime.[20] At around 243 K, ice particles occasionally formedin our experiments with no indication of the formation ofwater droplets prior to ice formation (a total of three times).However, in the few experiments where ice did form theRHiwas close to water saturation when ice nucleation wasobserved, suggesting water nucleation may have occurredfirst followed by ice nucleation during the condensationprocess. In other words, for the few experiments where icedid form we cannot rule out condensation freezing. In fact,at between 243 and 258 K all the results (including when icenucleated first) clustered around water saturation, suggest-ing water saturation is a prerequisite for both water and icenucleation.[21] For the experiments where water droplets firstformed, we estimated an upper limit to the depositionnucleation rate coefficient of ice on soot particles, belowwater saturation. This rate coefficient provides a quantita-tive measure of the ice nucleating ability, which may beused in modeling studies of ice formation in the atmosphere.On the basis of Poisson statistics, if ice nucleation did notoccur during the course of an experiment, an upper limit tothe heterogeneous nucleation rate coefficient, Jhetup, can becalculated with the following equation [Biermann et al.,1996; Koop et al., 1995, 1997]:Juphet¼1tAsln11C0 xC20C21ð2Þwhere t is the observation time, Asis the total surface areaavailable for heterogeneous nucleation, and x is theconfidence level (95% was used). In our RHirampexperiments t was approximately 60 s and Asranged from1 C2 105to 4 C2 105mm2. To calculate Aswe assumed aFigure 4. RHiat which liquid water droplets or ice particles were first observed as the RHiinside thecell was slowly increased. The open symbols indicate that water droplets were first observed, and thesolid symbols indicate that only ice particles were observed with no indication of the formation of waterdroplets prior to ice formation. (a) Results from a control experiment, where no particles were depositedon the hydrophobic substrate. (b–f) Results from the six soot types listed in Table 1. The error barsassociated with the data points were calculated from the uncertainties of PH2Oand Pice(Tcell).D04204 DYMARSKA ET AL.: ICE NUCLEATION ON SOOT5of9D04204geometric surface area (= 4pr2, where r is the radius of theparticles) for the soot particles. This is a conservativeestimate as the surface area exposed to the gas phase is inmost cases larger than the geometric surface area. On thebasis of a surface area of 1 C2105mm2, Jhetupwas calculated tobe 50 cmC02sC01.3.2. Ice Nucleation of Soot Particles Oxidized by O3[22] Lamp Black 101 was exposed to various amounts ofozone and then RHiramp experiments (constant ramp rateof 0.1 K minC01) were performed to test the ice nucleationproperties of these soots. The results are shown in Figure 5.The open symbols indicate that water droplets were ob-served first in all experiments. Also, the results for LampBlack 101 exposed to ozone are the same as the results fromunexposed Lamp Black 101. In all cases water dropletswere first observed indicating that ice did not nucleatebelow water saturation. Even after an O3exposure of9.5 C2 10C02atm s, which is equivalent to an exposure of80 ppb at atmospheric pressure (polluted conditions) for13.7 days, the results were not significantly different fromresults of unexposed Lamp Black 101. Either O3did notoxidize Lamp Black 101 significantly or the oxidationprocess did not change the IN ability significantly. Furtherresearch is needed to determine the extent of oxidation ofLamp Black 101 by O3. Also more research is needed todetermine if exposure to atmospherically relevant concen-trations of ozone, as well as other atmospheric oxidantssuch as OH and NO3radicals, can modify the IN propertiesof other types of soot in the deposition mode as well asother modes of ice nucleation. These initial experimentsshow that exposure to atmospherically relevant concentra-tions of ozone did not modify the IN ability of Lamp Black101 in the deposition mode below water saturation.3.3. Comparison With Other Results[23] As mentioned earlier there have been a few measure-ments of the ice nucleation ability of soot at temperaturesabove 238 K. In addition there have been a few studies atlower temperatures. DeMott et al. [1999] have investigatedice nucleation on Lamp Black 101 at temperatures rangingfrom 233 K to 213 K using a continuous flow diffusionchamber. In Figure 6, we compare these results with ourdata. At approximately 230 K, DeMott et al. observed icenucleation only at water saturation. Hence, at warmertemperatures it is highly unlikely that they would observeice nucleation below water saturation, which is consistentwith our findings.[24] Mo¨hler et al. [2005a] investigated ice nucleation onspark generated soot at temperatures less than 240 K using alow-temperature aerosol and cloud chamber (AIDA). Attemperatures between 235 K and 240 K, ice nucleation onlyoccurred on uncoated soot particles after approaching watersaturation. The authors commented that ice seems to formimmediately in this temperature range after liquid activationof the soot particles either by condensation freezing orhomogeneous freezing of the growing liquid water layer.This finding is consistent with our studies. More recently,Mo¨hler et al. [2005b] used the AIDA chamber to investigateice nucleation at low temperatures on soot produced fromcombustion of propane with various elemental carbon toFigure 5. RHiat which water droplets were observed forLamp Black 101 soot samples treated with a range of O3exposures. In all experiments, water droplets were observedfirst (i.e., ice particles did not form unless water dropletsfirst condensed). The exposure times indicated in the plotare equivalent atmospheric O3exposures at an atmosphericO3mixing ratio of 80 ppb (see text for details). The errorbars associated with the data points were calculated fromthe uncertainties of PH2Oand Pice(Tcell).Figure 6. A comparison of our results for Lamp Black 101(open squares) with those of DeMott et al. [1999] (solidtriangles). Our data points correspond to the conditions atwhich water droplets were observed using soot particlesranging in size from 1 to 40 mm in diameter. In theseexperiments, water droplets were always observed first. Ifice did form it was only after the appearance of waterdroplets. The results from DeMott et al. correspond to theonset for which 1% of Lamp Black soot particles (a numbermean diameter of 240 nm) nucleated ice.D04204 DYMARSKA ET AL.: ICE NUCLEATION ON SOOT6of9D04204organic carbon ratios. If these results are extrapolated towarmer temperatures they are also consistent with ourfindings.[25] DeMott [1990] investigated ice nucleation on sootproduced by combustion of acetylene at temperatures be-tween 253 K and 233 K using an expansion cloud chamber.Because of the experimental design and experimental con-ditions, mainly condensation and immersion freezing wereinvestigated. DeMott commented that there was someevidence of ice formation by deposition, but ice certainlydid not precede cloud droplet formation by much. One ofthe conclusions from this study was that immersion freezingis an efficient ice nucleation process after water has con-densed on soot particles.[26] Diehl and Mitra [1998] investigated ice nucleationon soot produced by the combustion of kerosene. Combineddeposition and condensation freezing (deposition/condensa-tion freezing) were studied in a single experiment using asoap film method. Ice nucleation occurred in these experi-ments at temperatures as high as 253 K. In these experi-ments, the relative humidity was not measured, so a directcomparison with our results is difficult. All the deposition/condensation experiments may have been carried outslightly above water saturation and condensation freezingmay have dominated. The authors also studied immersionfreezing and contact freezing, and they found that kerosene-burner exhaust particles are effective ice nuclei in thesefreezing modes.[27] Finally, Gorbunov et al. [2001] investigated sootproduced by the combustion of benzene and toluene, aswell as soot produced by thermal decomposition of benzeneusing a cloud chamber at temperatures ranging from 253 to268 K. All experiments were carried out close to liquidwater saturation: saturation with respect to liquid water wasequal to 1.02 ± 0.02. It was found that the fraction ofaerosol particles forming ice crystals was influenced by theconcentration of surface chemical groups that can formhydrogen bonds with water molecules. A large differencein the ice-forming activity (3 orders of magnitude in thefraction of soot particles forming ice crystals) was observedfor soot aerosols obtained with different generators. Sootparticles produced by combustion of benzene and toluenewere very potent ice nuclei, whereas soot produced bythermal decomposition of benzene were poor ice nuclei.They concluded that highly oxidized soot particles areextremely efficient ice nuclei. The difference between ourresults and the results from Gorbunov et al. may be due to adifference in experimental conditions: our research focuseson ice nucleation below water saturation and the workpresented by Gorbunov et al. was carried out at or slightlyabove water saturation. Alternatively, the soot particlesstudied by Gorbunov et al. were more effective IN thanthe soot studied in our experiments.[28] Combining all the previous results and those fromour studies, it appears at temperatures above 243 K andbelow water saturation, ice nucleation on many types ofsoot particles are not efficient [DeMott, 1990; DeMott et al.,1999; Mo¨hler et al., 2005a, 2005b]. In contrast, once theRHiis above liquid water saturation, water can condense onsoot particles, and then most types of soot may be importantice nuclei in the condensation or immersion mode [DeMott,1990; DeMott et al., 1999; Diehl and Mitra, 1998;Gorbunov et al., 2001; Mo¨hler et al., 2005a, 2005b].3.4. Ice Nucleation on Kaolinite[29] As mentioned in the introduction, for comparisonpurposes we also investigated the ice nucleating ability ofkaolinite particles. Kaolinite particles are believed to be asignificant component of dust particles in the atmosphere[Glaccum and Prospero, 1980; Pye, 1987]. The results fromthis study are shown in Figure 7, along with all the resultsfrom soot experiments conducted in this study. In thekaolinite experiments we used a similar density of particlesand particle sizes compared to our soot experiments, al-though the surface area available for nucleation is likelysignificantly higher in the soot experiments because of thelarge specific surface area of soot. Nevertheless, the RHivalues required for ice nucleation on kaolinite were signif-icantly less than the RHivalues required for ice nucleationon soot particles. The results for kaolinite are generallyconsistent with previous measurements [Bailey and Hallett,2002; Roberts and Hallett, 1968]. In a future publication wewill focus on the IN ability of mineral dust particlesincluding kaolinite.3.5. Constant RHiExperiments[30] In the constant RHiexperiments, n-hexane sootparticulates (air/fuel = 2.4) were held at 248 K and within5% RHiof water saturation (124 ± 4% RHi) for approxi-mately 8 hours. In this experiment the surface area of sootexposed to the vapor was 1.1 C2 105mm2. Even during thislong observation time at humidities close to water satura-tion, no ice was observed. From this we calculated an upperlimit to the heterogeneous nucleation rate coefficient usingFigure 7. RHiat which water droplets or ice particles wereobserved for soot and kaolinite experiments. The opensymbols indicate that water droplets were first observed,and the solid symbols indicate that only ice particles wereobserved with no indication of the formation of waterdroplets prior ice formation. The error bars associated withthe data points were calculated from the uncertainties ofPH2Oand Pice(Tcell).D04204 DYMARSKA ET AL.: ICE NUCLEATION ON SOOT7of9D04204equation (2). In this case the upper limit to Jhetupwascalculated to be 0.1 cmC02sC01. The upper limit is muchsmaller than the upper limit calculated from the RHirampexperiments, of 50 cmC02sC01, since the observation timewas much longer (8 hours compared with 1 min). Thisallows us to provide a better constraint on the rate coeffi-cient of ice nucleation in the deposition mode.[31] From Jhetupcalculated above we estimated the maxi-mum number of ice particles that can be produced in theatmosphere at 248 K and at RHi= 124%. The purpose is toplace the magnitude of Jhetupdetermined experimentally intocontext. The following equation can be used to estimate themaximum number of ice particles that can be producedduring a specified time period [Pruppacher and Klett,1997],nice¼ nsoot1 C0exp C0JuphetAptC0C1C2C3ð3Þwhere niceis the number density of ice particles produced(particles LC01), nsootis the number density of soot (LC01), Apis the surface area of a single soot particle (cm2), and t isthe total time (seconds). For ice nucleation in the atmo-sphere we assumed t was approximately 60 min, nsootwas1.5 C2105LC01, and Apwas 1.3 C210C09cm2. Avalue of 1.5 C2105LC01for nsootwas calculated by assuming a soot radius of0.1 mm, a soot density of 2 g cmC03, a geometric surface areafor soot, and an elemental carbon mass concentrations in theatmosphere of approximately 1.3 C2 10C06gmC03(whichcorresponds to urban-influenced rural areas [Seinfeld andPandis, 1998; Shah et al., 1986]). Avalue of 1.3C210C09cm2for Apwas calculated on the basis of a geometric surface areaand a soot radius of 0.1 mm. With these assumptions, amaximum number density of ice of 0.07 LC01was obtainedfrom equation (3). On the basis of field observations, Meyerset al. [1992] have developed empirical relationships betweenthe number concentration of ice nuclei and ice super-saturation. This parameterization, which accounts for thecombined effects of deposition nucleation and condensationfreezing,predictsat124%RHi,thenumberoficenucleiintheatmosphere is approximately 12 LC01. Ice nucleation on sootparticlesbelowwatersaturation(withpropertiessimilartothesootstudiedinourexperimentsandnsoot=1.5C2105LC01)canonly account for at most 0.6% of the ice nuclei densitypredicted by the Meyers et al. parameterization at 124% RHiand 248 K.4. Summary and Conclusions[32] The ice nucleating properties of soot particles havebeen investigated in the deposition mode below watersaturation. At 248 K and above, water droplets alwaysnucleated first. From this we conclude that at these temper-atures ice nucleation does not occur below water saturationunder our experimental conditions. In the experimentswhere water droplets were first observed ice nucleationwould occasionally occur at a later time. This occurred withboth the blank as well as with soot particles. Hence wecannot determine from our results if the formation of iceafter the formation of liquid droplets was due to the sootparticles or the substrate. Below 248 K, ice occasionallyformed in our experiments with no indication of theformation of water droplets prior to ice formation. However,even at these temperatures the RHiwas close to watersaturation when ice nucleation was observed.[33] The results of ice nucleation on Lamp Black 101exposed to ozone were similar to the results from unexposedLamp Black 101. Even after an O3exposure of 9.5 C2 10C02atm s, which is equivalent to an exposure of 80 ppb atatmospheric pressure for 13.7 days, the results were notsignificantly different from results of unexposed LampBlack 101.[34] Combining all the previous results and our studies, itappears that below water saturation at temperatures above243 K, ice nucleation on many types of soot particles is notefficient [DeMott, 1990; DeMott et al., 1999; Mo¨hler et al.,2005a, 2005b]. In contrast, once the RHiis above liquidwater saturation, water can condense on soot particles, andthen most types of soot may be important ice nuclei in thecondensation or immersion mode [DeMott, 1990; DeMott etal., 1999; Diehl and Mitra, 1998; Gorbunov et al., 2001;Mo¨hler et al., 2005a, 2005b].[35] For comparison purposes we also investigated the INability of kaolinite particles. The results show that the icenucleating ability of kaolinite in the deposition mode wassuperior to soot particles.[36] We also carried out experiments with a constant RHiand long observation times (8 hours) on n-hexane soot (air/fuel = 2.4). Even after a long observation time at close towater saturation, no ice was observed. From these measure-ments we calculated an upper limit to the heterogeneousnucleation rate coefficient of 0.1 cmC02sC01for a temperatureof 248K and RHi= 124 ± 4%. On the basis of this upperlimit, if the number of soot particles in the atmosphere is1.5 C2 105LC01, then the number of ice particles producedbelow water saturation (and at these conditions) is at most0.07 particles LC01. In contrast, the parameterization byMeyersetal.[1992],whichaccountsforthecombinedeffectsof deposition nucleation and condensation freezing, predictsthe number of ice nuclei in the atmosphere is approximately12 LC01at 124% RHi. Hence we conclude that depositionnucleation of ice on soot particles cannot account for asignificant proportion of the ice nuclei found in the Earth’stroposphere above 243 K and below water saturation.[37] Acknowledgments. The authors thank Dan Cziczo, PaulDeMott, and Boris Gorbonov for several helpful discussions regardingice nucleation on soot. The authors also thank Dwight Smith for helpfuldiscussions on the properties of soot as well as for providing us samples ofn-hexane soot. This work was funded by the Canadian Foundation forClimate and Atmospheric Sciences (CFCAS), the Natural Science andEngineering Research Council of Canada (NSERC), and the CanadaFoundation for Innovation (CFI).ReferencesAkhter, M. S., A. R. Chughtai, and D. M. Smith (1985), The structure ofhexane soot-I - spectroscopic studies, Appl. Spectrosc., 39(1), 143–153.Bailey, M., and J. Hallett (2002), Nucleation effects on the habit of vapourgrown ice crystals from C018 to C042C176C, Q. J. R. Meteorol. Soc., 128,1461–1483.Biermann, U. M., T. Presper, T. Koop, J. Mo¨ssinger, P. J. Crutzen, andT. Peter (1996), The unsuitability of meteoritic and other nuclei forpolar stratospheric cloud freezing, Geophys. Res. Lett., 23(13), 1693–1696.Choi, W., and M. T. Leu (1998), Nitric acid uptake and decomposition onblack carbon (soot) surfaces: Its implications for the upper troposphereand lower stratosphere, J. Phys. Chem. A, 102(39), 7618–7630.Chughtai, A. R., J. M. Kim, and D. M. 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