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Effects of sulfate coatings on the ice nucleation properties of a biological ice nucleus and several.. Bertram, Allan K.; Chernoff, Donna I. 2010-10-31

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Effects of sulfate coatings on the ice nucleation properties of a biological ice nucleus and several types of minerals Donna I. Chernoff1 and Allan K. Bertram1 Received 24 March 2010; revised 17 June 2010; accepted 28 June 2010; published 21 October 2010. [1] An optical microscope coupled to a flow cell was used to study the ice nucleation properties of uncoated and coated mineral dust and SNOMAX (a proxy for biological ice nucleators made from cells of Pseudomonas syringae) at temperatures ranging from 234 to 247 K. We define the onset conditions as the relative humidity (RH) and temperature at which the first ice nucleation event was observed. The results show that H2SO4 coatings modified the ice nucleation properties of all the minerals studied. For kaolinite and illite, the acid coatings increased the RH over ice (RHi) required for ice nucleation by ∼30% RHi; for montmorillonite and quartz, the acid coatings increased the RHi by ∼20% RHi. NH4HSO4 coatings also influenced the ice nucleation properties of kaolinite particles. In addition, our results indicate that SNOMAX is a reasonably good ice nucleus, having onset values between 110 to 120% RHi. In contrast to the mineral studies, sulfuric acid coatings did not hinder the ice nucleating ability of SNOMAX particles. Combined, our results support the idea that anthropogenic emissions of SO2 and NH3 may influence the icenucleating properties of mineral dust particles. From our laboratory data, we also determined contact angles () between the heterogeneous nuclei and ice embryos according to classical nucleation theory to parameterize the laboratory data for inclusion in atmospheric models. The data show that for uncoated ice nuclei the contact angles are small (below ∼20°), but for mineral particles coated with sulfuric acid, the contact angles are larger (above ∼60°). Citation: Chernoff, D. I., and A. K. Bertram (2010), Effects of sulfate coatings on the ice nucleation properties of a biological ice nucleus and several types of minerals, J. Geophys. Res., 115, D20205, doi:10.1029/2010JD014254. 1. Introduction [2] Aerosol particles are abundant in the atmosphere, and these aerosol particles can indirectly influence climate by modifying the formation conditions and properties of ice clouds and mixedphase clouds. To better understand this important topic, an improved understanding of the ice nucleation properties of atmospheric aerosols is required, and these properties need to be parameterized and incorporated into climate models [Baker and Peter, 2008; Cantrell and Heymsfield, 2005; Hegg and Baker, 2009; Houghton, 2001]. [3] Ice nucleation may occur in the atmosphere either homogeneously or heterogeneously. Homogeneous nucle- ation involves the spontaneous freezing of liquid droplets. In heterogeneous nucleation, ice forms on an insoluble or par- tially soluble aerosol particle. [4] Mineral dust particles are abundant in the atmosphere, and both laboratory and field studies have shown that min- eral dust particles are effective heterogeneous ice nuclei (IN). Laboratory studies indicate that mineral dust particles can lower the supersaturation required for ice formation com- pared to homogeneous nucleation. Field measurements have shown that these particles can have a significant effect on cloud formation and cloud properties [DeMott et al., 2003; Sassen, 2002; Sassen et al., 2003]. Measurements have also shown that the cores of ice crystals often contain mineral dust inclusions, indicating that these particles play an important role in atmospheric ice formation [Cziczo et al., 2004; Heintzenberg et al., 1996; Twohy and Poellot, 2005]. [5] While in the atmosphere, mineral dust particles can be coated with organic and inorganic material [Hinz et al., 2005; McNaughton et al., 2009; Sullivan et al., 2007; Usher et al., 2003; Wiacek and Peter, 2009]. These coatings may affect the ice nucleation properties of mineral dust [Gallavardin et al., 2008; Phillips et al., 2008]. Nevertheless, there have only been a few studies that have directly compared the ice nucleating ability of uncoated and coated mineral dust par- ticles at atmospherically relevant conditions [Archuleta et al., 2005; Cziczo et al., 2009; Eastwood et al., 2009; Gallavardin et al., 2008;Kanji et al., 2008;Knopf andKoop, 2006;Möhler et al., 2008a; 2008c; Salam et al., 2007]. A few other studies have measured the freezing properties of aqueous inorganic or organic solution droplets containing mineral dust particles [Ettner et al., 2004; Hung et al., 2003; Koop and Zobrist, 2009; Zobrist et al., 2008; Zuberi et al., 2002]. [6] Recently, we showed that sulfuric acid coatings (ammoniumtosulfate ratio (ASR) = 0) can have a significant 1Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada. Copyright 2010 by the American Geophysical Union. 01480227/10/2010JD014254 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D20205, doi:10.1029/2010JD014254, 2010 D20205 1 of 12effect on the ice nucleation properties of kaolinite particles [Eastwood et al., 2009]. Kaolinite particles can make up ∼5– 10% of mineral dust mass [Glaccum and Prospero, 1980]. In this previous study, we showed that a sulfuric acid coating on kaolinite particles increased the relative humidity over ice (RHi) required for ice nucleation compared to uncoated par- ticles by ∼30%, consistent with recent results on Arizona test dust (ATD) and illite [Möhler et al., 2008c].We also looked at the effect of ammonium sulfate coatings (ASR = 2) on kao- linite. These coatings had a different effect from sulfuric acid coatings. At the coldest temperature studied (236 K), the ammonium sulfatecoated particles were as efficient as the uncoated kaolinite particles, whereas at the warmer tempera- tures (240 and 245 K) the onset RHi values were significantly higher than the uncoated case. These results suggest that the ASR of the coating is important, at least for certain sizes. [7] In addition to mineral dust, biological particles may play an important role in the formation of ice clouds in the atmosphere [Ariya et al., 2009; Christner et al., 2008a; Christner et al., 2008b; Möhler et al., 2007; Phillips et al., 2009; Szyrmer and Zawadzki, 1997]. In particular, the bac- teria Pseudomonas syringae has been identified as an extremely efficient IN, demonstrating icenucleating activity at temperatures as warm as –2°C in purified samples [Möhler et al., 2007]. However, the abundance of biological particles in the atmosphere and their activity under atmospheric con- ditions remain poorly understood. The potential role of biological particles in atmospheric ice formation has recently been emphasized in field studies done in Wyoming and the Amazon basin [Pratt et al., 2009; Prenni et al., 2009]. In both studies, the ice nuclei composition was dominated by min- eral dust and biological particles. [8] Similar to mineral dust, biological particles can also be coated with inorganic and organic material in the atmosphere [DeMott et al., 2003; Lammel et al., 2005]. We are not aware of any studies in which the ice nucleation properties of uncoated and coated biological particles were directly com- pared. Studies have investigated the freezing of dilute aqueous solutions [Chen et al., 2002; Kawahara et al., 1996; Kawahara et al., 1995; Obata et al., 1993; Pouleur et al., 1992; Yin et al., 2005] and concentrated aqueous solutions [Koop and Zobrist, 2009] containing biological particles. [9] In the following sections, we expand on our previous work [Eastwood et al., 2009] and consider other mineral dusts which have been shown to be abundant in the atmo- sphere [Chester et al., 1972; Glaccum and Prospero, 1980; Usher et al., 2003], as well as a more complete range of sulfate coatings. The ice nucleation properties of SNOMAX, a proxy for biological ice nucleators, are also investigated. SNOMAX is produced from cells of P. syringae that have been grown under optimal conditions to maximize ice nucleation. We interpret our results using classical nucleation theory. The results from this study should prove useful for understanding the effects of anthropogenic emissions of SO2 and NH3 on climate by influencing the icenucleating properties of mineral dust and biological particles. 2. Experimental 2.1. Ice Nucleation Measurements [10] The apparatus used in these studies has been described in detail previously [Dymarska et al., 2006; Parsons et al., 2004]. The apparatus consisted of an optical microscope coupled to a flow cell in which the relative humidity could be accurately controlled. In a typical experiment, mineral dust or SNOMAX particles (coated or uncoated) were deposited on the bottom surface of the flow cell, the RHi inside the cell was increased, and the conditions under which ice first formed were determined with a reflected light microscope. The bottom surface of the flow cell, which supported the particles, consisted of a glass coverslide treated with dichlorodimethylsilane to make a hydrophobic layer, which reduced the probability of ice nucleation directly on the surface. Typical experimental RHi trajectories used in our experiments are illustrated in Figure 1. The three trajectories correspond to ice frost points of 237, 242, and 247 K, where the ice frost point is defined as the temperature at which the RHi = 100%. Included in this figure is the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]. At the beginning of the experiments, the particles were exposed to a flow of dry He gas (RHi <1%) at room temperature. Next, the temperature of the cell was rapidly lowered and the RHi was set to ∼80%. The nucleation experiments were then conducted by steadily decreasing the temperature and increasing the RHi. The RHi ramp rate was ∼1% min−1. In a previous study in our group, experiments were carried out using a ramp rate of ∼0.5% min−1. No difference in results was obtained, suggesting the aqueous coatings were in equilibrium with the water vapor [Eastwood et al., 2009]. Growth rate calculations of aqueous solution droplets have also been done in our group using the equations presented by Figure 1. Typical experimental trajectories. The tempera- ture was reduced at a rate of 0.1 K min−1 while the water vapor partial pressure was held constant. The trajectories correspond to ice frost points of 237, 242, and 247 K, where the ice frost point is defined as the temperature at which the relative humidity over ice (RHi) = 100%. Trajectories were calculated using the saturation vapor pressures of water and ice from the parameterization of Murphy and Koop [2005]. The dashed line represents the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]. CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 2 of 12Pruppacher [1997] to further confirm that the sulfuric coatings were in equilibrium with the gasphase water vapor in our experiments. The uncertainty in our measurements due to nonequilibrium conditions is at most 3% RHi [Eastwood et al., 2009]. 2.2. Sample Preparation and Thickness of the Coatings [11] The mineral dusts used in this study are listed in Table 1 along with their chemical formulae. Kaolinite and montmorillonite were purchased from Fluka, illite was provided by the Clay Minerals Society, and quartz was obtained from U.S. Silica. SNOMAX was purchased from York Snow, Inc. As mentioned, SNOMAX is produced from cells of P. syringae that have been grown under optimal conditions to maximize ice nucleation frequency. The cells are then concentrated using ultrafiltration and frozen into pellets. Next the pellets are freezedried and exposed to beta irradiation to make a sterile product [Lee et al., 1995]. Note that although SNOMAX is made from cells of P. syringae, it does not display the same freezing spectrum (fraction frozen vs. temperature) as some naturally occurring strains of P. syringae. Also note that the freezing spectrum of naturally occurring P. syringae can vary signif- icantly from strain to strain [Gross et al., 1983; Hirano et al., 1985; Möhler et al., 2008b; Ward and DeMott, 1989]. [12] Coated particles were prepared by mixing the mi- nerals or SNOMAX with the coating material in highpurity water to create a suspension. The composition of the sus- pension was 1 wt% mineral or SNOMAX and 0.2 wt% coating material. This suspension was placed into an ultra- sonic bath for 10 min and then stirred for approximately 2 to 4 days to ensure that the coating material had adequate time to interact with the heterogeneous IN. To deposit the particles on the glass slide, the suspension was passed through a nebulizer using highpurity nitrogen (N2) as a carrier gas. The flow from the nebulizer was directed at a hydrophobic glass slide, and droplets containing the particles were deposited on the surface of the slide upon impaction. Water then evaporated, leaving behind the coated particles. Coated particles pro- duced by this method had an average weight fraction of H2SO4 or NH4HSO4 of 0.167 under dry conditions. [13] Uncoated illite, quartz, and SNOMAX particles were prepared using a procedure similar to that described above. First, these minerals or SNOMAX particles were mixed in highpurity water (composition was 1 wt% mineral or SNOMAX) to create a suspension. The suspension was then placed in an ultrasonic bath for 10 min and then stirred for 2 to 4 days to be consistent with the experimental procedure for the coating experiments. These suspensions were then nebulized, creating droplets on the hydrophobic slides. Water then evaporated, leaving behind the uncoated particles. [14] For uncoated kaolinite andmontmorillonite, we use the results previously published by our group [Eastwood et al., 2008; Eastwood et al., 2009]. In these previous experi- ments, uncoated kaolinite was suspended in water and then nebulized as described above. For montmorillonite, the par- ticles were produced by dry dispersion. This involved placing the dry particles in a glass vessel immersed in an ultrasonic bath and a flow of ultrahighpurityN2was passed through the glass vessel to entrain the particles. The flowwas then directed at the hydrophobic glass slide, and the particles were depos- ited on the slide by impaction. [15] The thickness of the coatings in our experiments was estimated on the basis of the compositions of the starting suspensions and assuming a spherical core shell model (e.g., a kaolinite core surrounded by a uniformH2SO4 or NH4HSO4 coating). According to our calculations, under dry conditions (<1% RHi), a kaolinite core with a diameter of 15 mm will have a 0.7mmthick coating, and a 5 mm core will have a coating of 0.2 mm. A coating of 0.2 mm represents at least a few hundred sulfate layers covering the surface of the particle. [16] Previously in our group, the thickness of the coatings on kaolinite particles (using the same technique for particle pro- duction as discussed above) was further characterized by monitoring the change in particle size as the relative humidity with respect to water (RHw) was increased from <1% to 95% [Eastwood et al., 2009]. From the change in size, we estimated the total amount of water adsorbedwhen cycling between <1% and 95% RHw using the thermodynamic model of Clegg et al. [1998]. From this, we estimated the amount of H2SO4 on each particle and, in turn, the thickness of the H2SO4 coating under dry conditions. Measurements made for 15 individual particles yielded an average weight fraction for the coating of 0.12 ± 0.07 under dry conditions. The uncertainty in this value derives from the uncertainty in the relative humidity measurements. [17] Two different types of nebulizers were used in our studies. For the minerals we used an inhouse design. This resulted in average particle sizes ranging from 6–10 mm. For the SNOMAX particles, we also used this inhouse design, average sizes ranging from approximately 16 to 23 mm. To generate smaller SNOMAX particles we used a commercially available design (Meinhard Glass Products, Model Number TR30A1). The commercial nebulizer generated SNOMAX particles with average sizes of ∼6–7 mm. Note that the SNOMAX particles considered in our studies are most likely agglomerates of several SNOMAX cells, which are typically 1–2 mm, and/or cell fragments. See the next section for more details on particle size distributions used in the experiments. 2.3. Particle Number, Particle Sizes, and Total Surface Area [18] In typical freezing experiments, a sample held between 100 and 1000 individual particles. Hence, our results corre- spond to when 0.1 to 1% of the particles nucleated ice. The total surface area of mineral dust or SNOMAX deposited in any particular experiment ranged from 2 × 10−5 to 3 × 10−3 cm2. The mean diameters and standard deviations for all particle types considered in this study are presented in Table 2. 3. Results and Discussion 3.1. Effect of Sulfuric Acid Coatings on the Ice Nucleation Properties of Different Minerals [19] Shown in Figure 2 are the results for uncoated and sulfuric acid coated kaolinite, illite, montmorillonite, and Table 1. Chemical Formulae of the Four Minerals Studied Mineral Formula Kaolinite Al4Si4O10(OH)8 Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,H2O] Montmorillonite (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·nH2O Quartz SiO2 CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 3 of 12quartz particles. Each data point corresponds to one freezing event, and the error bars represent the uncertainty in measuring the frost point and temperature at the onset. All experiments were carried out with an ice frost point of 237 K. Included is the RHi at which liquid water saturation occurs for this experimental trajectory. The RHi necessary for water saturation was determined by calculating the RHi at which the experimental trajectory crosses the water saturation line in Figure 1. Also included in Figure 2 is the threshold for homogeneous freezing of sulfuric acid droplets. This threshold was determined by calculating the RHi at which the experimental trajectory crosses the homogeneous freezing line for sulfuric acid in Figure 1. As mentioned above, the data for kaolinite (uncoated and coated) and montmorillonite (uncoated) are taken from Eastwood et al. [2009] and Eastwood et al. [2008], respectively, and are plotted here again for comparison purposes. All other data were obtained during this study. The total surface area in any particular experiment ranged from approximately 2 × 10−5 to 3 × 10−3 cm2. Over this relatively narrow range, the onset results did not depend strongly on the surface area, so we combined the data and compared the averages and the 95% confidence intervals for the coated and uncoated cases in Figure 3. The data show that, for kaolinite and illite, the coatings have a major impact on the RHi required for ice nucleation; the coating increased the onset RHi value by ∼30%. The effect for montmorillonite and quartz is smaller; in this case, the increase in onset RHi is ∼20%, but there is still a statistically significant effect. It is interesting to note that for kaolinite, montmorillonite, and quartz the average onset values fall below water saturation and the conditions Table 2. Particle Sizes and Standard Deviations for the Mineral Dust and SNOMAX Particles Studied Sample Coating Size (mm) Standard Deviation Kaolinite None 7.7 5.3 Kaolinite H2SO4 7.8 5.7 Kaolinite NH4HSO4 10.3 7.2 Kaolinite (NH4)2SO4 6.9 5.0 Illite None 5.9 3.8 Illite H2SO4 7.2 5.2 Montmorillonite None 8.1 – Montmorillonite H2SO4 7.7 3.8 Quartz None 8.2 4.8 Quartz H2SO4 10.1 6.5 SNOMAXa None 5.8 3.7 SNOMAXa H2SO4 6.6 5.1 SNOMAX None 15.9 8.9 SNOMAX H2SO4 22.8 15.6 aThese particles were deposited using a commercially available nebulizer; all other particles were deposited using an inhouse made nebulizer. Figure 2. Ice nucleation measurements on uncoated (closed symbols) and H2SO4 coated (open symbols) kaolinite, illite, montmorillonite, and quartz particles. Data are plotted as onset RHi against surface area (cm2). All experiments were done at an ice frost point of 237 K. Kaolinite results are taken from Eastwood et al. [2009]; uncoated montmorillonite results are taken from Eastwood et al. [2008]. The dashed line represents the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]. CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 4 of 12necessary for homogeneous freezing of the H2SO4 coating. As a result, heterogeneous freezing on these mineral cores is most likely still the dominant mechanism for nucleation in our experiments. For the case of illite, the average onset RHi overlaps with the conditions necessary for homogeneous nucleation of the H2SO4 coating if one considers the con- fidence intervals. This suggests that the coatings on illite may be “shutting off” heterogeneous freezing and favoring homogeneous nucleation of the aqueous coating. [20] Recently, there have been several studies of the effect of sulfuric acid coatings on the ice nucleation properties of mineral dust particles. In a majority of the studies, a sig- nificant reduction in the ice nucleation efficiency after sul- furic acid coating was observed [Archuleta et al., 2005; Cziczo et al., 2009; Gallavardin et al., 2008; Möhler et al., 2008c]. Specifically, the coatings led to an increase in the RHi required for ice nucleation [Archuleta et al., 2005; Cziczo et al., 2009; Gallavardin et al., 2008; Möhler et al., 2008c], and the coated particles often required saturations approaching those for homogeneous freezing of aqueous solutions, as observed here [Cziczo et al., 2009; Möhler et al., 2008c]. However, in some cases, the impact of the coating was much less significant [Archuleta et al., 2005; Cziczo et al., 2009] or was not significant at all [Knopf and Koop, 2006], and for some minerals a decrease in RHi necessary for ice nucleation was observed [Archuleta et al., 2005]. Differences in particle size, particle type, coating thickness, and temperature range studied may account for the variation between these results. [21] Field measurements also support the hypothesis that atmospheric processing of mineral dust leads to a reduced ice nucleation ability. Phillips et al. [2008] compared field and laboratory data and concluded that atmospheric pro- cessing leads to a reduced ice nucleation efficiency. Prenni et al. [2009] noted the near absence of ice nuclei composed of mixed dust and sulfate, suggesting that coatings may affect the ability of these particles to act as ice nuclei. DeMott et al. [2003] noted that mineral dust particles that acted as good ice nuclei were relatively pure in form. [22] The mechanism responsible for the deactivation of the ice nucleation ability of mineral dusts may be related to the mineral surface and how this surface interacts with the sulfuric acid coatings. Kaolinite, illite, and montmorillonite are clay minerals composed of layers of aluminosilicate sheets. The structure of quartz consists of siliconoxygen tetrahedra linked by shared oxygen atoms [Gualtieri, 2000; Viani et al., 2002]. [23] The composition of the sulfuric acid coating at the beginning of each experiment was very acidic; the starting pH of the coating was below zero. The point of zero charge (PZC) is defined as the pH at which the net surface charge is zero for a particular material [Stumm, 1992], and there is a PZC associated with the surfaces of each mineral dust studied. Kaolinite surfaces have PZCs at pH 6 or above, Figure 3. Average onset values for uncoated and sulfuric acid coated kaolinite, illite, montmorillonite and quartz particles studied using a frost point of 237 K. Error bars represent the 95% confidence inter- vals. The dashed line represents the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]. CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 5 of 12depending on the crystalline face [Stumm, 1992]; the sur- faces of illite and montmorillonite have PZCs at 2.2 or higher [Kriaa et al., 2009; Parks, 1965; Rozalen et al., 2009]; and quartz has a PZC of 2.2 [Parks, 1965]. Accordingly, the exposed areas of the mineral particles would be protonated under the acidic conditions in our experiments. The posi- tively charged, protonated environment should facilitate strong adsorption of sulfate anions to the mineral surface, changing the chemical and physical properties of the surfaces [Eastwood et al., 2009]. It is therefore hypothesized that the ice nucleation properties of the mineral dusts would change when coated with sulfuric acid. Molecular simulations sim- ilar to recent simulations on uncoated mineral surfaces would be useful to better understand these processes [Croteau et al., 2008; Hu and Michaelides, 2007; 2008]. 3.2. Effect of AmmoniumtoSulfate Ratio on the Ice Nucleation Properties of Kaolinite [24] Shown in Figure 4 are the results for kaolinite coated with ammonium bisulfate. Also included for comparison are the results for uncoated, sulfuric acid coated, and ammo- nium sulfate coated kaolinite particles. Each data point in this figure is the average from at least six separate mea- surements done for each particle type and temperature. The error bars represent 95% confidence intervals of the onset RHi values. [25] From Figure 4 one can conclude that kaolinite par- ticles coated with ammonium bisulfate are less efficient ice nuclei than uncoated kaolinite particles; the coating increased the onset RHi by approximately 18 to 26% com- pared to uncoated kaolinite particles. Also, in general, it appears that sulfuric acid coatings (ASR = 0) have the largest effect on the nucleation properties of kaolinite. This is most clear at the lowest temperatures studied. Ammonium bisulfate coatings (ASR = 1) appear to be intermediate between sulfuric acid and ammonium sulfate. Again, these differences are clearest at the lowest temperatures. [26] As suggested above, the sulfuric acid coating may hinder ice nucleation by protonation of the kaolinite surface and by adsorption of sulfate anions to the protonated sur- face. The pH of the ammonium bisulfate coating at the beginning of each experiment was 0.87. Hence, a similar effect can occur for ammonium bisulfate, which would explain why ammonium bisulfate also changes significantly the ice nucleation properties of kaolinite particles. It is also possible that the acid solutions irreversibly react with the mineral surfaces. For example, recently it was shown that when mineral surfaces are exposed to pH values <1.0, an increase in dissolution of aluminosilicates with decreasing pH was observed as well as precipitation of an amorphous silica phase [Shaw et al., 2009]. These processes could be occurring in our experiments (and in the atmosphere) and potentially could explain the difference between the sulfuric acid and ammonium bisulfate coatings, since the sulfuric acid solutions have lower pH values and lead to increased dissolution rates. [27] Figure 4 also shows that ammonium sulfate coatings (ASR = 2) are most sensitive to temperature. This is most likely related to the phase of the coatings. H2SO4 does not undergo deliquescence and effluorescence and NH4HSO4 deliquescences at <70% RHi over the temperature range studied here. As a result, H2SO4 and NH4HSO4 coatings remained liquid during our experiments. In contrast, (NH4)2SO4 coatings can remain solid for some of the con- ditions used in our experiments. The deliquescence relative humidity for (NH4)2SO4 is shown in Figure 4 as a dotted line. If the (NH4)2SO4 coatings are crystalline, this solid can also act as a heterogeneous ice nuclei (at least for bigger particles) [Abbatt et al., 2006; Baustian et al., 2010; Shilling et al., 2006; Zuberi et al., 2002]. For a further discussion of the (NH4)2SO4 results as well as an explanation of the temperature trend observed for (NH4)2SO4 coatings, see Eastwood et al. [2009]. 3.3. Ice Nucleation on Uncoated SNOMAX [28] The onset results for uncoated and sulfuric acid coated SNOMAX particles are presented as a function of surface area in Figure 5. Each data point represents one freezing event, and the error bars represent the uncertainty in RHi. The onset data for SNOMAX have been summarized as a function of temperature in Figure 6. [29] The results for sulfuric acid coated SNOMAX are discussed in this next section. The results for the uncoated case, shown in Figures 5 and 6, show that ice nucleation occurs at ∼110–120% RHi, independent of temperature. This indicates that SNOMAX is a reasonably good ice nucleus at atmospherically relevant conditions. [30] The ice nucleation properties of SNOMAX have been investigated in other studies [Möhler et al., 2008b; Ward and DeMott, 1989; Wood et al., 2002], but these measurements were done at 258 K and above. To our knowledge, our studies are the first to look at freezing of these particles at lower temperatures. There have also been numerous studies Figure 4. Onset results for NH4HSO4 coated kaolinite par- ticles (open triangles). Included for comparison are previous results for uncoated kaolinite, H2SO4 coated and (NH4)2SO4 coated kaolinite particles (filled symbols) [Eastwood et al., 2009]. Error bars are given as 95% confidence intervals. Results shown are an average of at least six separate mea- surements. The dotted line represents the deliquescence relative humidity (DRH) for ammonium sulfate; the DRH line for NH4HSO4 lies below 70% RHi over the temperature range shown. The dashed line represents the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]. CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 6 of 12on the ice nucleation properties of unaltered P. syringae. These studies have also focused on warmer temperatures than employed in the current study, so the results are not directly comparable [Lindow et al., 1978; 1989; Möhler et al., 2008b; Vali, 1971; Wolber et al., 1986]. 3.4. Effect of Sulfuric Acid Coatings on the Ice Nucleation Properties of SNOMAX [31] The ice nucleation results for uncoated and sulfuric acid coated SNOMAX particles are compared in Figures 5 and 6. Unlike the mineral dust results, the sulfuric acid coat- ings did not hinder the icenucleating ability of SNOMAX particles. [32] The fact that the uncoated and coated results were similar was somewhat surprising in light of the mineral dust results presented earlier in this paper. We offer here a few different explanations. First, it is possible that a few SNOMAX particles were not completely covered with the acid solution, providing sites for ice nucleation. This did not occur in the mineral dust studies, which had identical experimental con- ditions. Nevertheless, it may have occurred in the SNOMAX studies. Unfortunately, it is not possible to verify that every particle is completely covered in our experiments. Even if some SNOMAX particles were not covered completely, these particles were still exposed to a dilute acid solution (2 × 10−2 M H2SO4, pH 1.6) for 2–4 days prior to nebuli- zation. At a minimum, our results show that long exposure to dilute acid solutions does not modify the ice nucleation properties of SNOMAX particles at the temperatures and relative humidities studied. Figure 5. Ice nucleation measurements on uncoated (closed symbols) and H2SO4 coated (open symbols) SNOMAX particles. Data are plotted as onset RHi against surface area (cm2). Squares and circles repre- sent particles made using a commercially available and inhouse built nebulizer, respectively. The dashed line in the left image represents the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]; this line lies above water saturation for the middle and right images. Figure 6. Onset results for uncoated (filled symbols) and H2SO4 coated (open symbols) SNOMAX. Open stars repre- sent freezing of aqueous acid solution drops containing SNOMAX inclusions from Koop and Zobrist [2009]. Error bars represent the 95% confidence intervals. Results shown are an average of at least three separate measure- ments. Squares and circles represent particles made using a commercially available and inhouse built nebulizer, respectively. The dashed line represents the threshold for homogeneous freezing of sulfuric acid droplets 8 mm in diameter at a freezing rate of 10 s−1 [Koop et al., 2000]. CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 7 of 12[33] Another possible explanation for the coated SNOMAX results is that the acid solution does not significantly modify active ice nucleation sites present on SNOMAX. It is thought that the reason SNOMAX and unaltered P. syringae are good IN is due to a certain protein located in the outer cell mem- brane. Experimental evidence has shown that the protein forms aggregates on the outer membrane in such amanner that the hydrophilic repetitive region of the protein provides a lattice match for the hydrogen bonding requirements of ice [Green andWarren, 1985;GurianSherman andLindow, 1993; Lee et al., 1995; Morris et al., 2004; Szyrmer and Zawadzki, 1997]. Our current results may suggest that sulfuric acid solutions do not modify this environment significantly enough to influence ice nucleation for the temperatures and RHi values explored. [34] Recently, the freezing properties of dilute and con- centrated acid solution droplets containing SNOMAX were studied using differential scanning calorimetry [Koop and Zobrist, 2009]. Koop and Zobrist reported results in terms of freezing temperatures and water activities. Shown in Figure 6 are the freezing conditions predicted by the Koop and Zobrist [2009] data for the temperatures used in our experiments. These predictions are in good agreement with the results we obtained for coated particles. This agreement provides some support for the finding that the particles in our experiments were completely coated and also that coatings by acids have relatively little effect on the freezing conditions, at least for the temperature range we studied. [35] Several studies also investigated the freezing properties of dilute acid solutions containing other species of bacteria, many in the Pseudomonas genera, known to be effective IN. These studies were done at warm temperatures and as a result are not directly comparable to our studies. Nevertheless, it is interesting to note that in the experiments where an acid effect was observed, the freezing temperature, even in the acid solutions, was still above −11 °C; which is above the temperature range employed in our studies [Chen et al., 2002; Kawahara et al., 1996; Obata et al., 1993; Pouleur et al., 1992; Yin et al., 2005]. Also, one study noted that the freezing temperature of one species of Pseudomonas was not sensitive to the pH range studied (3.5 to 5.0) [Kawahara et al., 1995]. 3.5. Nucleation rate, Jhet [36] The heterogeneous nucleation rate, Jhet, is defined as the number of nucleation events per unit surface area of solid material per unit time. Note that Jhet is referred to as both a rate [Pruppacher and Klett, 1997; Hung et al., 2003; Archuleta et al., 2005] and a rate coefficient [Dymarska et al., 2006; Marcolli et al., 2007] in the literature. The hetero- geneous nucleation rate is related to the onset data through equation (1): Jhet ¼ ! Ast ; ð1Þ where w is the number of ice crystals nucleated, As is the total mineral dust/SNOMAX surface area available for heterogeneous nucleation, and t is the observation time. At the onset of ice nucleation, w was equal to 1. [37] Table 3 lists the nucleation rates determined in our experiments. The uncertainty in Jhet was determined by considering the uncertainties in As and t. We used 10 s for the observation time with an upper limit of 20 s (the time between image captures) and a lower limit of 1 s. Note, however, that nucleation may have happened at a shorter time than 1 s. If this is the case, the calculated nucleation rates are lower limits to the true nucleation rates. For the surface area, we used the geometric surface area of the particles determined directly from the optical microscope. For an upper limit to the surface area, we multiplied the geometric surface area of the particles by a factor of 50 on the basis of scanning electron microscope measurements of kaolinite particles [Eastwood et al., 2008]. The data shown in Table 3 suggest that our experiments are typically sen- sitive to values of Jhet ranging from 2 to 13,000 cm−2 s−1. 3.6. Classical Nucleation Theory Parameters From Jhet [38] The applicability of standard classical nucleation theory to heterogeneous nucleation on minerals and bio- logical particles remains to be determined. In fact, some measurements show that, for precise predictions, active site theory is required [Archuleta et al., 2005; Hung et al., 2003; Marcolli et al., 2007]. Nevertheless, classical nucleation theory is a relatively convenient and simple way to param- eterize laboratory data. Hence, classical nucleation theory is a reasonable starting point for analyzing our experimental data. Below in this section, we analyze the nucleation rates using classical nucleation theory. From this analysis, we determined the contact angle between an ice embryo and the mineral surface. [39] In this analysis, we focus on the results for uncoated minerals, uncoated SNOMAX, and sulfuric acid coated minerals studied at an ice frost point of 237 K. We did not Table 3. Jhet Values and Contact Angles for Uncoated and Sulfuric Acid Coated Mineral Dusts and Uncoated SNOMAX Mineral Type Onset Temperature (K) RHi (%) Jhet (cm−2 s−1) Jhet, upper (cm−2 s−1) Jhet, lower (cm−2 s−1) lower  upper Kaolinite Pure 236 104 ± 2 281 2814 3 3 9 14 Kaolinite Coated 234 134 ± 5 224 2237 2 58 72 100 Illite Pure 237 112 ± 5 1292 12921 13 11 15 18 Illite Coated 235 142 ± 6 1000 10000 10 64 79 109 Montmorillonite Pure 236 108 ± 4 931 9307 9 8 12 16 Montmorillonite Coated 235 125 ± 5 1281 12808 13 48 60 80 Quartz Pure 236 120 ± 5 606 6057 6 17 20 23 Quartz Coated 235 140 ± 4 805 8046 8 63 78 108 SNOMAX Pure 236 116 ± 6 127 1271 1 15 18 21 SNOMAX Pure 242 112 ± 2 94 936 1 13 16 19 SNOMAX Pure 246 116 ± 1 137 1370 1 16 19 22 CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 8 of 12do a similar analysis for the ammonium bisulfate coated studies because the theromodynamic parameters of the ammonium bisulfate solution needed for the calculations are not readily available. Also, we excluded the sulfuric acid coated SNOMAX results because the coated results were not statistically different from the uncoated results. [40] All the uncoated results that we focused on for the classical nucleation theory analysis corresponds to deposi- tion freezing. To convert the nucleation rates for these deposition freezing results to contact angles, we used the procedure outlined in our previous paper [Eastwood et al., 2008]. For details on the calculations, please see this pre- vious publication. [41] All the sulfuric acid coated results that were analyzed using classical nucleation theory correspond to immersion freezing. According to standard classical nucleation theory, the rate of heterogeneous nucleation (Jhet) by immersion freezing is defined as follows [Archuleta et al., 2005; Pruppacher, 1997; Tabazadeh et al., 1997; 2000]: Jhet;imm ¼ A exp DFg;het Dg kT   ; ð2Þ where A is the preexponential factor in units of cm−2 s−1, DFg,het is the free energy of formation of the critical embryo on the surface in units of J, k is the Boltzmann constant in J K−1, T is the onset temperature in K, and Dg is the activation energy for the diffusion of a water molecule across the icewater interface in units of J. [42] The critical embryo is approximated as a spherical cap on a curved surface. The free energy of formation of the critical embryo for immersion freezing can be described by the following equation [Pruppacher, 1997]: DFg;het ¼ 4 3 r2g i=s f m; xð Þ; ð3Þ where rg is the radius of the ice embryo; si/s is the surface tension at the icesulfuric acid solution interface, f (m,x) is the geometric factor, m is the compatibility parameter for ice on a solid substrate, and x is the ratio of the radius of the substrate to the radius of spherical ice germ. Assuming the radius of the substrate to be much larger than the radius of the ice germ (a good approximation under our experimental conditions), f (m,x) is defined as follows: f m; xð Þ ¼ m 3  3m þ 2 4 : ð4Þ The compatibility parameter, m, is equal to cos, where  is the contact angle between an ice nucleus and the solid surface. [43] Also, the radius of the ice embryo is given (in cm) as follows [Archuleta et al., 2005; DeMott and Lynch, 2002; Khvorostyanov and Sassen, 1998]: rg ¼ 2i=s iLef ln T0T   aGw   ; ð5Þ where ri (g cm−3) is the temperaturedependent density of ice, si/s (erg cm−2) is as defined above, Lef (cal g−1) is the effective latent heat of formation, T0 is the triple point of water, aw is the water activity, and G is a dimensionless parameter equal to RT/LefMw [Khvorostyanov and Sassen, 1998], where Mw is the molecular weight of water. Figure 7. Average contact angles for uncoated and sulfuric acid coated mineral dust particles and uncoated SNOMAX particles studied at a frost point of 237 K. Error bars are given as 95% confidence intervals. The sulfuric acid coated SNOMAX results were excluded because the coated results were not statistically different from the uncoated results. CHERNOFF AND BERTRAM.: EFFECTS OF COATINGS ON ICE NUCLEATION D20205D20205 9 of 12[44] We calculated  according to the method used by Archuleta et al., [2005]. First, equation (2) was used to obtain DFg,het using the temperaturedependent expression for Dg (ergs × 1013) from Tabazadeh et al., [1997], the experimentally determined Jhet,imm values from equation (1), and a preexponential factor, A, equal to 1020 cm−2 s−1 [Fletcher, 1969; Hung et al., 2003]. Second, equation (5) was used to find rg. In this calculation, we used ri from Pruppacher, [1997], si/s from Tabazadeh et al., [2000], and Lef from Khvorostyanov and Sassen, [1998]. Third, equation (3) was used to find f(m,x). Fourth, equation (4) can be evaluated for m, and finally, the compatibility parameter can be used to find the contact angle, . [45] In Table 3, the nucleation rates and contact angles calculated using the procedures discussed above are listed for uncoated and sulfuric acid coated mineral dust particles, as well as the uncoated SNOMAX results. The upper and lower limits for Jhet (and hence ) were determined using the upper and lower limits to the observation times in our experiments and the upper limits to the surface area esti- mated from scanning electron microscopy (see section 3.5 for more details). The contact angle values for measure- ments done at an ice frost point of 237 K are also illustrated in Figure 7. The data show that for uncoated ice nuclei, the contact angles are small (below ∼20°). For mineral dust particles coated with sulfuric acid, the contact angles are larger (above ∼60°). These values may be useful for future modeling studies of ice nucleation in the atmosphere and for comparing results between different laboratories. However, keep in mind that our calculations assume one contact angle for a given sample type. In reality, particles within a given sample type may have a range of ice nucleation efficiencies and hence a range of contact angles [Archuleta et al., 2005; Hung et al., 2003; Marcolli et al., 2007]. If this is the case, contact angles determined from onset conditions (as done in the current study) may over- estimate the nucleation rate on the same sample exposed to longer nucleation times or RHi values above the onset values. For these reasons, extrapolation outside our exper- imental conditions should be done with caution. This will be addressed in more detail in a future publication [Wheeler et al., unpublished manuscript, 2010]. 4. Conclusions [46] An optical microscope coupled to a flow cell was used to study the heterogeneous ice nucleation properties of uncoated and coated mineral dust and SNOMAX particles at temperatures ranging from 234 to 247 K. The results show that H2SO4 coatings significantly modified the heteroge- neous ice nucleation properties of all the minerals studied. For kaolinite and illite, the acid coatings increased the onset RHi by ∼30%; for montmorillonite and quartz, the acid coatings increased the onset RHi by ∼20%. Our studies also show that NH4HSO4 coatings influence the heterogeneous ice nucleation properties of kaolinite particles. The coated particles are less effective at nucleating ice than uncoated particles, with the onset RHi increasing by approximately 18 to 26%, depending on temperature. [47] Onset results indicate that uncoated SNOMAX, a biological IN made from cells of P. syringae, is a reasonably good ice nucleus, having onset values between 110 and 120% RHi. Unlike the mineral dust results, the sulfuric acid coatings did not hinder the heterogeneous icenucleating ability of SNOMAX particles within experimental uncer- tainty. One possible explanation is that a few SNOMAX particles were not completely covered with the acid solution, providing a bare site for ice nucleation. However, the agreement between our coated results and the recent results by Koop and Zobrist [2009] for sulfuric acid solutions containing SNOMAX provides some support for the finding that the particles in our experiments are completely coated. Another possible explanation for the coated SNOMAX results is that the acid solution does not significantly modify the active ice nucleation sites for SNOMAX, which are thought to be certain proteins located in the outer cell membrane. [48] The heterogeneous nucleation rates (Jhet) and contact angles () were determined according to classical nucleation theory for all uncoated and sulfuric acid coated mineral dusts studied and for uncoated SNOMAX particles. The data show that for all uncoated ice nuclei, the contact angles are small (below ∼20°). For mineral dust particles coated with sulfuric acid, the contact angles are larger (above ∼60°). The contact angles presented here are average values; however, previous work has indicated that it is the probability distri- bution function (PDF) of contact angles that is important to measure in future work [Eidhammer et al., 2009; Marcolli et al., 2007; Phillips et al., 2008]. Marcolli et al. [2007] showed that the most active ice nucleation sites are rare in that they lie at the tail of the PDF, and furthermore, that it is these sites that are involved in ice nucleation in a population of particles of a given type. [49] Combined, our results support the idea that anthro- pogenic emissions of SO2 and NH3 may influence the het- erogeneous icenucleating properties of mineral dust particles by increasing the relative humidity required for ice nucleation. [50] Acknowledgments. We thank P. J. DeMott for helpful discus- sions on the immersion mode calculations. 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