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Ice nucleation on mineral dust particles: onset conditions, nucleation rates and contact angles. Eastwood, Michael L. 2011

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Ice nucleation on mineral dust particles: Onset conditions, nucleation rates and contact angles Michael L. Eastwood,1 Sebastien Cremel,1 Clemens Gehrke,1 Eric Girard,2 and Allan K. Bertram1 Received 20 June 2008; revised 4 September 2008; accepted 18 September 2008; published 20 November 2008. [1] An optical microscope coupled to a flow cell was used to investigate the onset conditions for ice nucleation on five atmospherically relevant minerals at temperatures ranging from 233 to 246 K. Here we define the onset conditions as the humidity and temperature at which the first ice nucleation event was observed. Kaolinite and muscovite were found to be efficient ice nuclei in the deposition mode, requiring relative humidities with respect to ice (RHi) below 112% in order to initiate ice crystal formation. Quartz and calcite, by contrast, were poor ice nuclei, requiring relative humidities close to water saturation before ice crystals would form. Montmorillonite particles were efficient ice nuclei at temperatures below 241 K but were poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between our data and previously published work. This can be explained by several factors including the mineral source, the particle sizes, the surface area available for nucleation, and observation time. Heterogeneous nucleation rates (Jhet) were calculated from the measurements of the onset conditions (temperature and RHi) required from ice nucleation. The Jhet values were then used to calculate contact angles (q) between the mineral substrates and an ice embryo using classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 to 12, whereas for quartz and calcite, the contact angles ranged from 25 to 27. The reported Jhet and q values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. Citation: 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. 1. Introduction [2] Ice nucleation can occur in the atmosphere by either homogeneous nucleation or heterogeneous nucleation. Het- erogeneous nucleation typically involves solid substrates, which are often called ice nuclei (IN). These ice nuclei have the potential to modify climate by changing the formation conditions and properties of ice and mixed-phase clouds. [Kärcher, 2004; Haag and Kärcher, 2004; Lohmann and Feichter, 2005; Lohmann and Diehl, 2006]. Our lack of knowledge on ice nucleation is a major obstacle for the simulation of the complex interactions between aerosols and cold clouds. It has been shown in various investigations that mixed-phase clouds often cannot be properly simulated with the existing IN parameterizations, such as those of Fletcher [1962] and Meyers et al. [1992] [Girard and Curry, 2001; Girard et al., 2005]. More physically based parameteriza- tions are needed to simulate heterogeneous nucleation in climate models. [3] Mineral dust particles are abundant in the atmosphere, and both laboratory [Pruppacher and Klett, 1997; Bailey and Hallett, 2002; DeMott, 2002; Zuberi et al., 2002; Hung et al., 2003; Archuleta et al., 2005; Mangold et al., 2005; Möhler et al., 2006; Field et al., 2006; Kanji and Abbatt, 2006; Knopf and Koop, 2006; Marcolli et al., 2007; Zimmerman et al., 2007] and field measurements [Sassen, 2002, 2005; DeMott et al., 2003a, 2003b; Sassen et al., 2003; Toon, 2003; Cziczo et al., 2004; Twohy and Poellot, 2005; Kanji and Abbatt, 2006] have shown that mineral dust particles are effective ice nuclei. Laboratory data have shown that mineral dust particles can lower the supersatu- rations required for ice formation compared to homoge- neous nucleation. At the same time, field measurements have shown that mineral dust particles can have a signifi- cant impact on cloud formation, cloud properties and precipitation [Sassen, 2002, 2005; DeMott et al., 2003a; Sassen et al., 2003]. Measurements have also shown that the cores of ice crystals often contain mineral dust particles, suggesting that ice nucleation is often initiated by mineral dust aerosols in the atmosphere [Heintzenberg et al., 1996; Cziczo et al., 2004; Twohy and Poellot, 2005]. Furthermore, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D22203, doi:10.1029/2008JD010639, 2008 Click Here for Full Article 1Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada. 2Department of Earth and Atmospheric Sciences, University of Quebec at Montreal, Montreal, Quebec, Canada. Copyright 2008 by the American Geophysical Union. 0148-0227/08/2008JD010639$09.00 D22203 1 of 9 measurements of the chemical composition of ice nuclei in the atmosphere show that mineral dust is composed of a significant fraction of atmospheric IN [Chen et al., 1998; Rogers et al., 2001; DeMott et al., 2003b; Richardson et al., 2007]. [4] Although there have been numerous studies on the ice nucleating properties of mineral dust particles, more work is still needed for a complete understanding of the ice nucle- ation properties of these particles [Vali, 1996; Martin, 2000; Demott, 2002; Cantrell and Heymsfield, 2005]. For exam- ple, only a few of the most abundant types of minerals found in the atmosphere have been studied in detail. In addition, in many of the previous studies, only the onset conditions for ice nucleation were reported and only a few studies considered ice nucleation rates (a key parameter for describing ice nucleation). Measurements of ice nucleation rates are needed to more accurately compare laboratory results and to extrapolate laboratory data to the atmosphere and for climate modeling. [5] Our studies focus on the ice nucleating properties of mineral dust particles at temperatures between 247 and 233 K, a temperature range relevant for the lower troposphere. As pointed out by Vali [1996] in a review on ice nucleation, the origin of ice in lower tropospheric clouds is not resolved, and it remains a question of great importance and in need of new efforts. Also, there have been very few measurements in the range of 247 to 233 K. In the following study we focus on the ice nucleation properties of muscovite (a mineral in the mica group), kaolinite, montmorillonite, quartz and calcite particles. These minerals were chosen since they are major components of aerosolized mineral dust found in the atmosphere. To illustrate this point, the mineralogies of Saharan dust collected in Sal Island, Bar- bados and Miami after three large Saharan dust outbreaks are shown in Table 1 [Glaccum and Prospero, 1980]. The minerals chosen for our studies represent over 90% of the total mass during these outbreaks. Montmorillonite compo- sition was below the detection limit (5%) in the Saharan dust studies illustrated in Table 1, but several studies have cited montmorillonite as one of the dominant clay minerals present in African and Asian dusts [Prospero, 1999; Hanisch and Crowley, 2001]. [6] This paper is organized as follows: first, we present measurements of the onset conditions for ice nucleation on muscovite, kaolinite, montmorillonite, quartz and calcite. Here we define the onset conditions as the humidity and temperature at which the first ice nucleation event was observed. Second, we compare this data with existing data in the literature. Third, from the onset conditions we determine heterogeneous ice nucleation rates (number of nucleation events per unit surface area of solid material per unit time) for each mineral type. Fourth, we parameterized the heterogeneous nucleation rates using classical nucle- ation theory. Classical nucleation theory is a reasonable starting point for analyzing laboratory data, as this theory has been used in the past for describing ice nucleation in atmospheric models [Kärcher, 1996, 1998; Jensen and Toon, 1997; Jensen et al., 1998; Kärcher et al., 1998; Martin, 2000; Demott, 2002; Morrison et al., 2005]. In this last step we determined the contact angle between an ice nucleus and the mineral surfaces. 2. Experimental [7] Micron-sized muscovite particles were generated by grinding muscovite flakes (1–10 mm in diameter, <1 mm in thickness, purchased from Alfa Aesar), and micron-sized quartz particles were produced by grinding a sample of quartz tubing (purchased from United Silica). Kaolinite (purchased from Fluka), montmorillonite K10 (purchased from Fluka) and calcite (purchased from Puratronic1, Alfa Aesar) particles were used as supplied. Table 2 lists the chemical formulae of the minerals studied. [8] The apparatus used in these studies consisted of an optical microscope coupled to a flow cell (see Figure 1) in which the humidity and temperature could be accurately controlled [Dymarska et al., 2006; Parsons et al., 2004a, 2004b]. Mineral dust particles were deposited on the bottom surface of the flow cell; the relative humidity with respect to ice (RHi) inside the cell was increased, and the conditions under which ice crystals formed were determined with a reflected-light microscope (Zeiss Axiotech 100) equipped with a 10 objective lens. From these measurements we determine the onset conditions (temperature and relative humidity) for ice nucleation. Table 1. Mineralogy of Saharan Dust Collected at Sal Island (in the Cape Verde Islands), Barabados, and Miami, Florida, After Three Large Saharan Dust Outbreaks [Glaccum and Prospero, 1980]a Mineral Sal Island (%) Barbados (%) Miami (%) Mica 53.8 64.3 62.1 Kaolinite 6.6 8.3 7.1 Chlorite 4.3 4.1 4.2 Quartz 19.6 13.8 14.2 Microcline 2.2 1.5 1.1 Plagioclase 5.4 4.1 4.5 Calcite 8.2 3.9 6.9 Montmorillonite 5 5 5 aPercentages shown are averages of the three outbreaks (assuming 100% crystalline material). Table 2. Chemical Formulae of the Five Minerals Studied [Anthony et al., 1995] Mineral Formula Muscovite KAl2(Si3Al)O10(OH,F)2 Kaolinite Al4Si4O10(OH)8 Quartz SiO2 Calcite CaCO3 Montmorillonite (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2 nH2O Figure 1. Schematic of the flow cell used for this study. D22203 EASTWOOD ET AL.: ICE NUCLEATION ON MINERAL DUST PARTICLES 2 of 9 D22203 [9] The temperature of the cooling stage and hence the flow cell was regulated with a refrigerating circulator (Thermo Neslab ULT-95). A hydrophobic slide (which supported the particles) was positioned inside the aluminum cell body. An insulating spacer, made from polychloro- trifluoroethylene (PCTFE), was placed between the hydro- phobic glass slide and the flow cell body. This ensured that the coldest portion of the flow cell was the glass substrate (by 10 K), thus preventing unwanted ice nucleation on other components of the cell. All seals within the cell were made with Viton O-rings with the exception of the seal between the glass slide and the PCTFE spacer, which was made with low vapor pressure chlorotrifluoroethylene grease (Series 28LT from Halocarbon Products, vapor pressure < 0.1 Torr at 293 K). The grease ensured that no space remained between the glass slide and the PCTFE spacer, where ice could nucleate without being detected by the microscope. For kaolinite and montmorillonite we also performed experiments using a lower vapor pressure grease (Krytox LVP from DuPont, vapor pressure <1013 Torr at 293 K) and the same results were obtained. Also, for kaolinite we previously carried out experiments without grease and the same results were obtained, suggesting the grease had little effect on the freezing results [Dymarska et al., 2006]. [10] The upper portion of the cell body and the inlet and outlet were made from stainless steel. A sapphire window (1 mm thick) was positioned at the top of the cell body, allowing optical access to the bottom surface of the cell. The reflected-light microscope was coupled to a high- resolution monochrome digital video camera (Sony, XCD- X700) which captured images of the particles deposited on the hydrophobic slide during the course of the experiments. The images were analyzed with the Northern Eclipse software package to determine particle size and total surface area available for ice nucleation. [11] The bottom surface of the flow cell was a hydropho- bic slide made from a glass cover slide treated with dichlorodimethylsilane (DCDMS). This hydrophobic layer was added to reduce the probability of ice nucleation directly on the surface. Prior to the treatment with DCDMS the glass slide was thoroughly cleaned in ‘‘piranha’’ solution (3:1 mixture by volume of sulfuric acid and hydrogen peroxide), rinsed in high-purity water (distilled water further purified with a Millipore system) and methanol (HPLC grade). Any remaining contaminant particles removed with a dry ice cleaning system (Sno Gun-IITM, Va-Tran Sys- tems). The treatment with DCDMS involved placing the slides in an airtight chamber together with 2–3 droplets of DCDMS solution (Fluka, 5% DCMS in heptane). The slides were not in direct contact with the DCDMS, rather the DCDMS coated the glass slides via vapor deposition. [12] All samples were prepared and the flow cell con- structed within a filtered air laminar flow hood. This greatly reduced the possibility of sample contamination by ambient atmospheric and laboratory particles. All mineral dust particles with the exception of calcite were deposited on a hydrophobic glass slide using the following technique: the dry dust particulates were placed in a glass vessel immersed in an ultrasonic bath. A flow of ultra-high-purity N2 was passed through the glass vessel, and vibrations from the ultrasonic bath caused the dust particles to be suspended in the flow of N2. This flow was then directed at the hydro- phobic glass slide, and the dust particles were deposited on the slide by impaction. Calcite particles were not readily suspended by the vibrations from the ultrasonic bath, so these were deposited on the hydrophobic slide simply by sprinkling them directly on the slide using a small spatula. In all cases, dust particles deposited on the slide were less than 50 mm in diameter. The optical resolution limit of the microscope was 1 mm. A typical sample held between 100 to 1000 individual particles, the majority of which were between 1 and 20 mm in diameter. The average sizes of the particles used in our experiments were 7.7 mm for kaolinite, 9.0 mm for muscovite, 8.1 mm for montmorillonite, 10.0 mm for quartz and 14.2 mm for calcite based on the optical microscope images. [13] During the ice nucleation experiments, a flow of humidified He gas was introduced to one side of the cell and exited on the other where its frost point was measured with a frost point hygrometer (General Eastern). From the frost point measurements, the water vapor pressure (pH2O) was calculated using the parameterization of Murphy and Koop [2005]. A flow of humidified gas was generated by passing a flow of ultra-high-purity He gas (99.999 %) over a reservoir of ultra-pure water (distilled water further purified using a Millipore system). The desired pH2O was adjusted by altering the temperature of the water reservoir and diluting the humidified flow with a second flow of dry He. A continuous flow of between 1900 to 2100 cm3 min1 (at 273.15 K and 1 atm) was maintained throughout the course of the ice nucleation experiments. For purification, the He gas used in these experiments was first passed through a trap containing molecular sieve (1/16’’ pellets, Type T4A) at 77 K and then through a 0.02 mm filter (Anodisc 25). [14] In our experimental apparatus, a Pt-100 resistance temperature detector (RTD) was located just beneath the slide containing the mineral dust particles. The RTD was calibrated against the dew point or ice frost point within the cell, similar to methods used by other researchers [Middlebrook et al., 1993; Parsons et al., 2004b; Dymarska et al., 2006]. Calibration involved observing the change in size of ice crystals on the slide as the temperature was ramped up and down. The temperature at which the size of the ice crystals remained constant was determined from these measurements; at that point the ice crystals were in equilibrium with the water vapor inside the cell, whose frost point was precisely known from the hygrometer measurements. Hence it was possible to determine the offset temperature between the temperature reported by the RTD and the temperature of the ice crystals formed on the hydrophobic glass slide in the cell, and use this offset to correct experimentally measured temperatures obtained with the RTD. [15] The RHi within the cell was then calculated using equation (1): RHi ¼ pH2O=piceðTcellÞ * 100 ð1Þ Where pice(Tcell) is the saturation vapor pressure of ice at the temperature of the cell, calculated using the parameteriza- tion of Murphy and Koop [2005]. [16] In each ice nucleation experiment, the RHi was ramped from below 100% to water saturation by decreasing D22203 EASTWOOD ET AL.: ICE NUCLEATION ON MINERAL DUST PARTICLES 3 of 9 D22203 the temperature of the cell at approximately 0.1 K min1 (corresponding to a change in RHi of approximately 1% min1) while maintaining a constant pH2O inside the cell. Typical experimental RHi trajectories are illustrated in Figure 2 for three different initial temperatures of 246.7 K, 241.7 K, and 236.7 K. Images of the dust particles were recorded digitally every 20 seconds or 0.033 K, while simultaneously recording pH2O and the cell temperature. From these images, the RHi and temperature at which ice crystals first formed was determined (i.e., the onset con- ditions of ice nucleation). Shown in Figure 3 are images of the kaolinite particles recorded in a typical freezing exper- iment before and after ice nucleation, respectively. The formation of ice crystals is clearly discernable. 3. Results and Discussion 3.1. Measurements of Onset Conditions of Ice Nucleation [17] As mentioned above, the onset conditions were determined for muscovite, kaolinite, montmorillonite, quartz and calcite. The total surface area of mineral dust exposed in any particular experiment ranged from 5 x 105 to 5 x 103 cm2, based on the geometric surface area of the particles. Shown in Figure 4 are the onset conditions for all five minerals at each of the three temperatures studied. The error bars represent 95% confidence intervals based on at least six measurements per data point. For kaolinite, mus- covite and montmorillonite ice nucleation occurred below liquid water saturation and there was no indication of liquid water condensing prior to ice nucleation as expected. This is also the case for quartz and calcite at the two lowest temperatures. For these experiments the data correspond to deposition freezing, which occurs when vapor absorbs onto a solid surface and is transformed into ice [Valie, 1985]. For quartz and calcite at the warmest temperature studied the data overlap with liquid water saturation. For approximately half of the quartz and calcite experiments at the warmest temperature we observed only ice nucleation with no indication of liquid water condensation prior to ice nucleation. For the other half of the quartz and calcite experiments at the warmest temperature we first observed the condensation of liquid water. In this case it appears that condensation freezing may be important. Condensation freezing refers to the sequence of events whereby liquid water first condenses followed by freezing of the liquid [Valie, 1985]. [18] On the basis of Figure 4, kaolinite and muscovite are effective ice nuclei at all temperatures studied, with onset RHi values below 112%. Quartz and calcite were poor ice nuclei at all temperatures studied, requiring relative humid- ities close to water saturation before ice nucleation occurred. Montmorillonite was an effective ice nucleus at the two lowest temperatures studied (236.0 and 240.8 K), but a relatively poor ice nucleus at the highest temperature studied (244.6 K). Overall, the data show significant differ- ences in the ice nucleating abilities of the five minerals studied over this temperature range. [19] The reason why some minerals are better ice nuclei than others is still relatively poorly understand. However, previous research suggests that it is likely a combination of the strengths of the chemical bonds at the mineral surface, the crystallographic match between the substrate and ice Figure 2. Typical experimental trajectories of RHi, where temperature was reduced at a rate of 0.1 K min1, while the water partial pressure was constant. The trajectories were calculated using the saturation vapor pressures of water and ice from the parameterizations of Murphy and Koop [2005]. Figure 3. Optical microscope images of kaolinite particles (a) before and (b) after ice nucleation. Figure 4. Onset conditions for all minerals studied. D22203 EASTWOOD ET AL.: ICE NUCLEATION ON MINERAL DUST PARTICLES 4 of 9 D22203 embryo, and the presence of active sites on the mineral surface, which can promote ice nucleation [Pruppacher and Klett, 1997]. For example, it has been speculated that the relatively good ice nucleation ability of kaolinite may be due to the pseudo-hexagonal arrangement of the hydroxyl (OH) groups at the kaolinite surface [Pruppacher and Klett, 1997]. Our laboratory results provide further data that can be used to test these various theories. [20] In our experiments, the mineral particles are sus- pended on a hydrophobic glass substrate. Before we discuss our results further, we first address the possible effect of the hydrophobic glass substrate on the ice nucleation results. [21] First, in blank experiments (when no mineral dust was used) ice nucleation did not occur below liquid water saturation, (see Figure 4a in the study of Dymarska et al. [2006]). Second, from direct observations of the optical images we confirmed that ice nucleation always occurred on a mineral particle, rather than a bare spot on the hydropho- bic glass substrate. Third, to further ensure that the hydro- phobic glass support was not influencing our results, we did some tests with other types of supports. Tests were carried out with bare glass slides that were not coated with the hydrophobic monolayer. In this case the glass slides were just cleaned as described above, resulting in a hydrophilic substrate. Also, tests were carried out using thin Teflon sheets as the bottom surface of the flow cell. In the test experiments we measured the onset conditions for ice nucleation on kaolinite particles using the different supports (bare glass, hydrophobic glass, and Teflon). Within the uncertainty of our measurements, the results were indepen- dent of the type of support used. This gives further confi- dence that the hydrophobic glass support is not influencing our results. [22] Below we compare our onset conditions with results published in the literature. The comparison below focuses mainly on results from six different studies: those of Roberts and Hallett [1968], Bailey and Hallett [2002], Dymarska et al. [2006], Kanji and Abbatt [2006], Salam et al. [2006] and Zimmerman et al. [2007]. Some of the experimental conditions from these measurements are listed in Table 3 for reference. 3.2. Comparison of Measured Onset Conditions for Kaolinite With Literature Data [23] In Figure 5, we compare our kaolinite results with previous measurements. At our temperature range, there have been three previous studies: those of Dymarska et al. [2006], Bailey and Hallett [2002], and Salam et al. [2006]. The data from Dymarska et al. [2006] are in agreement with our measurements, which is not surprising since the same instrument and experimental protocol were employed. The onset conditions for Salam et al. [2006] are also in agree- ment with our current studies at 233 to 246 K. These authors observed that 0.5% of the kaolinite particles acti- vated as ice nuclei even at the lowest supersaturations with respect to ice (close to 100%). The fraction of particles activated remained almost constant in these studies until above 120–130 % RHi at which point the fraction activated increased sharply. The results from Bailey and Hallett [2002] differ from our results by approximately 10–15% RHi. These differences may be due to variation in mineral surface area available in the different experiments, variation in particle diameters, or variability due to different exper- imental techniques or different observation times in the experiments. Also, differences in the source of the kaolinite particles may result in some variability in the interaction of the particles with water. Hoffer [1961] and Schuttlefield et al. [2007] reported that water uptake by kaolinite and montmorillonite varied significantly with the location of the mineral source. These points should be investigated in more detail in future experiments. Table 3. Summary of Experimental Conditions for Previously Published Results Study Size Range (mm) Number of Particles Surface Area Range (cm2) Current study 1–50 102–103 3  106–1  102 Roberts and Hallett [1968] 0.5–3 101–104 8  108–3  103 Bailey and Hallett [2002] 5–10 Not determined Not determined Kanji and Abbatt [2006] 0.5–5 4.6  102–3.2  104 9  105–3  102 Dymarska et al. [2006] 1–20 2  102–8  102 6  106–1  102 Salam et al. [2006] < 0.5–5 Not determineda Not determineda Zimmerman et al. [2007] 1–10 Not determinedb Not determinedb aThese authors used a continuous flow diffusion chamber. Typical total aerosol number concentrations were <15 particles cm3. bZimmerman et al. [2007] spread particles on a silicon plate (5  5 mm), and the particle density was 100–150 particles/ mm2. However, the exact fraction of particles activated could not be determined precisely because only a small part of the total silicon plate was imaged. Figure 5. Summary of ice nucleation results for kaolinite. D22203 EASTWOOD ET AL.: ICE NUCLEATION ON MINERAL DUST PARTICLES 5 of 9 D22203 [24] If we consider all the data shown in Figure 5, the following conclusions seem appropriate. At temperatures above 255 K, ice nucleation does not occur until liquid water saturation is reached. At temperatures below 250 K, all data suggest that kaolinite is an effective ice nucleus (i.e., RHi values less than water saturation are required for ice nucleation). Quantitatively, however, there are relatively large differences between the different experiments at tem- peratures below 250 K as mentioned above. 3.3. Comparison of Measured Onset Conditions for Montmorillonite With Literature Data [25] In Figure 6 we compare our ice nucleation data for montmorillonite with previous data from Kanji and Abbatt [2006], Salam et al. [2006] and Zimmerman et al. [2007]. Our results are consistent with the more recent study by Zimmerman et al. [2007] if we extrapolate our results to warmer temperatures, but appear to be inconsistent with the results from Kanji and Abbatt [2006]. At the warmer temperatures our results also appear to be inconsistent with the results from Salam et al. [2006]. These authors found that 1.2% of montmorillonite particles were active ice nuclei even at very small supersaturations with respect to ice (RHi 100%) over the temperature range of 258 K and 233 K. The fraction of particles activated remained almost constant until above approximately 107–115 % RHi, at which point the fraction activated, increased sharply. [26] Not shown in Figure 6 are the results from Roberts and Hallett [1968]. Roberts and Hallett [1968] observed ice nucleation at 248 K and at liquid water saturation when using approximately 104 particles. This result is consistent with our observations. [27] Overall, there appear to be large differences between some of the montmorillonite studies. Some possible reasons for these differences are discussed above. Kanji and Abbatt [2006] also presented other reasons why their results may differ from other measurements. First, they suggested a time dependence may be causing the observed differences, with longer exposure times leading to lower RHi values required for activation. Second, they suggested that some of the differences may be due to different preparation techniques. Kanji and Abbatt [2006] prepared their particles by nebu- lizing an aqueous suspension of the mineral dust. In contrast, all other studies mentioned above for montmoril- lonite used dry dispersion. Future studies that investigate the effect of these different parameters are needed for a com- plete understanding of ice nucleation on mineral particles. 3.4. Comparison of Measured Onset Conditions for Muscovite, Quartz, and Calcite With Literature Data [28] To our knowledge, these are the first measurements of the onset conditions (both RHi and temperature) for ice nucleation on muscovite. Mason and Maybank [1958] reported that muscovite was inactive as an ice nucleus at temperatures above 255 K, which does not contradict our results. [29] For quartz, there are no previous reports of the onset conditions for ice nucleation. Mason and Maybank [1958] reported that this mineral was inactive as an ice nucleus above 248 K, which is consistent with our data. [30] For calcite, Roberts and Hallett [1968] reported one temperature (255 K) and RHi (120%) at which one particle in 104 nucleated ice. These conditions correspond to liquid water saturation. At 244 K, the warmest temperature we investigated, we also observed that relative humidities close to liquid water saturation were needed for ice nucleation. We conclude that the limited results from Roberts and Hallett [1968] do not contradict our measurements. 3.5. Nucleation Rates, Jhet [31] Above we reported onset conditions (RHi and tem- perature), which may depend on several experimental parameters, such as observation time and surface area available for nucleation. A more useful parameter for describing ice nucleation is the heterogeneous nucleation rate, Jhet, which allows for a more direct comparison between laboratory studies and for extrapolation to the atmosphere. 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; Martin, 2000; Hung et al., 2003; Parsons et al., 2004a, 2004b; Archuleta et al., 2005; Pant et al., 2006] and a rate coefficient [Dymarska et al., 2006; Marcolli et al., 2007] in the literature. The heterogeneous nucleation rate is related to the onset data through equation (2): J het ¼ w=Ast ð2Þ where w is the number of ice crystals nucleated, As is the total mineral dust surface area available for heterogeneous nucleation, and t is the observation time. At the onset of ice nucleation, w was equal to one. [32] Table 4 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 second. If this is the case the calculated nucleation rates will be lower limits to the true nucleation rates. For the surface area available for nucleation we used the geometric surface area of the particles determined directly from the optical microscope images using digital Figure 6. Summary of ice nucleation results for mon- tmorillonite. D22203 EASTWOOD ET AL.: ICE NUCLEATION ON MINERAL DUST PARTICLES 6 of 9 D22203 image software (Northern Eclipse). For an upper limit to the surface area available for nucleation we multiplied the geometric surface area of the particles by a factor of 50, based on scanning electron microscope (SEM) measure- ments where we more accurately determined the specific surface area of a limited number of particles. The data shown in Table 4 suggests that our experiments are typically sensitive to values of Jhet ranging from 1 to 22000 cm 2s1. 3.6. Classical Nucleation Theory Parameters From Jhet [33] The applicability of standard classical nucleation theory to heterogeneous nucleation on minerals remains to be determined. In fact, some measurements show that for precise predictions, active site theory is required. See for example the studies of Hung et al. [2003], Archuleta et al. [2005], and Marcolli et al. [2007]. Nevertheless, classical nucleation theory has been used in the past to describe heterogeneous nucleation in atmospheric cloud models [Kärcher, 1996, 1998; Jensen and Toon, 1997; Jensen et al., 1998; Kärcher et al., 1998; Demott, 2002; Morrison et al., 2005]. Also classical nucleation theory is a relatively convenient and simple way to parameterize laboratory data. Hence classical nucleation theory is a reasonable starting point for analyzing our experimental data. Below we analyze the nucleation rates using classical nucleation theory. From this analysis, we determined the contact angle between an ice nucleus and the mineral surface. [34] For this analysis we only consider the case of deposition freezing. As a result, we do not consider any nucleation data where the onset conditions overlap the liquid water saturation line, since in this case nucleation may have occurred by either deposition or condensation freezing, as discussed above. [35] According to standard classical nucleation theory, the rate of heterogeneous nucleation (Jhet) by deposition freez- ing is defined as: [Pruppacher and Klett, 1997] J het ¼ A 	 expDFg;het kT ð3Þ where A is the pre-exponential factor in units of cm2 s1, DFg,het is the free energy of formation of the critical embryo in joules (J), k is the Boltzmann constant in J K1, and T is the temperature in K. Assuming that an ice embryo on a curved solid substrate can be described as a spherical cap model, the free energy of formation of the critical embryo is given by [Pruppacher and Klett, 1997]: DFg;het ¼ 16pM2ws 3 i=v 3½RTrInSi 	 fðm; xÞ ð4Þ where Mw is the molecular weight of water in g mol 1, si/v is the surface tension at the ice-vapor interface in mJ m2, R is the universal gas constant in J mol1 K1, r is the density of ice in g cm3, Si is the supersaturation ratio with respect to an ice surface, 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. The compatibility parameter, m, is equal to cosq, where q is the contact angle between an ice nucleus and the mineral surface. [36] 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 ðm; xÞ ¼ m 3  3mþ 2 4 ð5Þ To calculate q, we first calculated the free energy of formation of the critical nucleus using equation (3), our experimentally determined Jhet values, and assuming a pre- exponential term (A) equal to 1025 cm2 sec1 [Fletcher, 1958, 1959; Pruppacher and Klett, 1997]. Then, we calculated the contact angle (q) using equations (4) and (5), assuming the density of ice (r) is 0.92 g cm3 [CRC, 2001–2002], Mw is 18.015 g mol 1, and si/v equals 106 ± 5 mJ m2 [Pruppacher and Klett, 1997]. For si/v, we are using the surface tension appropriate for hexagonal ice. Recent work has shown that cubic ice is the first phase to nucleate when homogeneous nucleation dominates [Murray et al., 2005; Murray and Bertram, 2006], but further research is needed to determine if this also the case for heterogeneous nucleation. Table 4. J Values and Contact Angles for all Five Minerals Studied Mineral Onset Temperature (K) RHice Jhet (cm2 s1) Jhet, upper (cm2 s1) Jhet, lower (cm2 s1) qlower q qupper Kaolinite 246.1 105 ± 5 420 4200 4.2 5.2 9.7 13.9 Kaolinite 241.1 104 ± 5 200 2000 2.0 2.4 7.8 12.4 Kaolinite 236.4 104 ± 2 280 2800 2.8 3.4 9.2 13.5 Muscovite 246.2 107 ± 5 940 9400 9.4 7.1 10.7 14.7 Muscovite 241.3 106 ± 6 2200 22,000 22 5.5 9.5 14.3 Muscovite 237.0 102 ± 2 490 4900 4.9 0.7 6.2 11.2 Montmorillonite 244.6 124 ± 5 460 4600 4.6 19.6 22.3 25.3 Montmorillonite 240.8 110 ± 4 870 8700 8.7 10.9 14.3 17.6 Montmorillonite 236.0 106 ± 4 930 9300 9.3 7.7 12.0 15.6 Quartz 243.9 133 ± 1 360 3600 3.6 - - - Quartz 238.7 137 ± 3 340 3400 3.4 24.5 27.1 30.1 Quartz 234.0 136 ± 4 480 4800 4.8 23.7 26.3 29.3 Calcite 244.1 130 ± 3 620 6200 6.2 - - - Calcite 238.9 135 ± 3 250 2500 2.5 23.8 26.4 29.4 Calcite 234.2 132 ± 6 130 1300 1.3 22.2 24.9 27.9 D22203 EASTWOOD ET AL.: ICE NUCLEATION ON MINERAL DUST PARTICLES 7 of 9 D22203 [37] In Table 4, the contact angles calculated using the procedure discussed above are listed. The contact angle values are also illustrated in Figure 7. The data show that for efficient ice nuclei such as muscovite and kaolinite, the contact angles are small (below 18). For poor ice nuclei such as quartz and calcite, the contact angles are larger (above 20). These values may be useful for future model- ing studies of ice nucleation in the atmosphere and for comparing results between different laboratories. 4. Summary and Conclusions [38] An optical microscope coupled to a flow cell was use to characterize the ice nucleation ability of muscovite, kaolinite, montmorillonite, quartz and calcite over the temperature range of 233 to 247 K. Onset conditions for ice nucleation, nucleation rates and contact angles were determined. [39] Onset measurements indicate that muscovite and kaolinite are very good ice nuclei with onset RHi values of less than approximately 110%, well below water satura- tion. This can be explained by a better crystallographic match between the pseudo-hexagonal arrangement of the hydroxyl (OH) groups at the kaolinite surface and the hexagonal ice structure which allows a better hetero-epitax- ial growth of the ice structure though a H-bond framework [Pruppacher and Klett, 1997]. Onset measurements indicate that quartz and calcite are the poorest ice nuclei. For the temperature range studied, the RHi values needed to induce ice nucleation on quartz and calcite are approximately 20 to 40% higher than those needed for kaolinite and muscovite. In contrast, montmorillonite was an effective ice nucleus at the two lowest temperatures studied (236.0 and 240.8 K), but a relatively poor ice nucleus at the highest temperature studied (244.6 K), based on the onset data. Overall, the data show significant differences in the ice nucleating abilities of the five minerals studied over this temperature range. [40] The measured onset conditions for the mineral dusts were compared with previously published data. In several cases, there was a lack of quantitative agreement among published work. This can be explained by several factors including the mineral source, the particle sizes, the surface area available for nucleation, observation and equilibrium times. Future studies that investigate the effect of these different parameters are needed for a complete understand- ing of ice nucleation on mineral particles. [41] The heterogeneous nucleation rates (Jhet) and contact angles (q) were determined according to classical nucleation theory for all five minerals studied. The contact angles measured for kaolinite and muscovite ranged from 6 to 12; whereas for quartz and calcite the contact angles ranged from 25 to 27. The reported Jhet and q values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. 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