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Ice nucleation on uncoated and coated atmospheric mineral dust particles Eastwood, Michael Logan 2008

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ICE NUCLEATION ON UNCOATED AND COATED ATMOSPHERIC MINERAL DUST PARTICLES by MICHAEL LOGAN EASTWOOD B. Sc., The University of British Columbia, 2002  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Chemistry)  UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  August 2008 © Michael Logan Eastwood, 2008  ii  ABSTRACT An optical microscope coupled to a flow cell was used to investigate ice nucleation on five atmospherically relevant mineral dusts at temperatures ranging from 233 to 247 K. Kaolinite and muscovite particles 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 particles, 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 poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between these data and previously published work. This can be explained by several factors including mineral source, particle size, observation time and surface area available for nucleation. Heterogeneous nucleation rates (Jhet) were calculated from the onset data. Jhet values ranged from 60 to 1100 cm-2s-1 for the five minerals studied. These values were then used to calculate contact angles (θ) for each mineral according to classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 to 12º; for quartz and calcite the contact angles were much higher, ranging from 25 to 27º. The contact angles measured for montmorillonite were less than 15º at temperatures below 241 K, and above 20º at higher temperatures. The reported Jhet and θ values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere. The roles of H2SO4 and (NH4)2SO4 coatings on the ice nucleating properties of kaolinite were also investigated. Onset data was collected for H2SO4 coated and (NH4)2SO4 coated kaolinite particles at temperatures ranging from 233 to 247 K. In contrast to uncoated kaolinite particles, which were effective ice nuclei, H2SO4 coated particles were found to be poor ice nuclei, requiring relative humidities close to water saturation before nucleating ice at all temperatures studied. (NH4)2SO4 coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K.  iii  TABLE OF CONTENTS  ABSTRACT ........................................................................................................................ ii TABLE OF CONTENTS ................................................................................................... iii LIST OF TABLES .............................................................................................................. v LIST OF FIGURES ........................................................................................................... vi ACKNOWLEDGEMENTS ............................................................................................. viii 1. Introduction ................................................................................................................. 1 1.1 Aerosols ................................................................................................................. 1 1.1.1 Aerosol impacts on climate ......................................................................... 3 1.1.2 Aerosols as atmospheric ice nuclei ............................................................. 7 1.2 Mineral Dust .......................................................................................................... 8 1.3 The Troposphere.................................................................................................. 10 1.4 Motivation for Research ...................................................................................... 12 1.5 Thesis Overview .................................................................................................. 13 2. Ice nucleation on mineral dust particles: onset conditions, nucleation rates and contact angles ......................................................................... 14 2.1 Introduction ......................................................................................................... 14 2.2 Experimental ....................................................................................................... 16 2.2.1 Overview ................................................................................................... 16 2.2.2 Slide preparation ....................................................................................... 17 2.2.3 Sample preparation ................................................................................... 17 2.2.4 Flow cell.................................................................................................... 19 2.2.5 Humidity control ....................................................................................... 20 2.2.6 Temperature calibration ............................................................................ 23 2.2.7 Experimental procedure ............................................................................ 26 2.3 Results and Discussion ........................................................................................ 28 2.3.1 Onset conditions........................................................................................ 28  iv  2.3.2 Heterogeneous nucleation rates ................................................................ 35 2.3.3 Classical nucleation theory ....................................................................... 36 3. The effects of sulphuric acid and ammonium sulphate coatings on the ice nucleating properties of kaolinite ........................................................... 41 3.1 Introduction ......................................................................................................... 41 3.2 Experimental ....................................................................................................... 42 3.2.1 Overview ................................................................................................... 42 3.2.2 Sample preparation ................................................................................... 43 3.2.3 Coated particle characterization ................................................................ 45 3.3 Results and Discussion ........................................................................................ 46 3.3.1 Uncoated kaolinite particles ...................................................................... 46 3.3.2 H2SO4 coated kaolinite particles ..................................................................... 48 3.3.3 (NH4)2SO4 coated kaolinite particles ............................................................. 52 4. Concluding remarks ................................................................................................. 55 4.1 Uncoated mineral dust ......................................................................................... 55 4.2 Coated mineral dust ............................................................................................. 56 4.3 Suggestions for future work ................................................................................ 57 REFERENCES ........................................................................................................... 58  v  LIST OF TABLES Table 1.1: Source strengths of various aerosol species [adapted from Forster et al., 2007] ................................................................. 2 Table 1.2: Common minerals present as atmospheric aerosols and their chemical formulae [Berry et al., 1983; Anthony et al., 1995] ......................... 8 Table 1.3: Mineralogy of Saharan dust collected at Sal Island (in the Cape Verde Islands), Barabados, and Miami, FL, after three large Saharan dust outbreaks. Percentages shown are averages of the three outbreaks (assuming 100% crystalline material) [Glaccum and Prospero, 1980]. ...................................................................... 9 Table 2.1: Chemical formulae of minerals studied [Anthony et al., 1995]..................... 18 Table 2.2: Summary of experimental conditions for previously published results ........ 31 Table 2.3: J values and contact angles for all five minerals studied. ............................. 39  vi  LIST OF FIGURES Figure 1.1: Global mean RFs from selected agents and mechanisms, including levels of scientific understanding [adapted from Forster et al., 2007] .......... 4 Figure 1.2: Schematic diagram of two aerosol indirect effects. The cloud on the left is representative of a normal atmospheric aerosol concentration, while the one on the right reflects an elevated concentration [adapted from Forster et al., 2007] ................................................................ 6 Figure 1.3: Heterogeneous modes of ice nucleation ........................................................ 7 Figure 1.4: Mean temperature profile of the Earth’s atmosphere as a function of altitude [adapted from Ahrens, 1994] ...................................................... 11 Figure 2.1: Schematic of the flow-cell used for these studies ........................................ 16 Figure 2.2  Variation of pliq – pice as a function of temperature. The shaded region highlights the temperature range studied in these experiments ........ 21  Figure 2.3: Ice crystal area and temperature as a function of time for a typical calibration experiment. By averaging the three highlighted results, the offset between hygrometer and the RTD at this temperature was determined to be 0.43 ± 0.19ºC ............................................................ 24 Figure 2.4: Calibration curve used to correct for the temperature difference between the RTD in the aluminum stage and the surface of the slide in the flow cell ..................................................................................... 25 Figure 2.5: Experimental trajectories used in these studies. Each trajectory is labelled with its corresponding frost point................................................... 26  vii  Figure 2.6: Optical microscope images (100×) of kaolinite particles before (left) and after (right) ice nucleation ..................................................................... 27 Figure 2.7: Onset conditions for all minerals studied..................................................... 29 Figure 2.8: Summary of ice nucleation results for kaolinite .......................................... 32 Figure 2.9: Summary of ice nucleation results for montmorillonite .............................. 34 Figure 2.10: Contact angle as a function of temperature for each mineral studied .......... 40 Figure 3.1: Image of H2SO4-coated kaolinite particles prior to an ice nucleation experiment, collected using an optical microscope with a 10× objective.... 44 Figure 3.2: (A) Image of two selected kaolinite particles coated with H2SO4 at RHw < 1%. (B) Image of the same two kaolinite particles coated with H2SO4 at RHw = 95%............................................................................................... 46 Figure 3.3: Onset RHi as a function of surface area for uncoated and coated kaolinite particles at 244 K .......................................................................... 47 Figure 3.4: Summary of ice nucleation results for coated and uncoated kaolinite particles ........................................................................................................ 48 Figure 3.5: Experimental trajectories and lines of constant droplet composition (in wt% H2SO4) ............................................................................................ 49 Figure 3.6: The crystal structure of kaolinite. Red = oxygen (O2-), grey = aluminum (Al3+), blue = silicon (Si4+), yellow = hydrogen (H+). Dotted lines represent hydrogen bonds holding two alumina-silicate sheets together..... 51 Figure 3.7: Schematic of the possible phase behaviour of an (NH4)2SO4 coating during a typical freezing experiment ........................................................... 54  viii  ACKNOWLEDGEMENTS  First and foremost, I would like to express my deep gratitude to Dr. Allan Bertram for giving me the opportunity to work in his lab as a graduate student for the past two and a half years. His advice was invaluable, his patience was always appreciated and his guidance is largely what made my research career at UBC so rewarding. Allan, thank you for all your support! To my friends and colleagues from the Bertram group, both past and present, I extend my sincere thanks. To Dr. Ben J. Murray, for his thoughtful mentoring and tremendous assistance in getting my experimental work started so early. To Dr. Daniel A. Knopf, for his rigorous questioning (especially during talks) that kept me on my toes. To Dr. Matthew T. Parsons, for being a good friend who was always willing to share a laugh (or an enormous platter of barbequed meat). To Michael Wheeler, Sarah Hanna, Emily Simpson, Simone Gross, Lori Anthony, Dr. Sébastien Cremel and Dr. Pedro CampuzanoJost, for being such friendly and helpful colleagues. I would also like to express my appreciation to all the staff members from the Mechanical Shop, the Electronics Shop and the Glassblowing Shop for their assistance in various aspects of this project. Finally, I would like to thank my family and friends for being so supportive during my return to UBC for a Master’s degree. Especially during my final year when I was balancing a teaching job and research responsibilities, it was very helpful to receive encouragement from the people closest to me.  Chapter 1  1  1. Introduction 1.1 Aerosols Aerosols are suspensions of solid, liquid, or mixed solid-liquid particles in a gas [Seinfeld and Pandis, 1998]. Aerosols are ubiquitous in the atmosphere, present in concentrations from 102 to 106 cm-3 [Finlayson-Pitts and Pitts, 2000]. Aerosol particles have sizes ranging from clusters of a few molecules to 100 µm and larger [Pruppacher and Klett, 1997], but those with diameters ranging from 0.002–10 µm play the most significant role in terms of atmospheric chemistry and physics [Finlayson-Pitts and Pitts, 2000]. Aerosol particles have both natural and anthropogenic sources, with approximately 90% of emissions coming from natural sources [Andreae, 1995]. Atmospheric aerosols can be emitted directly as particles (primary aerosols) or formed in the atmosphere by gas-to-particle conversion processes (secondary aerosols) [Seinfeld and Pandis, 1998]. Table 1.1 provides the source strengths of various natural and anthropogenic species. Soil dust, sea salt and oceanic sulphate represent the largest fraction of the total natural aerosol abundance, while industrial dust and sulphates are the major components of aerosols produced by anthropogenic activities [d’Almeida et al., 1991; Andreae, 1995]. In the atmosphere, these particles have impacts in areas ranging from human health and visibility to atmospheric chemistry and climate.  Chapter 1  Table 1.1:  2  Source strengths of various aerosol species [adapted from d’Almeida et al., 1991; Andreae, 1995].  Source  Column Emission, burden, -1 Tg yr mg m-2  Natural Primary Soil dust Sea-salt Volcanic dust Biological debris  1500 1300 33 50  32.2 7.0 0.7 1.1  Secondary Sulphates Organic matter Nitrates  102 55 22  2.7 2.1 0.5  Total Natural  3060  46  Primary Industrial dust Black carbon  100 20  2.1 0.6  Secondary Sulphates Biomass burning Nitrates Organic matter  140 80 36 10  3.8 3.4 0.8 0.4  Total Anthropogenic  390  11.1  TOTAL Anthropogenic fraction  3450 11%  57 19%  Anthropogenic  Chapter 1  3  1.1.1 Aerosol impacts on climate In its Fourth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) stated that “it is extremely likely1 that humans have exerted a substantial warming influence on climate” [Forster et al., 2007]. Radiative forcing (RF) is a concept used to quantitatively compare the strengths of various human and natural agents in causing climate change, and is defined as “a change (usually expressed in Wm-2) in the average net radiation at the tropopause due to a particular perturbation of interest” [Forster et al., 2007]. Figure 1.1 illustrates the current best estimates for the RFs of a number of climate system components from 1750 to 2005. There are two major reasons for researching the potential climate impacts of atmospheric aerosols. First, despite recent improvements in atmospheric models to represent all aerosol components of significance, the Fourth Assessment Report lists the level of scientific understanding for the total aerosol RF as low to medium-low. Second, the estimated cooling effect due to aerosols (RF = –0.4 to –2.7 Wm-2) is potentially as large as the estimated warming effect due to rising greenhouse gas concentrations (RF = +2.4 to +2.9 Wm-2) [Forster et al., 2007]. Further study in this field will strengthen the scientific community’s understanding of the mechanisms by which aerosols contribute to climate change and decrease the relatively large uncertainties associated with the current best estimates for the total aerosol RF.  1  The use of ‘extremely likely’ is an example of the calibrated language used in the IPCC’s Fourth Assessment Report; it represents a 95% confidence level or higher.  –2  –1.5  –1  0  0.5 Radiative Forcing (W m-2)  –0.5  1.5  1  2  High  Halocarbons  Cloud albedo effect  High  N2O  Low  Low  Med-low  High  CH4  Direct effect  High  CO2  Figure 1.1: Global mean RFs from selected agents and mechanisms, including levels of scientific understanding. [adapted from Forster et al., 2007]  Solar irradiance  Total aerosol  Long-lived greenhouse gases  Radiative Forcing Terms  Anthropogenic  Natural  Scientific understanding  Chapter 1 4  Chapter 1  5  The total aerosol RF can be considered as resulting from two different effects. The direct aerosol effect is defined as “the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system” [Forster et al., 2007]. Of greater relevance to this thesis is the indirect aerosol effect, defined as “the mechanism by which aerosols modify the microphysical and hence the radiative properties, amount and lifetime of clouds” [Forster et al., 2007]. The aerosol indirect effect can be further classified into: (1) the cloud albedo effect, which includes the effects of aerosols on the cloud droplet number concentration and hence cloud droplet size for fixed water content, [Twomey, 1974] and (2) the cloud lifetime effect, relating to changes in the water content, cloud height and lifetime of clouds caused by aerosols [Albrecht, 1989] Aerosols contribute to these two effects primarily through their ability to act as cloud condensation nuclei (CCN) or ice nuclei (IN). An increase in aerosol concentration and hence an increase in the concentration of IN may lead to smaller cloud particles for a fixed water content (cloud albedo effect). A greater concentration of IN can also affect the precipitation efficiency, lifetime and optical depth of clouds (cloud lifetime effect). These processes are illustrated in Figure 1.2.  increased optical depth → less radiation reaches Earth’s surface  smaller cloud particles → less precipitation  more reflection = higher albedo  Figure 1.2: Schematic diagram of two aerosol indirect effects. The cloud on the left is representative of a normal atmospheric aerosol concentration, while the one on the right reflects an elevated concentration [adapted from Forster et al., 2007].  solar radiation  cloud particle (water droplet or ice crystal)  aerosol particle (ice nucleus)  Chapter 1 6  Chapter 1  7  1.1.2 Aerosols as atmospheric ice nuclei The formation of ice in the atmosphere occurs by one of two mechanisms: homogeneous nucleation or heterogeneous nucleation. Homogeneous ice nucleation is defined as the formation of ice from the gas or the liquid phase in the absence of foreign particles [Seinfeld and Pandis, 1998], and as such will not be discussed any further. Heterogeneous ice nucleation refers to the formation of an ice germ on a foreign substance such as an aerosol particle [Seinfeld and Pandis, 1998], and is thus of particular relevance to the studies presented hereafter. Figure 1.3 summarizes the four modes of heterogeneous ice nucleation proposed by Vali [1985]. Deposition nucleation occurs when water vapour in an environment that is supersaturated with respect to ice and at temperatures below 0ºC is adsorbed directly from the vapour phase onto the surface of an ice nucleus, where it transforms into ice. Condensation freezing occurs at relative humidities above water saturation, where an aerosol first acts as a cloud condensation nucleus to form a droplet, which then freezes. Immersion freezing refers to the process where an ice nucleus first becomes immersed in a water droplet, then freezes once its temperature has become sufficiently low. In contact freezing, ice nucleation is initiated when the ice nucleus comes in contact with a supercooled droplet.  Deposition nucleation Condensation freezing Immersion freezing Contact freezing  Liquid phase  Time ↑, Temperature ↓, RH ↑  Ice phase IN (solid, insoluble)  Figure 1.3: Heterogeneous modes of ice nucleation  Chapter 1  8  1.2 Mineral Dust Mineral dust particles are ubiquitous and abundant in the atmosphere, with estimates of global dust emissions to the atmosphere ranging from 500 to 5000 Mt/year [Peterson and Junge, 1971; d’Almeida, 1986; Schütz, 1987; Andreae, 1995; Duce, 1995; Tegen and Fung, 1995]. Mineral dust aerosols are soil particles that have been mobilized by strong wind currents and entrained into the atmosphere, and thus contain many of the same components as crustal rock [Usher et al., 2003]. Table 1.2 lists some common minerals found in aerosolized dust and their chemical formulae. Table 1.2: Common minerals present as atmospheric aerosols and their chemical formulae [Berry et al., 1983; Anthony et al., 1995]. Mineral  Formula  Calcite  CaCO3  Chlorite  Y6Z4O10(OH)8  Corundum  α–Al2O3  Feldspar  XZ4O8  Gypsum  CaSO4 ⋅ 2H2O  Hematite  α–Fe2O3  Kaolinite  Al4Si4O10(OH)8  Mica  XY2-3Z4O10(O,F,OH)2  Montmorillonite  (Na,Ca)(Al,Mg)6(Si4O10)3(OH)6 ⋅ nH2O  Palygorskite  (Al,Mg)2Si4O10(OH) ⋅ 4H2O  Quartz  SiO2 where X = Na, Ca or K Y = Al, Mg or Fe Z = Al or Si  Chapter 1  9  Table 1.3 lists the relative mineral composition of aerosol transported from North Africa and collected in Sal Island in the Cape Verde Islands, Barbados, and Miami [Glaccum and Prospero, 1980]. As dust is transported farther away from the source region, the larger, coarser particles typically composed of quartz and feldspars fall out of the atmosphere via gravitational settling. As a result, the overall composition tends to become enriched with the smaller, finer particles such as clays (e.g. kaolinite, montmorillonite) and micas (e.g. muscovite) [Usher et al., 2003].  Table 1.3: Mineralogy of Saharan dust collected at Sal Island (in the Cape Verde Islands), Barabados, and Miami, FL, after three large Saharan dust outbreaks. Percentages shown are averages of the three outbreaks (assuming 100% crystalline material) [Glaccum and Prospero, 1980].  Mineral  Sal Island  Barbados  Miami  Mica  53.8%  64.3%  62.1%  Quartz  19.6%  13.8%  14.2%  Kaolinite  6.6%  8.3%  7.1%  Calcite  8.2%  3.9%  6.9%  Plagioclase†  5.4%  4.1%  4.5%  Chlorite  4.3%  4.1%  4.2%  Montmorillonite  ≤5%  ≤5%  ≤5%  †  member of the feldspar mineral group  Chapter 1  10  1.3 The Troposphere The region of the atmosphere extending from the Earth’s surface to the height where air ceases to become colder with height is called the troposphere [Finlayson-Pitts and Pitts, 2000]. Figure 1.4 illustrates the mean temperature of the Earth’s atmosphere as a function of altitude and gives the height of the troposphere as approximately 11 km [Ahrens, 1994], but its height ranges from 8 to 18 km depending on the latitude and season [Wayne, 2000]. The troposphere is kept well stirred by rising and descending air currents, and contains all of the weather we are familiar with on Earth [Ahrens, 1994]. In order to maximize the atmospheric relevance of the research presented in this thesis, the experiments described herein were performed over a temperature range (233 to 247 K) that is relevant for the troposphere.  Figure 1.4: Mean temperature profile of the Earth’s atmosphere as a function of altitude [adapted from Ahrens, 1994]  Chapter 1 11  Chapter 1  12  1.4 Motivation for Research Particulate matter such as mineral dusts can have a number of effects in the atmosphere, but here the focus is on climate effects. Climate forcing by particulates is currently recognized as one of the greatest uncertainties in global circulation models, which are used to make predictions about climate change [Haywood and Boucher, 2000; Sokolik et al., 2001]. Particulates can alter the chemical balance of the atmosphere, changing its thermodynamic and optical properties, which can result in climate forcing as the absorption and scattering of solar radiation is affected [Usher et al., 2003]. The relative impact of atmospheric mineral dusts on climate forcing remains poorly quantified due to an incomplete understanding of the transport and removal processes, and the chemical and physical properties of the particles [Sokolik, 2001]. In its latest assessment report, the IPCC only considered the contribution of CCN to the RF resulting from the indirect aerosol effect. Although the IPCC acknowledges that IN such as mineral dust particles likely play a key role in cloud formation processes, understanding of this role was deemed insufficient [Forster et al., 2007]. This lack of understanding, which in turn contributes to the large uncertainties associated with the estimated RF resulting from aerosol effects, is the key motivation for the research presented in this thesis.  Chapter 1  13  1.5 Thesis Overview This thesis is presented in four chapters. This chapter discussed relevant background information relating to aerosols, ice nuclei, mineral dust and the atmosphere. Chapter 2 focuses on the ice nucleating properties of uncoated mineral dusts. Onset relative humidities are reported, heterogeneous nucleation rates (Jhet) are calculated, and then classical nucleation theory is employed to determine the contact angles (θ) between the critical ice nuclei and the mineral surfaces. Chapter 3 focuses on the effects of inorganic coatings (H2SO4 and (NH4)2SO4) on the ice nucleating properties of kaolinite. Finally, a brief summary of the results and suggestions for future experiments are presented in Chapter 4.  Chapter 2  14  2. Ice nucleation on mineral dust particles: onset conditions, nucleation rates and contact angles 2.1 Introduction The scientific community’s lack of data on ice nucleation processes is a major obstacle for the simulation of the complex interactions between aerosols and cold clouds. It has been shown in various investigations [Girard and Curry, 2001; Girard et al., 2005; Avramov, 2006] that mixed-phase clouds often cannot be properly simulated with the existing parameterizations such as those of Fletcher [1962] and Meyers et al. [1992]. More physically-based parameterizations are needed to simulate heterogeneous nucleation in climate models. Mineral dust particles are abundant in the atmosphere, and both laboratory [Pruppacher and Klett, 1997; Bailey and Hallett, 2002; DeMott, 2002; Zuberi, 2002; Hung et al., 2003; Archuleta et al., 2005; Mangold et al., 2005; Möhler et al., 2006; Field et al., 2006; Knopf and Koop, 2006; Marcolli et al., 2007; Zimmermann et al., 2007] and field measurements [Sassen et al., 2002; DeMott et al., 2003a; DeMott et al., 2003b; Sassen et al., 2003; Toon, 2003; Cziczo et al., 2004; Sassen, 2005; 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 supersaturations and temperatures required for ice formation compared to homogeneous nucleation. At the same time, field measurements have shown that mineral dust particles can have a significant impact on cloud formation, cloud properties and precipitation [Sassen et al., 2002; DeMott et al., 2003a; Sassen et al., 2003; Sassen, 2005]. 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, measurements have shown that mineral dust comprises a significant fraction of atmospheric ice nuclei. [Chen et al., 1998; DeMott et al., 2003b]. 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 role  Chapter 2  15  these particles play as atmospheric ice nuclei [Vali, 1996; Martin, 2000; Cantrell and Heymsfield, 2005]. Only a few of the most abundant types of minerals found in the atmosphere have been studied in detail. In addition, many of the previous studies only reported the onset conditions for ice nucleation; only a few considered nucleation rates (a key parameter for describing ice nucleation). Measurements of nucleation rates are needed in order to more accurately compare laboratory results and to extrapolate laboratory data to the atmosphere. 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. This chapter focuses on the ice nucleating properties of mineral dust particles at temperatures between 233 and 247 K, a temperature range relevant for the troposphere. To date, few measurements have been made at these particular temperatures. Five commercial mineral dusts were selected for this study. Specifically, the ice nucleation properties of muscovite (a common mineral in the mica group), kaolinite, montmorillonite, quartz and calcite particles were investigated. These minerals were chosen since they are major components of aerosolized mineral dust found in the atmosphere. To illustrate this point, the mineralogy of Saharan dust collected in Sal Island, Barbados and Miami after three large Saharan dust outbreaks are shown in Table 1.3 on page 9 [Glaccum and Prospero, 1980]. The minerals chosen for this study represent over 90% of the total mass during these outbreaks. Montmorillonite composition was below the detection limit (5%) in the Saharan dust studies illustrated in Table 1.3, but several studies have cited montmorillonite as one of the dominant clay minerals present in African and Asian dusts [Schütz, 1989; Schütz, 1997; Prospero, 1999; Hanisch and Crowley, 2001]. This chapter is organised as follows: first, measurements of the onset conditions for ice nucleation on muscovite, kaolinite, montmorillonite, quartz and calcite are presented. Here, the onset conditions are defined as the relative humidity with respect to ice (RHi) and temperature at which the first ice nucleation event was observed. Second, these data are compared with existing data in the literature. Third, heterogeneous ice nucleation rates (number of nucleation events per unit surface area of solid material per  Chapter 2  16  unit time) are determined from the onset data for each mineral type. Fourth, the heterogeneous nucleation rates are parameterized using classical nucleation theory, and contact angles between the mineral surfaces and the critical ice nuclei are calculated.  2.2 Experimental 2.2.1 Overview The apparatus used in this study consisted of an optical microscope coupled to a flow-cell (see Figure 2.1) in which the humidity and temperature could be accurately controlled. Mineral dust particles were deposited on the bottom surface of the flow cell and the relative humidity inside the cell was slowly increased. Ice crystal formation was observed with a reflected-light microscope (Zeiss Axiotech 100) equipped with a 10× objective lens. From these measurements, the onset conditions (temperature and relative humidity) for ice nucleation were determined.  Figure 2.1: Schematic of the flow-cell used for these studies.  Chapter 2  17  2.2.2 Slide preparation The bottom surface of the flow cell was a glass cover slide that had been treated with dichlorodimethylsilane (DCDMS) to make a hydrophobic layer, which reduced the probability of ice nucleation directly on the surface. Each glass slide was first cleaned with a dry ice cleaning system (Sno Gun-IITM, Va-Tran Systems) to remove any coarse impurities from the slide surface. The slides were then immersed in “piranha” solution (3:1 mixture by volume of concentrated sulphuric acid and 30% hydrogen peroxide) for approximately 10 minutes, then rinsed with high purity water (distilled water further purified with a Millipore Simplicity 185 system, 18.2 MΩ) and methanol (HPLC grade). The slides were then dried with a flow of high-purity N2 (99.999%). Finally, each glass slide was treated with the dry ice cleaning system and rinsed with high purity water and methanol for a second time. After being dried with high-purity N2 gas once again, the clean slide was placed in an airtight glass chamber. The final step involved adding 3 droplets of DCDMS solution (Fluka, 5% DCDMS in heptane) to the chamber, but not directly on the slides. Left overnight, the DCDMS coated the glass slides via vapour deposition. 2.2.3 Sample preparation Five different types of mineral dust were studied in these experiments. Muscovite particles were generated by grinding muscovite flakes (1–10 mm in diameter, <1 mm in thickness, purchased from Alfa Aesar) with a mortar and pestle. Quartz particles were produced by grinding a sample of quartz tubing (purchased from United Silica) with a mortar and pestle. Kaolinite (purchased from Fluka), montmorillonite (purchased from Fluka) and calcite (purchased from Puratronic®, Alfa Aesar) particles were used as supplied. The chemical formulae of these minerals are listed in Table 2.1.  Chapter 2  18  Table 2.1: Chemical formulae of 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)(Al,Mg)6(Si4O10)3(OH)6 • nH2O  All samples were prepared and the flow cell constructed 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 the 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 hydrophobic 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 µm in diameter. The optical resolution limit of the microscope was ~1 µm. A typical sample held between 100 to 1000 individual particles, the majority of which were between 1 and 20 µm in diameter.  Chapter 2  19  2.2.4 Flow cell The flow cell, illustrated previously in Figure 2.1, was positioned on a cooling stage. The temperature of the cooling stage and hence the flow cell was regulated with a refrigerating circulator (Thermo Neslab ULT-95). The 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 hydrophobic glass slide and the flow cell body. This ensured that the coldest component of the flow cell was the glass substrate (by ~10 K), thus preventing ice nucleation from occurring anywhere else in the cell. All seals within the cell were made with Viton Orings with the exception of the seal between the glass slide and the PCTFE spacer, which was made with low vapour pressure chlorotrifluoroethylene grease (Series 28LT from Halocarbon Products). 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. 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.  Chapter 2  20  2.2.5 Humidity control During the ice nucleation experiments, a flow of humidified helium gas was introduced to one side of the cell and exited on the other where its frost point was measured with a hygrometer (General Eastern 1311DR). Helium was chosen as the carrier gas because it diffuses to the particles on the slide more rapidly than nitrogen or argon. From the frost point measurements, the saturation vapour pressure over ice (pice) in units of Pa was calculated using the parameterization of Murphy and Koop [2005], given by: pice = exp(9.550426 – 5723.265/T + 3.53068ln(T) – 0.00728332T);  T > 110 K  (2.1)  The saturation vapour pressure of supercooled water (pliq) in units of Pa was calculated from the temperature using another parameterization proposed by Murphy and Koop [2005], given as follows: ln(pliq) = 54.842763 – 6763.22/T – 4.210ln(T) + 0.000367T + tanh{0.0415(T – 218.8)}(53.878 – 1331.22/T – 9.44523ln(T) + 0.014025T) for 123 < T < 332 K  (2.2)  Figure 2.2 illustrates the difference between pliq and pice as a function of temperature. For the temperature range used in these studies (233 to 247 K), the saturation vapour pressure over liquid water is 6 to 16 Pa greater than that over ice.  Chapter 2  21  30 25  pliq – pice  20 15 10 5 0  220  230  240  250  260  270  Temperature (K) Figure 2.2: Variation of pliq – pice as a function of temperature. The shaded region highlights the temperature range studied in these experiments.  Chapter 2  22  The relative humidity with respect to ice (RHi) inside the flow cell was determined as follows: p ice (Tfrost ) × 100% p ice (Tcell )  RH i =  (2.3)  where pice(Tfrost) is the vapour pressure of water inside the cell, calculated from Equation 2.1 using frost point (Tfrost) measurements made by the hygrometer, and pice(Tcell) is the saturated vapour pressure (with respect to ice) inside the cell, also calculated from Equation 2.1, but at the temperature of the cell (Tcell). The relative humidity with respect to water (RHw) is determined as follows:  RH w =  p ice (Tfrost ) × 100% p liq (Tcell )  (2.4)  where pliq(Tcell) is the saturated vapour pressure (with respect to water) inside the cell, calculated from Equation 2.2 at the temperature of the cell. The 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 humidity was set 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 min-1 (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 µm filter (Anodisc 25).  Chapter 2  23  2.2.6 Temperature calibration In the experimental apparatus (illustrated previously in Figure 2.1), a Pt-100 resistance temperature detector (RTD) was located beneath the slide supporting the mineral dust particles. The temperature reported by the RTD was slightly lower than that of the dust particles being studied. Calibrations were performed to determine the offset between the temperature given by the RTD and the temperature of the particles by observing the change in size of ice crystals on the slide as the temperature was ramped up and down. The RTD was calibrated against the frost point within the cell, as done previously by Middlebrook et al. [1993] and Parsons et al. [2004]. The temperature at which the size of the ice crystals remained constant was determined. At this temperature, the ice crystals were in equilibrium with the water vapour inside the cell, whose frost point was precisely known. The temperature offset determined using this technique was used to correct experimentally measured temperatures obtained with the RTD. The results from one calibration experiment performed at a frost point of 236.7 K are shown in Figure 2.3. The offset between the RTD and the hygrometer was determined to be 0.21ºC at 246.7 K, 0.31ºC at 241.7 K, and 0.43ºC at 236.7 K. A calibration curve, illustrated in Figure 2.4, was generated from these data and used to correct the RTD reading in all experiments.  Chapter 2  24  Ice crystal area (µm2 )  3000 2500 2000 1500 1000 500 –36.0 –36.2 –36.4  hygrometer  Temperature (ºC)  –36.6 –36.8 –37.0 –37.2 –37.4 –37.6  RTD  –37.8 –38.0 0  15  30  45  60  75  90  105  120  135  Time (minutes)  Figure 2.3: Ice crystal area and temperature as a function of time for a typical calibration experiment. By averaging the three highlighted results, the offset between hygrometer and the RTD at this temperature was determined to be 0.43 ± 0.19ºC.  Chapter 2  25  0.7 Offset between stage and RTD  Temperature offset (K)  0.6 0.5 0.4 0.3 0.2 0.1 0.0 230 232 234 236 238 240 242 244 246 248 250 Cell temperature (K) Figure 2.4: Calibration curve used to correct for the temperature difference between the RTD in the aluminum stage and the surface of the slide in the flow cell.  Chapter 2  26  2.2.7 Experimental procedure In each nucleation experiment, the RHi in the flow cell was ramped from approximately 95% to water saturation (135–150%, depending on the temperature) by decreasing the temperature of the cell at approximately 0.08 K min-1 while maintaining a constant frost point inside the cell. During these experiments, the RHi increased at an approximate rate of 1% min-1. Figure 2.5 illustrates the three experimental RHi trajectories used in these studies, taken at frost points of 236.7, 241.7, and 246.7 K.  160 150  236.7 K 241.7 K  140 RHi (%)  246.7 K  wate r sat urat  130 120  io n  110 ice saturation  100 90 230  235  240  245  250  Temperature (K) Figure 2.5: Experimental trajectories used in these studies. Each trajectory is labelled with its corresponding frost point.  255  Chapter 2  27  Images of the dust particles were recorded digitally every 20 seconds (corresponding to a temperature decrease of ~0.027 K or an RHi increase of ~0.3%), while simultaneously recording pice and the cell temperature. From these images, the RHi at which ice crystals first formed was determined (i.e. the onset of ice nucleation). Shown in Figure 2.6 are images of kaolinite particles recorded in a typical freezing experiment before and after ice nucleation, respectively. The formation of ice crystals is clearly discernable.  (a)  (b)  200 µm Figure 2.6: Optical microscope images (100×) of kaolinite particles before (left) and after (right) ice nucleation.  Chapter 2  28  2.3 Results and Discussion 2.3.1 Onset conditions Shown in Figure 2.7 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. Based on Figure 2.7, 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 humidities 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 differences in the ice nucleating abilities of the five minerals studied over this temperature range. The reason why some minerals are better ice nuclei than others is still relatively poorly understood. 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 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 kaolinite’s ability to act as an ice nucleus may be due to the pseudo-hexagonal arrangement of the hydroxyl (–OH) groups at the kaolinite surface [Pruppacher and Klett, 1997]. These results provide further data that can be used to test these various theories.  Chapter 2  29  150%  Water saturation Kaolinite Quartz Montmorillonite Calcite Muscovite  145% 140% 135%  RHi  130% 125% 120% 115% 110% 105% 100% 230  235  240 245 Temperature (K)  250  255  Figure 2.7: Onset conditions for all minerals studied  For all results where the onset conditions were below the water saturation line in Figure 2.7, the data correspond to deposition freezing, which occurs when water vapour adsorbs onto a solid surface and is then transformed into ice [Vali, 1996]. The data that overlap with the water saturation line in Figure 2.7, on the other hand, may have occurred by either deposition freezing or condensation freezing. Condensation freezing refers to the sequence of events whereby liquid water first condenses followed by freezing of the liquid [Vali, 1996]. In these experiments, there was no observation of liquid water droplet formation prior to ice, but the size of the water droplets may have been below the detection limit of the microscope, or else the water droplets only formed for a short amount of time and were not observed.  Chapter 2  30  The mineral particles in these experiments are suspended on a hydrophobic glass substrate. Before further discussing the data, the possible effect of the hydrophobic glass substrate on the ice nucleation results must be addressed. First, in blank experiments (when no mineral dust was used) ice nucleation did not occur below liquid water saturation, and liquid water droplets always formed (and were clearly visible with the microscope) when liquid water supersaturation was reached [Dymarska et al., 2006]. Occasionally, ice would form after the formation of liquid water droplets, but liquid water droplets were always observed first. This is very different from the results observed with mineral dust particles, where in many cases ice nucleation occurred below water saturation, and there was never any indication of the formation of liquid water droplets. Second, direct observations of the optical images confirmed that ice nucleation always occurred on a mineral particle, rather than a bare spot on the hydrophobic glass substrate. Third, tests were performed with two other types of supports to further ensure that the hydrophobic glass slide was not influencing the results. The ice nucleation experiments were repeated using bare glass slides that were cleaned as usual, but not coated with the hydrophobic DCDMS layer, resulting in a hydrophilic substrate. Also, experiments were performed using thin Teflon sheets as the bottom surface of the flow cell. Within the uncertainty of the measurements, the onset results were independent of the type of support used, providing further confidence that the hydrophobic glass support is not influencing the results. Below, the onset conditions from this study are compared with results published in the literature. The comparison below focuses mainly on results from five different studies: Roberts and Hallett [1968], Bailey and Hallett, [2002], Dymarska et al. [2006], Kanji and Abbatt [2006], and Zimmermann et al. [2007]. Some of the experimental conditions from these studies are listed in Table 2.2 for reference.  Chapter 2  31  Table 2.2: Summary of experimental conditions for previously published results Size range (µm)  Number of particles  Surface Area Range (cm2)  Current study  1 – 50  102 – 103  3×10-6 – 1×10-2  Roberts and Hallett, [1968]  0.5 – 3  101 – 104  8×10-8 – 3×10-3  Bailey and Hallett, [2002]  5 – 10  Not determined  Not determined  Kanji and Abbatt, [2006]  0.5 – 5  5×102 – 3×104  9×10-5 – 3×10-2  Dymarska et al., [2006]  1 – 20  2×102 – 8×102  6×10-6 – 1×10-2  Zimmermann et al., [2007]  1 – 10  Not determineda  Not determineda  Study  a  Zimmermann et al. [2007] spread particles on a silicon plate (5mm×5mm), 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. In Figure 2.8, the ice nucleation results for kaolinite are compared with previous measurements. There have only been two previous studies [Dymarska et al., 2006; Bailey and Hallett, 2002] conducted in the same temperature range as these experiments. The data from Dymarska et al. [2006] are in agreement with the results presented in this thesis, which is not surprising since the same instrument and experimental protocol were employed. The results from Bailey and Hallett [2002], however, differ from the results presented in this thesis 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 experimental 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.  Chapter 2  32  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.  155%  Water saturation Roberts & Hallett, 1968 Schaller & Fukuta, 1979 Bailey & Hallett, 2002 Dymarska et al., 2006 Zimmermann et al., 2007 This study  150% 145% 140% 135%  RHi  130% 125% 120% 115% 110% 105% 100% 95% 230  235  240  245  250  255  260  265  Temperature (K)  Figure 2.8: Summary of ice nucleation results for kaolinite  Not included in Figure 2.8 are the results from Salam et al. [2006]. These authors investigated ice nucleation between 233 K and 258 K using a continuous flow diffusion chamber. They observed that 0.5% of the kaolinite particles activated as ice nuclei even at the lowest supersaturations with respect to ice (close to 100% RHi). The fraction of particles activated remained almost constant until above 120–130% RHi, at which point the fraction activated increased sharply. The conditions they report for 0.5% of particles being activated are consistent with the results presented in this thesis.  Chapter 2  33  Considering all of the data from Figure 2.8, 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 temperatures below 250 K, as mentioned above. In Figure 2.9, the ice nucleation results for montmorillonite from these experiments are compared with previous data from Kanji and Abbatt [2006] and Zimmermann et al. [2007]. The results are consistent with the more recent study by Zimmermann et al. [2007] when extrapolated to warmer temperatures, but appear to be inconsistent with the results from Kanji and Abbatt [2006]. Not shown in Figure 2.9 are results from Roberts and Hallett [1968] and Salam et al. [2006]. Roberts and Hallett [1968] observed ice nucleation at 248 K and at liquid water saturation when using approximately 104 particles, a result that is consistent with those presented in this thesis. Salam et al. [2006] 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 233 to 258 K. The fraction of activated particles remained almost constant until above approximately 107–115% RHi, at which point the fraction of activated particles increased sharply. The conditions they report for 1.2% of particles being activated are inconsistent with the data presented for montmorillonite in Figure 2.7 at the warmest temperature (245 K), but consistent at the colder temperatures (236 K and 241 K).  Chapter 2  34  160% Water saturation Kanji & Abbatt, 2006 Zimmermann, 2007 This study  150% 140%  RHi  130% 120% 110% 100% 215  220  225  230  235  240  245  250  255  260  265  Temperature (K) Figure 2.9: Summary of ice nucleation results for montmorillonite  Overall, there appear to be large differences between some of the montmorillonite studies. Some possible reasons for these differences are listed above in the discussion of the kaolinite results. 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 nebulizing an aqueous suspension of the mineral dust. In contrast, all other studies mentioned above for montmorillonite used a dry deposition technique. Future studies that investigate the effect of these different parameters are needed for a complete  Chapter 2  35  understanding of ice nucleation on mineral particles. At this point, it is reasonable to say that the scientific community’s understanding of the ice nucleating properties of montmorillonite remains poor, with results appearing dependent on both the experimental protocol and experimental apparatus. 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. This result does not contradict the results for muscovite presented in Figure 2.7, but it should be noted that the temperature range for these experiments does not overlap with the range reported by Mason and Maybank [1958]. There are no previous reports of the onset conditions for ice nucleation on quartz particles. Mason and Maybank [1958] reported that this mineral was inactive as an ice nucleus at temperatures above 248 K, which is consistent with the results presented for quartz in Figure 2.7. For calcite, Roberts and Hallett [1968] reported one temperature (255 K) and RHi (120%) at which one particle in 104 nucleated ice. Since the temperature measured by these authors is significantly above the temperature range used in these experiments, a direct comparison is not possible; nevertheless, the results from Roberts and Hallett [1968] do not contradict the results presented for calcite in Figure 2.7. 2.3.2 Heterogeneous nucleation rates Onset conditions (RHi and temperature) 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. The heterogeneous nucleation rate is related to onset data through the following equation [Pruppacher and Klett, 1997]:  J het =  ω As t  (2.5)  Chapter 2  36  where ω 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 in these experiments, ω was equal to one. The surface area available for nucleation was determined directly from the optical microscope images using digital imaging software (Northern Eclipse). The observation time for the nucleation event was 20 ± 10 s (equal to the time between image captures). The uncertainty in Jhet was calculated by considering the uncertainties in As and t. 2.3.3 Classical nucleation theory  The applicability of classical nucleation theory to heterogeneous nucleation on mineral dust particles remains to be determined. In fact, some measurements show that for precise predictions, active site theory is required [Hung et al., 2003; Archuleta et al., 2005; Marcolli et al., 2007]. Nevertheless, classical nucleation theory is a relatively convenient and simple way to parameterize laboratory data, and has been used in the past to describe heterogeneous nucleation in atmospheric cloud models [Karcher, 1996; Jensen and Toon, 1997; Jensen et al., 1998; Karcher et al., 1998; Karcher, 1998; Hung et al., 2003]. The significance of applying classical nucleation theory in order to calculate  parameters such as contact angles (θ) lies in the fact that these data can be used to make predictions about ice nucleation for varying particle size, shape, composition, or observation time. These data are necessary to extrapolate laboratory data to the atmosphere and predict ice cloud formation accurately for a variety of conditions. Hence, classical nucleation theory is a reasonable starting point for analyzing the onset data from these experiments. For this analysis, only the case of deposition freezing is considered. As a result, contact angles were not reported for any nucleation data where the onset conditions surpassed or overlapped the liquid water saturation line, since in these cases nucleation may have occurred by either deposition or condensation freezing, as discussed previously.  Chapter 2  37  According to classical nucleation theory, the rate of heterogeneous nucleation by deposition freezing is defined as [Pruppacher and Klett, 1997]:  J het = A· exp  – ∆Fg ,het kT  (2.6)  where A is the pre-exponential factor in units of cm-2 s-1, ∆Fg,het is the free energy of formation of the critical embryo in J, k is the Boltzmann constant in J K-1, 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]:  ∆Fg ,het  16πM 2w σ i3/ v = · f ( m, x ) 3 [R T ρ ln Si ]  (2.7)  where Mw is the molecular weight of water in g mol-1, σi/v is the surface tension at the icevapour interface in mJ m-2, R is the universal gas constant in J mol-1 K-1, ρ is the density of ice in g cm-3, 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 cosθ, where θ is the contact angle between the critical ice nucleus and the mineral surface. Assuming the radius of the substrate to be much larger than the radius of the ice germ (a good approximation under the experimental conditions), f(m,x) is defined as follows [Pruppacher and Klett, 1997]:  f ( m, x ) =  m 3 – 3m + 2 4  (2.8)  To calculate θ, the free energy of formation of the crucial embryo was calculated from the experimentally determined Jhet values using Equation 2.6, assuming a pre-  Chapter 2  38  exponential term (A) equal to 1025 cm-2 s-1 [Pruppacher and Klett, 1997]. Then, θ was calculated using Equations 2.7 and 2.8, assuming the density of ice (ρ) is 0.92 g cm-3 [CRC, 2001-2002], Mw is 18.015 g mol-1, and σi/v equals 106 ± 5 mJ m-2 [Pruppacher and Klett, 1997]. For σi/v, the surface tension appropriate for hexagonal ice was used. 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 2.3 lists the nucleation rates and contact angles determined from the onset data. The uncertainties in the surface area and observation time were used to calculate upper and lower limits for Jhet and θ. The data show that the experiments are typically sensitive to values of Jhet ranging from 60 to 1100 cm-2s-1, and contact angles ranging from 0º to 30º. The contact angle results are also plotted as a function of temperature in Figure 2.10. The data show that for efficient ice nuclei such as muscovite and kaolinite, the contact angles are small (less than 15º). For poor ice nuclei such as quartz and calcite, the contact angles are larger (greater than 20º). The contact angles measured for montmorillonite were less than 15º at temperatures below 241 K, and greater than 20º at higher temperatures. These data may be useful for future modeling studies of ice nucleation in the atmosphere and for comparing results between different laboratories.  Chapter 2  39  Table 2.3: J values and contact angles for all five minerals studied.  θlower  θ  θupper  (º)  (º)  (º)  370  5.3  9.7  13.7  160  350  2.5  7.8  12.2  70  110  240  3.4  9.2  13.0  107 ± 5  280  470  1030  7.2  10.7  14.5  241.3  106 ± 6  700  1100  2400  5.6  9.5  14.0  Muscovite  237.0  102 ± 2  150  240  540  0.7  6.2  11.1  Montmorillonite  244.6  124 ± 5  140  230  500  19.8  22.4  24.8  Montmorillonite  240.8  110 ± 4  260  440  960  11.1  14.4  17.3  Montmorillonite  236.0  106 ± 4  280  470  1020  7.8  12.0  15.4  Quartz  243.9  133 ± 1  110  180  400  –  –  –  Quartz  238.7  137 ± 3  100  170  380  24.7  27.2  29.6  Quartz  234.0  136 ± 4  150  240  530  23.9  26.4  28.8  Calcite  244.1  130 ± 3  190  310  680  –  –  –  Calcite  238.9  135 ± 3  80  130  280  24.0  26.5  28.9  Calcite  234.2  132 ± 6  40  60  140  22.5  25.0  27.5  Temp.  RHi  Jhet, low  (K)  (%)  (cm-2s-1) (cm-2s-1) (cm-2s-1)  Kaolinite  246.1  105 ± 5  100  170  Kaolinite  241.1  104 ± 5  100  Kaolinite  236.4  104 ± 2  Muscovite  246.2  Muscovite  Mineral  Jhet  Jhet, up  Chapter 2  40  40 Quartz Calcite Montmorillonite Muscovite Kaolinite  35  Contact Angle (degrees)  30 25 20 15 10 5 0 230  235  240  245  Temperature (K)  Figure 2.10: Contact angle as a function of temperature for each mineral studied.  250  Chapter 3  41  3. The effects of sulphuric acid and ammonium sulphate coatings on the ice nucleating properties of kaolinite 3.1 Introduction  When first emitted into the atmosphere, mineral dust particles are likely uncoated. During their lifetime in the atmosphere, which can be hours to weeks depending on particle size [Seinfeld and Pandis, 1998], mineral dusts can become coated with inorganic and/or organic material [Usher et al., 2003]. The following chapter summarizes an investigation into the effects of H2SO4 and (NH4)2SO4 coatings on the ice nucleating properties of mineral dust particles. This research should be useful when trying to determine if anthropogenic emissions of gases such as SO2 and NH3 affect climate by “poisoning” natural ice nuclei such as mineral dust. Additional motivation for this study comes from field measurements made in the Arctic that showed that sulphate coatings drastically decrease the ice nucleating properties of aerosol particles. Borys [1989] reported that the fraction of ice nucleating particles in Arctic haze, which is composed of a wide variety of pollutants including sulphate, was 10 to 1000 times smaller than in Arctic aerosol unaffected by pollution. This investigation focuses on the ice nucleating properties of kaolinite particles in the presence and absence of H2SO4 and (NH4)2SO4 coatings at temperatures between 233 and 247 K, a temperature range relevant for the troposphere. Prior to this work, there have only been two studies that investigate the effect of H2SO4 coatings on the ice nucleating properties of mineral dust particles at temperatures relevant for the troposphere. Knopf and Koop [2006] studied the ice nucleating properties of Arizona test dust (ATD) with and without coatings of H2SO4 over the temperature range 197–260 K, and concluded that there was no significant difference between coated and uncoated particles. Archuleta et al. [2005] studied ice nucleation on aluminum oxide, amorphous aluminum silicate, iron oxide and Asian dust with and without coatings of H2SO4 over the temperature range 213–228 K. Based on their results, they concluded that treatment with H2SO4 can either increase or decrease the onset conditions for ice nucleation, depending on the mineral. There have also been two studies of the ice nucleating  Chapter 3  42  properties of mineral dust particles coated with (NH4)2SO4 [Zuberi et al., 2002; Hung et al., 2003], but in neither case were the ice nucleating properties of the uncoated particles  determined, so a direct comparison between uncoated and coated results was not possible. Also related, Georgii [1963] showed that gases such as NH3 and NO2 can reduce the ice nucleation ability of atmospheric aerosol particles, while Salam et al. [2007] observed that exposure to ammonia gas enhanced the activation of montmorillonite particles as ice nuclei. Given the dearth of information on the ice nucleating properties of coated mineral dusts, experiments were conducted to determine the onset conditions for kaolinite particles coated with either H2SO4 or (NH4)2SO4. This chapter summarizes the results for coated particles and compares them to those for uncoated particles. Additionally, differences between the results from this study and previously published measurements are discussed in light of varying particle size, coating thickness, and nature of the mineral. 3.2 Experimental 3.2.1 Overview  The technique used in the following investigation has been described in detail in Chapter 2. Here, a brief overview of the technique is provided, with an emphasis on the experimental conditions and procedures specific to the measurements made for H2SO4 and (NH4)2SO4 coated kaolinite particles. The apparatus used in this study consisted of an optical microscope coupled to a flow-cell (see Figure 2.1, page 16) in which the humidity and temperature could be accurately controlled. Mineral dust particles were deposited on the bottom surface of the flow cell and the relative humidity with respect to ice (RHi) inside the cell was slowly increased. Ice crystal formation was observed with a reflected-light microscope (Zeiss Axiotech 100) equipped with a 10× objective lens. From these measurements, the onset conditions (temperature and relative humidity) for ice nucleation were determined.  Chapter 3  43  3.2.2 Sample preparation  The uncoated kaolinite particles were prepared by first mixing kaolinite in highpurity water (1% kaolinite by mass of water) to create a suspension. The suspension was stirred and then placed in an ultrasonic bath for 10 minutes. To deposit the particles on the glass slide, the suspension was passed through a nebulizer using high-purity N2 (99.999%) as a carrier gas. The flow from the nebulizer was directed at the hydrophobic glass slide and droplets containing the particles were deposited on the surface of the slide upon impact. Any water then evaporated, leaving behind the kaolinite particle. The coated kaolinite particles were prepared by mixing the kaolinite and coating material in high-purity water (1% kaolinite and 0.2% coating material by mass in water). This suspension was then stirred and placed in the ultrasonic bath prior to nebulization. The estimated weight fraction of the coatings on particles produced from these suspensions was 0.167. The optical resolution limit of the microscope was ~1 µm, and a typical sample held between 100 and 1000 individual particles. Shown in Figure 3.1 is an image of kaolinite particles coated with H2SO4 deposited on the hydrophobic slide, just prior to an ice nucleation experiment. It should be noted that the results from the ice nucleation experiments using coated mineral dust particles were compared to results from uncoated particles using the sample preparation procedure outlined above, not the dry deposition procedure described in Chapter 2.  Chapter 3  Figure 3.1: Image of H2SO4-coated kaolinite particles prior to an ice nucleation  experiment, collected using an optical microscope with a 10× objective.  44  Chapter 3  45  3.2.3 Coated particle characterization  Using the Northern Eclipse software package, the average sizes of the coated and uncoated particles used in this investigation were determined. Based on an analysis of all the particles studied and assuming a normal distribution of particle size, the mean diameters for the uncoated particles, H2SO4 particles and (NH4)2SO4 particles were 7.7 µm, 7.8 µm and 6.9 µm with standard deviations of 5.3 µm, 5.7 µm and 5.0 µm, respectively. The thicknesses of the coatings were estimated based on the measured particle sizes and compositions, assuming a spherical core shell model (a kaolinite core surrounded by a uniform H2SO4 coating). According to these calculations, under dry conditions (<1% RHi) a kaolinite core with a diameter of 15 µm will have a 0.7 µm coating, and a 5 µm core will have a coating of 0.2 µm, representing several hundred sulphate layers covering the surface of the particle. To further characterize the thickness of the coatings, the change in particle size was monitored as the relative humidity with respect to water (RHw) was increased from <1% to 95% at 255 K. These measurements were only meant to provide an additional rough estimate of the coating thickness. For these studies, a Zeiss Axiotech 100 transmitted-light microscope equipped with a 50× objective lens was used. Shown in Figure 3.2 are images of two kaolinite particles coated with H2SO4 at <1% and 95% RHw. From the change in size, the total amount of water adsorbed when cycling between <1% and 95% RHw was estimated using the thermodynamic model presented by Clegg et al. [1998]. From this, the amount of H2SO4 in each particle and thus the thicknesses of the H2SO4 coatings under dry conditions were estimated, again assuming a core shell model. Measurements made for 15 individual particles yielded an average weight fraction for the coating of 0.12 ± 0.07. The uncertainty in this value derives from the uncertainty in the relative humidity measurements.  Chapter 3  46  A.  B.  10 µm  10 µm  Figure 3.2:  (A) Image of two selected kaolinite particles coated with H2SO4 at RHw < 1%. (B) Image of the same two kaolinite particles coated with H2SO4 at RHw = 95%.  3.3 Results and Discussion 3.3.1 Uncoated kaolinite particles  The total surface area of mineral dust deposited in any particular experiment ranged from 1×10-4 to 2×10-3 µm2. Over this narrow range, the limited data suggest that the onset results did not depend on the surface area. To illustrate this point, onset data for coated and uncoated kaolinite particles recorded at a temperature of 244 K are presented in Figure 3.3. Each point in this figure corresponds to one freezing experiment. Since no clear dependence on surface area was observed, an average and standard deviation for the onset conditions is reported for each temperature. Since approximately the same surface area of mineral was used in the uncoated and coated experiments, the results should be directly comparable.  Chapter 3  47  150%  uncoated kaolinite  145%  H2SO4 coated  140%  (NH4)2SO4 coated  135%  RHi  130% 125% 120% 115% 110% 105% 100% 95% -4 10  -3  10 2  Surface area (µm ) Figure 3.3: Onset RHi as a function of surface area for uncoated and coated kaolinite  particles at 244 K.  The onset results for uncoated kaolinite particles are shown in Figure 3.4. The data show that only a small supersaturation (<110% RHi) is required for an ice nucleation event to occur in these experiments. The results obtained here are in excellent agreement with results presented in Chapter 2 for uncoated kaolinite particles. It should be noted, however, that the method of sample preparation in the current experiments differs from the method described in Chapter 2. In the current study, particles were produced by nebulizing a suspension of kaolinite in pure water. The water was then allowed to evaporate, leaving behind the solid kaolinite. By contrast, in Chapter 2 the kaolinite particles were not exposed to an aqueous solution; they were dry when deposited on the  Chapter 3  48  slides. The agreement between the current results and the results from Chapter 2 suggests that the preparation method does not significantly impact the results.  160% 150%  wa te  r sa  H2SO4 coated kaolinite  tur at i o n  (NH4)2SO4 coated kaolinite uncoated kaolinite  RHi  140% 130% 120% 110% 100% 225  230  235  240  245  250  255  Temperature (K) Figure 3.4: Summary of ice nucleation results for coated and uncoated kaolinite particles  3.3.2 H2SO4 coated kaolinite particles  Onset results for kaolinite particles coated with H2SO4 are also shown in Figure 3.4. In contrast to the uncoated results, the H2SO4 coated particles required much higher supersaturations before ice nucleation occurred. The data show that H2SO4 coatings drastically altered the ice nucleating ability of kaolinite particles, increasing the RHi required to initiate ice nucleation by approximately 30%.  Chapter 3  49  Before discussing a possible explanation for the shift in the ice nucleating ability of H2SO4 coated particles, the composition of the H2SO4 coatings during the ice nucleation experiments must be discussed. Shown in Figure 3.5 are the trajectories used in these experiments and the concentrations of the H2SO4 coatings, assuming the vapour is in equilibrium with the aqueous phase. The coloured lines represent RHi and temperature conditions at which the compositions of the coatings are 10, 20, and 30 wt% H2SO4 (the remainder being H2O), calculated using the model of Clegg et al. [1998]. This figure illustrates that the H2SO4 coatings are concentrated aqueous solutions (approximately 30 wt% H2SO4 at RHi = 100%) and that an increase in RHi results in a decrease of the H2SO4 concentration in the solutions surrounding the particles.  160% 150%  experimental trajectories H2SO4 coated kaolinite  236.7 K 241.7 K  246.7 K  RHi  140% 130%  wate r  120%  10 wt%  110%  20 wt%  ice saturation  100% 90% 230  s at u ratio n  30 wt%  235  240  245  250  255  Temperature (K) Figure 3.5: Experimental trajectories and lines of constant droplet composition (in wt%  H2SO4).  Chapter 3  50  It is possible that the poor ice nucleating ability of kaolinite particles coated with H2SO4 may be a direct result of the organization of sulphate anions at the solid-aqueous interface. Kaolinite is a 1:1 layered clay mineral whose unit cell is best described as layers of aluminosilicate sheets held together by hydrogen bonds. The “top” surface of these sheets is a gibbsite-like surface with exposed hydroxyl groups. The “bottom” surface of these sheets is a siloxane surface with exposed oxygen atoms [Bergaya et al., Brady et al., 1996]. The Point of Zero Charge (defined as the pH at which the net surface  charge is zero) for the gibbsite-like surface is approximately 6, while the Point of Zero Charge for the edge surfaces is approximately 7–8 [Stumm, 1992]. For this discussion, the focus is on the gibbsite-like surface and the edge surface, since the siloxane surface is thought to be rather hydrophobic and hence not a good site for ice nucleation. As mentioned above, the H2SO4 coatings in these experiments are very concentrated. The pH of the H2SO4 solutions coating the kaolinite particles was calculated to be close to or below 0 under the experimental conditions. As a result, the kaolinite particles should exhibit highly protonated, positively charged surfaces with a proton surface density ~1.2×1014 cm-2 [Stumm, 1992; Zhou and Gunter, 1992]. Sulphate is known to strongly interact with mineral surfaces under these conditions [Ali and Dzombak, 1996]. Under these conditions, sulphate anions will likely be attracted to the positively charged kaolinite surface and potentially block ice nucleation sites, which could explain the higher onset RHi values observed in the experiments with H2SO4 coatings. A strong interaction between sulphate anions and positively charged kaolinite surfaces is also consistent with several spectroscopic studies that have shown that sulphate can be involved in strong mono- or bi-dentate chemisorbed complexes with surface protonated hydroxyl groups on iron and aluminum oxide or hydroxide surfaces [Hug, 1997; Peak et al., 1999; Wijnja and Schulthess, 2000; Fukushi and Sverjensky, 2007; Zhang and Peak,  2007]. Future experiments that focus on the partitioning of the sulphate group to the solid-aqueous interface would be helpful, as would investigations into how the arrangement of water molecules close to the kaolinite surface is perturbed by the presence of H2SO4.  Chapter 3  Figure 3.6: The crystal structure of kaolinite. Red = oxygen (O2-), grey = aluminum  (Al3+), blue = silicon (Si4+), yellow = hydrogen (H+). Dotted lines represent hydrogen bonds holding two alumina-silicate sheets together.  51  Chapter 3  52  As mentioned in the introduction to this chapter, there have only been two previous studies of the effects of H2SO4 coatings on mineral dust particles. Knopf and Koop [2006] studied ATD particles with and without H2SO4 coatings, but they did not  observe significant differences due to the coatings. A possible explanation for this apparent discrepancy with the data presented in this chapter may be due to the nature of the minerals studied. For example, perhaps the ATD samples studied by Knopf and Koop [2006] contained very little kaolinite or very little kaolinite surface was exposed for ice nucleation. Archuleta et al. [2005] studied different oxides, with and without H2SO4 coatings. For aluminum oxide and iron oxide, no statistical difference was observed between uncoated and coated particles at 228 K. For amorphous aluminum silicate at the same temperature, they reported that the RHi required for formation of ice on 1% of 50 nm particles was 10.6% higher for the coated particles. Again, the difference between the results presented in this chapter and the measurements of Archuleta et al. [2005] may be due to the different minerals studied. Alternatively, the differences may be related to experimental conditions. For example, Archuleta et al. [2005] studied particles with three different monodispersed size distributions centered on 50, 100 and 200 nm, while this study investigated much larger particles. Also, the coatings used by Archuleta et al. [2005] were between 2.9 and 7.1 layers thick, which are much smaller than the calculated coating thicknesses for these experiments. 3.3.3 (NH4)2SO4 coated kaolinite particles  The onset results for (NH4)2SO4 coated particles are also shown in Figure 3.4. At 240 and 245 K, the RHi values at the onset of ice nucleation are significantly higher than the uncoated case. However, at 236 K, the coated particles are just as efficient at nucleating ice as the uncoated kaolinite particles. Clearly, the (NH4)2SO4 coatings are influencing the ice nucleating ability of the kaolinite particles much differently than the H2SO4 coatings. A possible explanation for this difference may be related to the phase (i.e. solid or liquid) of the coatings. For the H2SO4 coatings, ice nucleation occurs on a kaolinite particle coated with an aqueous solution. This is not necessarily the case for (NH4)2SO4  Chapter 3  53  coatings, which can undergo deliquescence and crystallization. Deliquescence occurs when the solid coating absorbs surrounding water vapour and dissolves into a solution at high relative humidity. Crystallization occurs when the supersaturated aqueous coating precipitates to form a solid at low relative humidity. Thus, depending on the temperature and the relative humidity, the (NH4)2SO4 coating may be solid or liquid. This can complicate the situation, since solid (NH4)2SO4 is expected to be a good ice nucleus under certain conditions [Zuberi et al., 2002; Abbatt et al., 2006; Shilling et al., 2006]. The phase of the coating is discussed in more detail below. Figure 3.7 illustrates the possible phase behaviour of an (NH4)2SO4 coating during a typical freezing experiment. The experiments first start at a very low relative humidity (RHi <1%). Under these conditions, the (NH4)2SO4 particles are expected to be crystalline, based on previously reported values for the efflorescence relative humidity of pure (NH4)2SO4 [Martin, 2000] and (NH4)2SO4 droplets containing kaolinite [Pant et al., 2006]. The RHi is then increased and above 100% RHi, ice can nucleate on the solid (NH4)2SO4 coating. In this case, ice nucleation will occur by deposition freezing, which occurs when water vapour adsorbs onto a solid surface and is transformed into ice [Vali, 1996]. If ice nucleation does not occur, the solid (NH4)2SO4 coating can take up water at the deliquescence relative humidity (DRH), which is represented by the dashed line in Figure 3.7. Above this relative humidity, the (NH4)2SO4 coating is a liquid and ice nucleation can occur by immersion freezing. Based on the discussion above, the data suggest that solid (NH4)2SO4 coatings are good ice nuclei at approximately 236 K. This is consistent with previous laboratory data that show that solid (NH4)2SO4 particles are good ice nuclei below 239 K [Zuberi et al., 2002; Abbatt et al., 2006; Shilling et al., 2006].  Chapter 3  54  150%  (NH 4)2 SO4 -coated kaolinite Water saturation line (NH 4)2 SO4 DRH  140%  Ice saturation line Experimental trajectories  RHi  130% Liquid coating  120%  Deliquescence  110% 100%  Solid coating  90% 230  235  240  245  250  255  Temperature (K) Figure 3.7: Schematic of the possible phase behaviour of an (NH4)2SO4 coating during a  typical freezing experiment  Chapter 4  55  4. Concluding remarks 4.1 Uncoated mineral dust  The ice nucleating properties of five mineral dusts were studied at temperatures ranging from 233 to 247 K. Kaolinite and muscovite, two alumina-silicate clays, were determined to be effective ice nuclei at all temperatures studied. The observed onset of ice nucleation was below 110% RHi (well below water saturation) for both minerals. Montmorillonite, the third alumina-silicate clay, behaved similarly to kaolinite and muscovite at lower temperatures and was determined to be an effective ice nucleus below 241 K, but a poor ice nucleus at 245 K and above. Quartz and calcite were poor ice nuclei at all temperatures studied. For the temperature range studied, the RHi values needed to induce ice nucleation on quartz and calcite were approximately 20 to 40% higher than those needed for kaolinite and muscovite. Overall, the data show significant differences in the ice nucleating abilities of the five minerals studied over this temperature range. 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 size, the surface area available for nucleation, observation and equilibrium times. Future studies that investigate the effects of these different parameters are needed for a complete understanding of ice nucleation on mineral particles. The heterogeneous nucleation rates (Jhet) and contact angles (θ) were determined according to classical nucleation theory for all five minerals studied. The contact angles measured for kaolinite and muscovite ranged from 6 to 12º, whereas for quartz and calcite the contact angles ranged from 25 to 27º. For montmorillonite, the contact angles measured below 241 K ranged from to 15º, while the contact angle at 245 K was 21º. These data may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere.  Chapter 4  56  4.2 Coated mineral dust  The ice nucleating properties of kaolinite particles at temperatures ranging from 233 to 247 K change drastically when coated with either H2SO4 or (NH4)2SO4. Uncoated kaolinite particles are effective ice nuclei, but kaolinite particles coated with H2SO4 are relatively poor ice nuclei, requiring RHi values approximately 30% higher than uncoated particles in order to initiate ice nucleation. One possible explanation for this difference is the protonation of the kaolinite surface and the adsorption of sulphate anions onto surface hydroxyl groups resulting in a blocking of ice nucleation sites. The ice nucleating properties of particles coated with (NH4)2SO4 depend on the temperature: at low temperatures (i.e. 235 K), the ice nucleating ability for (NH4)2SO4 coated particles is comparable to that of uncoated particles with an onset RHi of 107%; however, at 240 K and above, the (NH4)2SO4 coated particles are poor ice nuclei, much like the H2SO4 coated particles. The differences in the results obtained for H2SO4 and (NH4)2SO4 may be explained by the deliquescence and efflorescence properties of (NH4)2SO4. These initial results support the idea that anthropogenic emissions of SO2 and NH3 may influence the ice nucleating properties of mineral dust particles by increasing the relative humidity required for ice nucleation on these particles. This shift in the conditions necessary for ice nucleation may influence the frequency and properties of ice clouds. Calculations using a cloud parcel model are needed to explore this possibility further. Field measurements made in the Arctic region show a drastic reduction in the number of ice-forming nuclei when sulphate concentrations are high [Borys, 1989]. The results presented in this thesis do not provide information on the change in number of ice nuclei since only the onset conditions for ice nucleation were determined. More work is needed to explain the measurements by Borys [1989], but the results presented herein support the idea that H2SO4 and (NH4)2SO4 coatings can significantly influence the ice nucleating properties of mineral dust particles.  Chapter 4  57  4.3 Suggestions for future work  Several experiments are recommended to further our understanding of mineral dusts and the role they play as ice nuclei in the troposphere: (1) The ice nucleating abilities of five uncoated minerals were determined in these studies; however, atmospheric mineral dust has many components of varying concentration. Future experiments to determine the ice nucleating abilities of other major components of mineral dust such as chlorite, corundum, hematite, gypsum and feldspar should be conducted. (2) The effects of H2SO4 and (NH4)2SO4 coatings on the ice nucleating ability of dust were only determined for one mineral – kaolinite. 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