Ice nucleation of bare and sulfuric acid-coated mineral dust particles and implication for cloud properties

2.1 Particle Generation

Figure 1 shows the experimental setup used to investigate the ice nucleating properties of size-selected mineral dust particles. ATD (A1 Ultrafine test dust), illite, montmorillonite, quartz, and kaolinite mineral dust types were dry-dispersed using a dust disperser (dry powder dispersion; TSI, 3433). ATD and illite dust samples were purchased from Powder Technology Inc., and Clay Minerals Inc., respectively. The montmorillonite and kaolinite particles were purchased from Sigma-Aldrich Co. LLC, and the K-feldspar was obtained from Excalibur Mineral Inc. The chemical compositions, as provided by the vendors, are presented in Table S1 in the supporting information. The particles were coated with sulfuric acid in a coating apparatus [Friedman et al., 2011] that was maintained at a temperature of either 100°C or 150°C. In this unit, particles entered the glass tube and passed over a heated sulfuric acid bath. The bath section was heated using flexible heating tapes, and its temperature was controlled using a thermostat. The residence time of the particles within the tube was < 5 s, which was achieved by maintaining constant air flow (0.3 liter per minute (Lpm)) for all experiments. Coating thickness was roughly estimated based on a simple acid molecule condensation parameterization (Appendix A). We assumed acid molecules condensed for a short period and bulk vapor concentration was fixed at all times during vapor condensation. We also assumed that particles were spherical with a homogeneous coating around them. At acid bath temperatures of 100°C and 150°C, our estimation showed coating thicknesses of less than 1 nm (2 monolayers) and 40 nm (80 monolayers) of acid molecules, respectively. We note that our assumptions affect the coating thickness estimates, such that these estimates could be higher than the actual coatings (Appendix A). We will refer to the particles coated at the lower temperature and the higher temperature as thin- and thick-coated particles, respectively. Coated particles with a mobility diameter of 200 nm were size selected by a differential mobility analyzer (TSI, 3080), and they were transported to the condensation particle counter (CPC) and compact ice chamber (CIC; 4). We observed that the differential mobility analyzer produced multiple charged particles, but not a monodispersed particle distribution; therefore, large (multiple charged) particles (>200 nm) were also transmitted. Theoretical calculations [Baron and Willeke, 2001] indicate that double and triple charged particles were 324 nm and 440 nm in size, respectively. Using a sample flow rate of CIC and CPC measurements, the total numbers of particles or condensation nuclei (CN) available for IN measurements were calculated. For X-ray diffraction (XRD) analysis, the coated particles were diverted to the PIXE cascade impactor (Model I-1 L), where they were collected on carbon tape that was attached to the mounting support of stage L2. According to the impactor specifications, this stage had a particle diameter cut point between 0.25 to 0.12 µm. To confirm that we did not modify the physical and chemical properties of the dust particles due to the heating effect within the coating apparatus, we also collected (for XRD analyses) particles that had passed through the same coating apparatus, but with no sulfuric acid solution. The empty coating apparatus was maintained at either 100°C or 150°C.

2.2 XRD Analysis

XRD is an analytical technique that is widely used for the identification of a crystalline material and its structural state. MicroXRD data were collected using a Rigaku D/Max Rapid II instrument equipped with a 2-D image plate detector. X-rays were generated with a MicroMax 007HF generator fitted with a rotating Cr anode (Lambda = 2.2897 Å) and focused on the specimen through a 300 µm diameter collimator. Samples were dispersed on double-sided carbon tape on a flat-top sample holder. We used 2DP, Rigaku 2-D Data Processing Software (Version 1.0, Rigaku, 2007), to integrate the diffractions rings captured by the detector. We then performed an analysis of the diffraction data using JADE 9.5 (Materials Data, Inc.) and the PDF4+ 2012 database from International Centre for Diffraction Data (ICDD). We verified the correct sample-to-detector distance by measuring the lattice constant of a Si standard (Silicon powder, National Institute of Standards and Technology 640c). The observed lattice parameters deviated from the standard by 0.002 Å. We also determined lattice parameters and average crystallite sizes from Le Bail fitting in TOPAS v4.2 (Bruker AXS GmbH, Germany).

2.3 Ice Nucleation Experiments

We obtained ice nucleation properties of size-selected mineral dust particles using CIC. The working principle of CIC has been noted in the literature [Stetzer et al., 2008; Friedman et al., 2011; Kulkarni et al., 2012], but a brief description of its design details follows. The chamber consists of two vertical parallel plates with an evaporation section attached at the bottom of the chamber to remove water droplets [Stetzer et al., 2008]. A continuous flow water vapor diffusion chamber ensures that, when aerosol particles are placed between the layers of two sheath flows, they are exposed to constant temperature and relative humidity with respect to ice (RH_ice) over the length of the chamber. Saturation vapor pressures over ice and water are calculated using formulations published by Murphy and Koop [2005]. The chamber wall temperatures are controlled by using two external cooling baths (Lauda Brinkmann Inc.), and temperature data are logged using the National Instrument CompactRIO programmable automation controller (cRIO-9114 combined with cRIO-9022). The chamber plates are temperature controlled independently to develop a linear temperature gradient across them, which, according to the principle of thermal gradient diffusion theory, produces a RH_ice profile between the plates [e.g., Rogers, 1988]. At the beginning of the experiment, the chamber walls are coated with an ~ 0.5 mm thick ice layer, and the temperature gradient is set at zero, which creates ice-saturation conditions inside the chamber (RH_ice = 100%). Then, the refrigeration system cools one plate and warms the other to increase the RH_ice. The total flow used was 11 Lpm; sheath and sample flows used were 10 and 1 Lpm, respectively, which limits the aerosol residence time to ~ 12 s within the CIC. Ice nucleates on the aerosol particles, and the newly formed ice crystal grows to a size greater than the original aerosol size. Ice crystals at > 1.5 µm exiting the chamber are counted with an optical particle counter (OPC; CLiMET, model CI-3100). The ice active fraction (F_ice) was calculated as the ratio of number of ice crystals measured by the OPC to the CN available for nucleation. Background IN concentrations were calculated to estimate the lower detection limit of F_ice. To calculate the background IN, we ran the CIC maintained at RH_w = 110% and three temperature conditions (−25, −30, and −35°C) conditions while sampling only filtered dry air for at least 30 min at the beginning, middle, and toward the end of the experiment. The lower detection limit of F_ice was 0.01%. To make sure that our background IN concentration was less than ~ 0.01%, we restricted our experiment duration time to less than 3 h.

These ice nucleation measurements were further analyzed using the concept of ice-active surface site (N_s) density theory. In this analysis, the ice-active sites are assumed to be distributed over the dust surface at a certain activation energy density, such that nucleation will occur at a particular site with a characteristic temperature and RH_ice [Connolly et al., 2009; Murray et al., 2012; Niemand et al., 2012]. According to equation (1) from Niemand et al. [2012], the density of N_s can be expressed as

(1)

where A is the surface area per particle calculated, assuming dust particles are spherical in shape, and F_ice (RH_ice) is the fraction of ice particles observed as a function of RH_ice, as discussed above.

2.4 Cloud Modeling

We used the single-column mode of the Community Atmosphere Model CAM5.1 [Neale et al., 2010] to examine the sensitivity of cloud properties to ice nucleation on bare and coated particles, such as ice crystal concentration and ice water content, under both water-subsaturated and water-supersaturated conditions. The atmospheric column that we simulated is located at the Southern Great Plain (SGP) site near Lamont, Oklahoma. We used the National Oceanic and Atmospheric Administration's rapid update cycle analyses to derive the large-scale forcing terms for the model column using the constrained variation objective analysis approach described by Zhang and Lin [1997] and Zhang et al. [2001]. We used surface radiation and precipitation observed at the SGP site and top-of-atmosphere radiation measurements by the Geostationary Operational Environmental Satellite to adjust the atmospheric state variables to conserve column-integrated quantities such as mass, moisture, and static energy. In addition to ice nucleation treatment processes, the model also considers complex ice microphysical processes, such as water vapor deposition, ice sublimation, melting, autoconversion from cloud ice to snow, riming, accretion, and sedimentation.

The model has 30 vertical levels up to 10 hPa and a time step of 10 min. The ice cloud microphysics processes are described in Liu et al. [2012a, 2012b] and Zhang et al. [2013]. The simulation was performed for the month of April 2010 during the period of the Department of Energy (DOE) Small Particles in Cirrus (SPARTICUS) field campaign [Zhang et al., 2013], with a spin-up period of three model days.

3.1 XRD Analysis

The XRD patterns of four bare and coated dust types are shown in Figure 2. Different peaks or degrees 2 theta (2θ) reflections indicate the crystalline structure or characteristic identities of different minerals presented within each dust type [e.g., Panda et al., 2010; Melo et al., 2010; Volzone and Ortiga, 2011]; that is, each peak corresponds to one mineral (for convenience, we name them as minor minerals). Several minor minerals were detected and quantified, as shown and listed in Figure 2 and Table 2, respectively. Particles from each dust type were mostly composed of various minor minerals such as illite1, illite2, montmorillonite, quartz, calcite, albite, anorthite, and orthoclase. Albite and orthoclase, from the K-feldspar group, were found to be common minor minerals in ATD and illite. In ATD, the compositions of albite and orthoclase were found to be 8 and 12%, respectively. Illite was primarily composed of illite2 and quartz, with only ~ 3% of orthoclase. A recent study [Murray et al., 2012] showed that the K-feldspar group minerals are common minerals found in most atmospheric mineral dust particles and estimated that, on average, ~ 10% of atmospheric dust is composed of K-feldspar minerals.

The XRD patterns of bare particles showed a number of intense peaks, indicating that these particles are crystalline in nature [Warren, 1990; Cullity and Stock, 2001]. Once the particles were coated with sulfuric acid, the peak intensity of some of these minor minerals decreased significantly, indicating that the particles were amorphous in nature. For example, the positions of 2θ reflections in the region between 30° < 2θ < 35° for ATD were found to decrease after coating, and well-defined peaks in this region were not observed. In this sample, we also observed that the calcite mineral (43° < 2θ < 47°) was completely lost after coating. The XRD patterns of the bare and coated quartz particles were nearly identical (Figure S1). However, for thickly coated particles, we observed a peak at 2θ = ~ 42°, which could be from a new phase that had formed on the surface. It was not possible to identify the mineral associated with this peak, but we speculate that any minor impurities (nonquartz minerals undetected by XRD) that were sensitive to the high acid concentration could have produced these compounds.

In addition, the XRD patterns of the heated ATD particles are shown in Figure 2a. We observed negligible structural changes. For example, peaks such as illite2, orthoclase, albite, and calcite are clearly visible, and their intensities are very similar to bare particles. These results confirm that we did not inadvertently modify the physical and chemical properties of the dust particles before their ice nucleating properties were investigated. However, we cannot neglect any additional chemistry that may have occurred if the hot sulfuric acid vapors had condensed on the dust particle.

3.2 Ice Nucleation Measurements of Bare and Coated Mineral Dust Particles

Figure 3 shows the F_ice of four bare and coated mineral dust species, (1) ATD, (2) illite, (3) montmorillonite, and (4) K-feldspar, as a function of RH_w and RH_ice at different temperatures: −25, −30, and −35°C. This figure shows the fraction of particles that induce ice nucleation in the deposition mode (water-subsaturated conditions; RH_w < 100%) and the condensation/immersion freezing mode (water-supersaturated conditions; RH_w ≥ 100%). We assume that only deposition ice nucleation occurs at water-subsaturated conditions. However, we also note that a recent study [Marcolli, 2014] hypothesized that condensation freezing may be occurring at water-subsaturated conditions. The vertical error bars shown in the figure are equal to the one standard deviation of the measurements (n = 8), and the horizontal error bars are equal to the ±3% uncertainty in the RH_w measurements.

F_ice spectra, which represent the measurement of ice nucleation behavior, varied depending on temperature and RH_w (Figure 3). At water-subsaturated conditions, our results indicate that F_ice increases with increasing RH_w, which is in agreement with many previous studies that have also observed this ice nucleation behavior for dust particles [e.g., Welti et al., 2009; Sullivan et al., 2010c; Tobo et al., 2012; Kanji et al., 2013]. We observed (Figure 3) that all dust species at −25 and −30°C initiate ice at ~ 85% and ~ 80% conditions, respectively. However, at a lower temperature (−35°C), ice nucleation on ATD and illite particles was observed at RH_w = ~ 77%, and on montmorillonite and K-feldspar minerals at ~ 80%. At water-subsaturated conditions, the IN abilities of acid-treated particles was found to be lower compared to uncoated particles (Figure 3). The sensitivity at −25°C was so acute that ice nucleation was observed only near water saturation conditions. At the other two temperatures considered, (−30 and −35°C), the minimum RH_w required for ice nucleation onto dust particles was ~ 90% (including error limits). At water-supersaturated conditions, where particles are immersed in water droplets, changes in F_ice are also observed. We observed that a maximum F_ice is reached at −35°C for all four dust species: at 105% RH_w conditions, the F_ice of ATD particles at −25, −30, and −35°C are ~ 20%, 22% and 35%, respectively. Under these conditions, the results also show that the F_ice values for bare and coated dust species are very similar.

The sensitivity of bare dust species to temperature and ice-saturation conditions can be attributed to the variability in the efficiency of active sites present on their surfaces that promote ice embryo formation as previously reported; e.g., by Marcolli et al. [2007]. Marcolli et al. [2007] found that, for dust particles with a diameter of at least 100 nm, nucleated ice consists of one active site on average, and they suggested that ice nucleation occurs on active sites that have a certain probability distribution. This suggests that active sites are required to nucleate ice, and according to Pruppacher and Klett [1997] the rate of ice embryo formation at these sites rises either with an increase in humidity at any particular temperature or with a decrease in temperature at fixed humidity conditions. Our result shows that at 85% F_ice was ~ 0.1%, and near the water saturation condition, F_ice increased from 1 to 10%, depending on the temperature. These results suggest that the dust particles tested here had varying a quality of active sites, such that some sites induced ice nucleation at lower and some at higher ice-saturation conditions.

Except for quartz particles, the IN abilities of acid-treated particles were found to be lower compared to bare particles in the deposition mode of ice nucleation. The sensitivity at −25°C was so strong that ice nucleation was observed only near water saturation conditions. At the other two temperatures (−30 and −35°C), the minimum RH_w required for ice nucleation on dust particles was ~ 90% RH_w (including error limits). In the condensation/immersion freezing conditions (particularly above RH_w = 105%), coated particles did not show a significant decrease in the IN ability compared to bare particles. These observations are in agreement with Sullivan et al.'s [2010c] findings, which suggested that, at water-supersaturated conditions, surface chemical reactions might not change the original ice nucleating properties permanently because coating material could be removed by dissolution and could potentially expose surface active sites. These sites may induce ice nucleation via condensation freezing, and they showed that, in this freezing regime, ice nucleation properties of dust particles depend on the dust composition and the uptake mechanisms of condensed soluble materials. Additionally, they suggested that surface chemical reactions may lead to reaction products with different solubility levels. For example, Panda et al. [2010] concluded that sulfuric acid-treated kaolinite particles can result in a reaction product (i.e., aluminum sulfate) that can be easily dissolved in water. We speculate that such soluble reaction products might have been produced in our experiments and may be the reason that we did not observe the influence of a coating in this regime.

Thin- and thick-coated dust particles had very similar ice nucleating behaviors (Figure 3) in both the deposition and the condensation/immersion ice nucleation modes. This result suggests that condensation of sulfuric acid vapor occurring on the dust particles when the coating apparatus was maintained at 100°C was sufficient to modify the surface so that further condensation of vapor has no effect on ice nucleation efficiency of coated particles.

In general, acid coating leads to the dissolution or fragmentation of surface layers, ultimately modifying the surface. Recently, Reitz et al. [2011] suggested that chemical reactions between sulfuric acid and ATD particles reduced the hydrogenated fragments of the sulfate species and produced reaction products such as metal and ammonium sulfates. Sulfate anions can adsorb to these reactions products due to strong electrostatic attractions and modify the structural properties [e.g., Stumm, 1992]. It should also be noted that dust minerals undergo various structural changes, depending on the concentration of the acid. At lower acid strengths, there is an exchange between the cations of the dust and H⁺ ions of the sulfuric acid [Volzone et al., 2005; Volzone and Ortiga, 2006; Zhao et al., 2013]. At a higher acid concentration, metallic ions can leach out from the interlayer structure of the dust minerals and may alter the structural and crystallite properties completely [Panda et al., 2010; Makó et al., 2006; Rajesh et al., 2008; Volzone et al., 2005; Volzone and Ortiga, 2011].

We performed XRD experiments (section 7) to identify any particular reasons for the decrease in the IN ability of the coated dust particles in the deposition ice nucleation mode. In our experiments, we clearly observed that the acid treatment distorted the atomic positions and thus modified the structural order. We know that large crystallographic differences or dislocations or structural disorderliness at the ice-substrate interface will increase the interface free energy and can reduce the ability of the dust particle to induce ice formation [Pruppacher and Klett, 1997]. To quantify these crystallographic effects, we calculated crystallographic properties such as lattice parameters and crystallite sizes of montmorillonite and K-feldspar (Table 3). The results show that the crystal lattice dimensions of bare and coated dust particles do not match. For example, the length of the lattice parameters or the lattice constants—a, b, and c—of bare and coated montmorillonite are 5.187, 8.935, 14.899 and 5.446, 9.034, 13.924, respectively, in Å. The montmorillonite also consists of quartz, and the lattice parameters of bare and coated quartz were identical; this is expected because quartz does not react with acid (Figure S1). For comparison, we also tabulated the lattice parameters of ice-Ih and AgI. Numerous previous studies [e.g., Baklanov et al., 1991; Edwards and Evans, 1960; Vonnegut, 1947] have shown that AgI is efficient IN because the crystal lattice closely matches ice-Ih, indicating the importance of crystallographic properties on the IN ability of aerosol particles. The crystallite size of the minerals, which is the measure of the average size of a single crystal, was observed to decrease after acid treatment. For example, the crystal sizes of bare and coated montmorillonite particles were 3.8 and 2.8 nm, respectively. Detailed discussions of the influence of lattice fits on heterogeneous ice nucleation are beyond the scope of this paper.

In summary, our observations show that the acid treatment caused structural deformation or disorderliness in four dust test samples (ATD, illite, montmorillonite, and K-feldspar) (Figure 2 and Table 3), but the acid treatment did not modify the structural properties of the quartz particles (Figure S1 and Table 3). Acid treatment affected the K-feldspar group minerals (albite, orthoclase, and anorthite; Figure 2), which are suggested to be the important ice nucleating minerals for atmospheric ice nucleation [Atkinson et al., 2013; Yakobi-Hancock et al., 2013]. The peak intensities of the orthoclase and albite minerals were reduced in ATD, whereas the orthoclase from illite was completely lost. K-feldspar also showed a similar sensitivity toward acid treatment. The intensity of all three minor minerals was reduced, and the peaks were broad. These structural changes indicate that the ice nucleation sites of these commonly observed minor minerals in the test dust species were modified. These results indicate that destruction of the structured order led to the reduction of the ice nucleating properties of various dust species (Figure 3), but only at water-subsaturated conditions. These conclusions are in agreement with the experimental spectroscopic studies reported by Yang et al. [2011], in which they reported that ordered structures at dust (i.e., mica) surfaces are important for ice nucleation. In their experiments, they found that structured order becomes distorted or particle surface become amorphous with exposure to sulfuric acid. Ice nucleation experiments were not performed, but their study provides molecular-level insights into why dust particles coated with sulfuric acid are poor IN.

Previously, Sullivan et al. [2010a] reported that heated bare particles (ATD) had a minimal influence on IN properties (within experimental uncertainties), in both the deposition and condensation/immersion freezing modes. In their experiments, the authors used a thermodenuder to heat the particles, where temperatures were maintained at 45, 70, 85, or 250°C, and the residence time of the particles was 4 to 5 s. In our experimental setup, the coating apparatus was maintained at 100 or 150°C, and the flow was 0.3 Lpm, which limited the particle residence time to ~ 5 s. In our study, we observed very little effect of only heated particles on ice nucleation properties (not shown; F_ice spectra of heated and bare particles was very similar, as shown in Figures 3a–3c). In addition to measuring the ice nucleating properties of heated dust particles, we also subjected these particles to XRD analysis and found that heat treatment caused negligible surface modifications (Figure 2a). On the other hand, we noted that hot sulfuric acid vapors within the coating apparatus may influence the chemical and physical properties of the dust particles. This possibility should be investigated in future studies.

3.3 Comparison of Ice Nucleation Properties With Previous Studies

The direct comparison of our results with previous investigations is difficult because of differences in the ice crystal detection threshold, nucleation mechanism, experimental setup, and the particle generation method. As a result, we calculated the N_s density (Figures 4 and 5) for comparison, as discussed in 3. Figure 4 displays the N_s densities for the ATD particles calculated in the deposition mode at two different temperatures: −25 and −30°C. Our measurements indicate that the N_s densities for bare particles are higher than those for coated particles. Data reported by Sullivan et al. [2010a] also show that bare particles had higher N_s densities. However, their coated N_s densities are at least an order of magnitude smaller than those from our study at −25 and −30°C. At −30°C, however, the N_s densities for bare particles are in better agreement. Additionally, our results are in agreement with those of Cziczo et al. [2009] for bare particles at lower humidity conditions (RH_ice < 110%; Figure 4, right), but at RH_ice = 120%, our data are an order of magnitude smaller than theirs.

Figure 5 shows the N_s densities calculated for the condensation/immersion freezing. We did not observe significant N_s density differences between the bare and coated particles at −25, −30, and −35°C. Niedermeier et al. [2010, 2011] also reported similar results, but only at temperatures lower than −32°C. At warmer temperatures of −30 and −28°C, they showed that bare and coated particles had different N_s densities. At −25°C, Sullivan et al. [2010a] reported 3 orders of magnitude variability in the N_s values calculated for coated particles, while, at −30°C, they observed a variability of 2 orders of magnitude.

Figure 6 shows the critical RH_ice required to activate 1% of the illite, montmorillonite, and quartz particles at −35°C. Results from the study by Chernoff and Bertram [2010] are also plotted. Our results show that coated particles require higher RH_ice conditions, which are in agreement with the results of Chernoff and Bertram [2010] study. Illite particles were investigated by Cziczo et al. [2009], and their data show a negligible influence of coating. However, their data are in agreement with our results for coated particles (illite).

The majority of previous studies report a decrease in the ice nucleation activity of particles coated with sulfuric acid (Table 1), and our measurements agree with those findings. In general, previous studies [Niedermeier et al., 2011; Wex et al., 2013; Sullivan et al., 2010a; Tobo et al., 2012; Eastwood et al., 2009; Chernoff and Bertram, 2010] concluded that condensed sulfuric acid could react with and alter the mineral particle's surface properties, thus reducing the efficiency of ice nucleation active sites that are needed to nucleate the ice embryo in deposition ice nucleation mode. Our XRD results (note that these previous studies did not conduct XRD analysis) confirm that the physical and chemical properties of the surfaces are modified. This may be the reason that coated particles have lower ice nucleation efficiency in the deposition mode. On the other hand, Knopf and Koop [2006] reported that coating does not influence the ice nucleation properties of ATD particles. They suggest that these dust species, which are a complex mixture of different minor minerals, may or may not be reactive toward sulfuric acid. However, Cziczo et al. [2009] found that coating was more effective for ATD particles, but for illite particles, it was less pronounced. The degree of surface modification also may depend upon the amount of acid coating. For example, in the immersion freezing mode, Niedermeier et al. [2010] found that ATD particles coated with small amounts of acid could efficiently act as IN at higher temperatures, compared to particles that had larger amounts of the coating. More recently, Wex et al. [2013] suggested that surface morphology and composition could be important. They observed that kaolinite particles from two different sources showed different sensitivities toward ice nucleating ability in the immersion freezing mode: particles from one source showed that coating reduces the IN ability, while particles from another source showed no such reduction in the IN ability. Thus, past studies also demonstrate that mineralogy and a sufficient amount of coating to alter the particles' surfaces are important factors that can control the ice nucleation efficiency of acid-treated dust particles.

In addition, a number of previous studies examined the influence of mineral dust treated with various compounds (nitric acid, organics and ammonium sulfate) on their ice nucleation efficiency. The effect of nitric acid coating on ice nucleation ability of ATD particles was investigated by Sullivan et al. [2010a], which found that coating suppressed ice nucleation activity at water-subsaturated conditions, but observed no effect above water saturation. The influence of organic-coated dust particles showed that water saturation conditions are mostly required to nucleate ice [Koehler et al., 2010; Möhler et al., 2008]. Recently, Tobo et al. [2012] examined the effect of levoglucosan and sulfuric acid coatings on the ice nucleating properties of kaolinite particles. They observed that, at water-subsaturated conditions, the coating treatment reduced the ice nucleating ability of the kaolinite; however, at water-supersaturated conditions, only sulfuric acid coated dust particles showed a reduction in ice nucleation efficiency. Levoglucosan is a non-reactive material compared to sulfuric acid, which reacts vigorously with dust minerals, and their study suggests surface chemical reactions play an important role toward inducing ice formation on aged particles. Immersion freezing experiments by Niedermeier et al. [2011] showed that the ice nucleation ability of sulfuric acid-coated dust particles was reduced compared to bare particles. These studies at water saturation conditions indicate that the sulfuric acid-treated dust particles had poor ice nucleation efficiency compared to bare particles. The immersion freezing efficiency of the aqueous solutions of ammonium sulfate compounds containing dust minerals was investigated by Zobrist et al. [2008] and Zuberi et al. [2002]. Their results also suggested that the ice nucleation efficiency of coated dust particles were significantly reduced.

3.4 Sensitivity of Cloud Microphysical Properties to Ice Nucleation on Bare and Coated Particles

Using Single column mode Community Atmospheric Model version 5, we investigated the influence of coated particles on the ice crystal number concentration and the ice water content in clouds. The IN concentrations were parameterized using the N_s density approach (4) at water-subsaturated and -supersaturated conditions. For these calculations, we used illite particles as a surrogate for natural dust particles, because illite has been shown to be an abundant mineral component observed in atmospheric dust particles [from Murray et al., 2012, Figure 5]. Figure S3 shows N_s density fits as a function of RH_ice at different temperatures for the deposition nucleation (a to c), and at RH_w = 105% for the condensation/immersion freezing (d). These temperature and RH_ice-dependent parameterizations were used to simulate the deposition and condensation/immersion freezing modes, respectively.

The model is configured such that particles can induce ice formation via deposition ice nucleation parameterization where there are no cloud droplets in the model grid box or via the condensation/immersion freezing mode when there are cloud droplets and the air is saturated with respect to water (RH_w > 100%). The initial number concentrations of bare and coated illite particles are prescribed as 200 L⁻¹. In all these idealized simulations, both the bare and coated illite particles are assumed to be monodisperse and have a diameter of 200 nm. N_s density parameterizations were used to calculate the heterogeneous ice nucleation rate. We note that the simulated ice-number concentrations cannot be directly compared to the observations; however, the model sensitivity to different N_s densities derived from the laboratory experiments will provide insight on how the changes in ice nucleation activity of dust particles due to coating will affect the ice cloud properties in the atmosphere.

Figure 7 shows the simulated monthly mean vertical profiles of the ice crystal number concentration and the ice water content at the SGP site during April 2010. At water-subsaturated conditions, the maximum monthly mean ice crystal number concentration for both bare and coated cases was observed at ~ 450 hPa. Compared to the coated case, the modeled ice crystal number for the bare case was about 1 order of magnitude higher at each level. At water-supersaturated conditions, the modeled ice crystal number and ice water content are higher in the bare case than in the coated case. However, the difference between the bare and coated case above 600 hPa is much smaller. This is mainly due to that fact that the relative difference between the N_s densities of bare and coated illite particles is smaller at lower temperatures (Figure S3d), and the water-saturated air appears less frequently in the upper troposphere.

In the ice cloud regime, coated dust particles will lead to fewer nucleated ice crystals; thus, air can be supersaturated more easily because less water vapor is consumed. As a result, homogeneous ice nucleation can happen more easily in the upper troposphere (below −38°C), promoting the formation of small ice crystals. The actual impact on radiation will depend on how frequently the homogeneous ice nucleation occurs, which is further dependent on the updraft velocity and water vapor fluctuation in this region. In the mixed-phase cloud regime, where both cloud droplets and ice crystals exist, the smaller number of nucleated ice crystals from coated dust particles leads to a weaker Bergeron process, so the liquid water content would be higher than for the uncoated case. This would further delay the initiation of precipitation and increase the lifetime of the mixed-phase clouds, which leads to higher cloud albedo and larger shortwave cloud forcing.

Recently, Girard et al. [2013] performed cloud modeling studies to assess the effect of sulfuric acid-coated kaolinite dust particles on Arctic clouds and radiation budget. These authors calculated single contact angles for bare and coated particles and simulated the resulting numbers of ice crystals in a model using Classical Nucleation Theory. Girard et al. also observed that coated particles significantly modified the mixed-phase and ice cloud properties. Under mixed-phase cloud condition coated-particle simulations, there was an increased frequency of liquid water content, indicating very few ice crystals formed, whereas at ice cloud conditions (> −40°C), coated particles that nucleated liquid water froze homogeneously, and thus increased the ice water content.