Article pubs.acs.org/JPCC
Heterogeneous Ice Nucleation on Simulated Sea-Spray Aerosol Using Raman Microscopy Gregory P. Schill and Margaret A. Tolbert* Cooperative Institute for Research in Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: The deliquescence and heterogeneous ice nucleation behavior of simulated sea-spray aerosol was probed from 215 to 235 K using Raman microscopy coupled to an environmental cell. Water uptake prior to deliquescence was also probed on sea-salt particles along deliquescence/ice nucleation experimental trajectories. Synthetic sea-salt particles were generated from solutions of aquarium salt, which emulates all major and minor ions that are found in saltwater. Our results indicate that, under cirrus cloud conditions, sea-salt particles will be in an internally mixed liquid−solid phase, with a brine layer surrounding a crystalline core. Above 230 K, the core of the internally mixed liquid−solid particle fully deliquesces prior to ice nucleation. Below 225 K, however, the crystalline core can act as an efficient immersion-mode ice nuclei prior to full liquefaction. To simulate the presence of carbohydrate-like organics in sea-spray aerosol, sea-salt particles were mixed with the disaccharide sucrose 1:1 by mass. At low relative humidity, the sea-salt particles effloresced into an amorphous organic-salt matrix. Above 225 K, sea-salt/sucrose particles fully deliquesce prior to ice nucleation; however, at lower temperatures, the organic-salt matrix was found to be glassy and could act as an efficient heterogeneous ice nucleus. This work suggests the possible role of sea-salt aerosol in low-temperature ice nucleation. Consistent with this, data from aircraft campaigns suggests sea-salt aerosol is enhanced in ice residuals from low-temperature anvil cirrus whose convective inflow regions are primarily marine environments.
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INTRODUCTION Cirrus clouds are composed almost entirely of ice and are present over 50% of the time in the tropics;1 up to half of cirrus clouds in tropical regions are associated with anvil cirrus, which form at high altitudes in the moist outflows of deep convective clouds.2 Ice clouds can affect climate by scattering incoming solar radiation and scattering and absorbing outgoing terrestrial radiation. The balance of these radiative effects, however, is dependent on the microphysical properties of the ice particles,3 which are affected by the presence of ice freezing nuclei.4 Thus, understanding the sources of ice freezing nuclei in deep convective clouds is integral to unraveling the effects of anvil cirrus on climate. Previously, it was thought that homogeneous ice nucleation would dominate cirrus formation; however, modeling5,6 and field studies7,8 have indicated that heterogeneous pathways may be the dominant mechanism for cirrus cloud formation, especially in the Northern Hemisphere. For example, in a compilation of data from four aircraft measurement campaigns in regions of high cirrus abundance, heterogeneous ice nucleation accounted for 94% of the cirrus cloud encounters, based on analysis of ice residuals.8 In two of the four campaigns, sea salt in ice residues was largely enhanced above sea salt in both background upper tropospheric aerosol and interstitial aerosol. Because it was assumed that sea-salt particles would be deliquesced under cirrus conditions, it was previously © 2014 American Chemical Society
concluded that these residuals were the result of homogeneous freezing.9 Two separate studies have re-examined this assumption and found that crystalline NaCl and NaCl·2H2O particles can depositionally nucleate ice prior to deliquescence.10,11 To the authors’ knowledge, this is the only proposed mechanism for heterogeneous ice nucleation on sea-salt particles under cirrus conditions. One limitation of this mechanism is that natural saltwater contains additional inorganic ions that may cause sea-salt particles to take up water under cirrus conditions and form brine layers on their surface. For such internally mixed liquid−solid particles, the surface properties of the sea-salt aerosol could differ greatly from pure NaCl particles;12 however, the ability of liquid−solid sea-salt particles to nucleate ice remains unexplored. Sea-spray particles also contain organic species that are often enriched in the aerosol phase compared to natural seawater.13 Several studies have shown that carbohydrate-like organic species may form a large portion of the organic mass in seaspray aerosol. For example, laboratory studies using a 33 m wave breaking channel have shown that 200 nm to 1 μm seaSpecial Issue: John C. Hemminger Festschrift Received: May 31, 2014 Revised: July 17, 2014 Published: August 1, 2014 29234
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Raman spectra were obtained using a 532 nm frequencydoubled Nd:YAG laser as the excitation source. Spectra were taken from 200 to 4000 cm−1 with a typical resolution of 2−4 cm−1. Single spectra typically consisted of 256 coadded scans. Points in line maps typically consisted of 16 coadded scans. In all cases, the single-scan collection time was 1 s. Single spectra and line maps were taken with 50× and 100× long-range objectives, which focus the laser to a spot size of approximately 1.3 and 1.1 μm, respectively.23 Unless otherwise noted, Raman spectra were taken at the center of each particle and Raman line maps consisted of Raman spectra taken every micron in the Y direction. Materials. Sea-salt solutions were prepared by adding the synthetic sea-salt (SSS) mixture Instant Ocean to high purity water. Instant Ocean is a commercially available product that is used as an aquarium salt and seeks to contain every major, minor, and trace element necessary for aquatic life. Furthermore, Instant Ocean has previously been used as a proxy for sea-salt particles in heterogeneous oxidation24 and low temperature deliquescence experiments.25 To simulate mixed sea-salt dissolved organic carbon (DOC) particles found in marine environments, SSS was mixed with the disaccharide sucrose [Mallinckrodt, AR (ACS)] in a 1:1 ratio by mass in high purity water. The total weight percent of all solutions was 0.2%. Sucrose was chosen as a proxy for DOC as it has been previously shown that DOC can be up to 80% carbohydratelike organics.26 Further, sucrose has been used in the past as a proxy for atmospheric organic glasses,27 including in experiments regarding water uptake and evaporation 28 and heterogeneous ice nucleation.19,20 Particle Generation. SSS and SSS/sucrose particles were generated by nebulizing droplets from 0.2 wt % solutions of SSS and 1:1 SSS/sucrose, respectively. Solutions were selfaspirated into a Meinhard TR-50 glass concentric nebulizer. The nebulized spray was directed at a hydrophobically treated fused silica disk, where the droplets were allowed to settle and coagulate into supermicron droplets. This droplet-laden disk was then transferred to the environmental cell, where the temperature and relative humidity (RH) were 25 °C and approximately 0%, respectively. The particles were exposed to these conditions for at least 10 min to ensure that the particles were nominally dry. The lateral diameters of the resulting dry particles ranged from 1 to 20 μm. Deliquescence and Ice Nucleation Experiments. Each deliquescence and ice nucleation experiment began at 25 °C and approximately 0% RH. During the experiment, the RH inside the environmental cell was controlled by keeping a constant frost point and lowering the temperature in a controlled manner. Specifically, after the frost point was steady for at least 10 min, the temperature was lowered at a rate of 10 K min−1 until the ice saturation ratio (Sice = Pwater/VPice) was approximately 0.9. At 215 to 235 K, the RH with respect to water at Sice = 0.9 ranged from 53 to 63%. The cell was then cooled at a rate of 0.1 K min−1 until deliquescence or ice nucleation was observed visually and confirmed spectrally. In this work, deliquescence is defined as the RH when all crystalline material inside the particle dissolves. This is an important distinction for sea-salt particles, which begin to take up water at RHs much lower than the full deliquescence RH (DRH). Deliquescence observations were conducted using the 50× objective for 10−20 particles at a time. Experiments were conducted on various sized particles at various positions on the fused-silica disc to minimize systematic errors. Additionally,
spray aerosol are dominated by a particle type that contains sea salt and organic carbon.14 Further, solid-state NMR studies on aerosol generated from Atlantic seawater suggest that submicron sea-spray particles contained up to 73% organics by mass during times of high biological activity.15 Additionally, infrared spectroscopy and positive matrix factorization studies on aerosol collected in the North Atlantic and Arctic seas have determined that an ocean-derived factor accounted for 68 and 37% of the organic mass of submicron aerosol, respectively.16 Moreover, 88% of this ocean-derived factor was due to an organic hydroxyl signature whose calculated infrared spectrum was similar to reference carbohydrates. Using scanning transmission X-ray microscopy near edge absorption fine structure, a majority of aerosol collected during the same campaign were identified as cuboids with thin, homogeneous coatings of organic hydroxyls and potassium. This type of morphology suggests that NaCl crystallized out of a homogeneous solution containing dissolved carbohydrate-like organics and additional inorganic ions. Finally, despite having lower mass fractions of organics in supermicron particles, Raman microscope images and spectra suggest that supermicron particles may also exist as cuboids with thin, organic coatings that can affect ice nucleation.17 Although the presence of dissolved sugars in sea-spray particles is well-known, no studies on the ice nucleation behavior of mixed sea-salt/sugar particles have been reported. In the present study, we have examined the ice nucleation properties of effloresced synthetic sea-salt particles. To emulate the inorganic ions of saltwater, the commercially available aquarium salt Instant Ocean was used. This salt mixture attempts to replicate the concentration of every major and minor inorganic ion found in natural seawater. Our results indicate that sea-salt particles will be in an internally mixed liquid−solid state at ice saturation from 215 to 235 K. Despite having a liquid layer surrounding the crystalline core, these particles can still act as efficient heterogeneous mode ice nuclei at lower temperatures through immersion nucleation. To simulate the effect of adding dissolved carbohydrate-like organic material to sea-salt particles, we mixed Instant Ocean with sucrose 1:1 by mass. For the 1:1 Instant Ocean/sucrose mixtures, we find that the particles effloresce into crystalline sea-salt particles with an organic coating. This organic coating was found to be glassy under low temperature conditions and act as an efficient heterogeneous ice nucleus in agreement with previous ice nucleation studies on sugar-like organic glasses.18−20
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EXPERIMENTAL SECTION Raman Microscope. The experimental apparatus has been described previously in detail.21,22 Briefly a Thermo Nicolet Almega XR Raman spectrometer has been coupled to an Olympus BX-51 microscope with 10×, 20×, 50×, and 100× capabilities, which correspond to numerical apertures of 0.25, 0.40, 0.50, and 0.60, respectively. The Raman microscope has also been outfitted with a Linkam THMS600 environmental cell. A temperature stage inside the cell can be controlled from −196° to 600 °C, with an accuracy of ±0.1 K. Water partial pressure inside the cell is controlled by mixing flows of dry and humidified nitrogen. A Buck research CR-A1 dew point hygrometer in line with the cell measures the frost point with an accuracy of ±0.1 K. A Gast diaphragm pump at the exit of the hygrometer ensures that the flow through the cell is at least 1 L min−1. 29235
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several experiments were reconducted at the coldest temperatures using a cooling rate of 0.01 K min−1, to ensure that solvation kinetics were not artificially inflating reported DRH. For ice nucleation experiments, the initial observation of ice was monitored by scanning the entire disc using the 10× objective. Great care was taken to ensure that the first nucleation event was observed; however, it cannot be discounted that other ice particles had formed outside of the field of view after the observation of this event. If more than one particle had nucleated ice within the field of view, that value was discarded, and the experiment was repeated. After the first ice particle was noted, the 50× objective was used to verify the existence of ice both visually and spectrally. The presence of ice was confirmed in the Raman spectra by the emergence of a broad band centered near 3250 cm−1 (−OH stretch of water− ice). Only after confirmation of ice, the Sice was set to approximately 1.0, and a line map of the entire particle was taken. The ice was then sublimed by turning off the flow of the humidified nitrogen and an image of the dry, bare nucleus was recorded along with an additional Raman spectrum. In all cases, ice had formed on a particle rather than the fused-silica disc. Water Uptake Experiments. During each deliquescence/ ice nucleation experiment, it was noted that pure SSS particles had taken up water and individual particles were in an internally mixed liquid−solid state. To quantify the transition from solid to liquid−solid, we conducted separate water uptake experiments on effloresced SSS particles along the deliquescence/ice nucleation experimental trajectories used in this study. As in deliquescence/ice nucleation experiments, each water uptake experiment began at 25 °C and approximately 0% RH, and the RH inside the environmental cell was controlled by keeping a constant frost point and lowering the temperature. In contrast to the deliquescence/ice nucleation experiments, however, the temperature was lowered in 0.5 K steps. At each step, the temperature was held for at least 5 min; at the end of this 5 min, optical images and Raman spectra were taken. An example of a typical water uptake experiment can be found in Figure 1. At approximately 0% RH, the Raman spectra contains hydrate (3400 cm−1) and sulfate (1010 cm−1) peaks, likely due to MgSO4, CaSO4, and their respective hydrates (Figure 1a).17 A 50× gray-scaled image of a particle at approximately 0% RH is shown in Figure 1b. The irregular, cuboidal shape and bright opacity due to light scattering suggest that the particle is crystalline. The degree of brightness, however, is difficult to assess using the gray-scaled image. Therefore, a binary contrast enhanced image (grayscale cutoff value = 120) found to the right of each gray-scaled image is included to qualitatively assess the degree of white light scattering. As the RH surrounding the sea-salt particle is increased up to 15%, there are minimal changes in both the Raman spectra (Figure 1a) and the binary image (Figure 1c); at 17% RH, however, the particle visually darkens as indicated by an increase in black pixels in the binary contrast image (Figure 1d). Additionally, a small peak centered at 981 cm−1 appears, and the shoulders of the hydrate peak (3200−3600 cm−1) increase in the Raman spectra (Figure 1a). The small peak centered at 981 cm−1 corresponds to unbound or dissolved sulfate,29 and the broad peak in the −OH region indicates uptake of water. By 25% RH, the sharp peak at 981 cm−1 and the broad peak from 3200 to 3600 cm−1 have both increased in intensity, and the particle has darkened considerably (Figure 1e). For reference, a fully deliquesced SSS particle from a separate experiment has also been provided (Figure 1f). Thus, at 17% RH, it appears that the cuboidal
Figure 1. Raman spectra (a) and 50× gray-scaled and binary contrast enhanced images (b−e) from a typical water uptake experiment along the 215 K experimental trajectory. As shown, the particle undergoes minimal change both visually and spectrally from 0 to 15% RH. At 17% RH, however, the particle visually darkens. Further, a small peak at 981 cm−1 (dotted vertical line) appears and the intensity of the −OH stretch (3200−3600 cm−1) increases. These changes correspond to the appearance of unbound, dissolved sulfate29 and the uptake of water, respectively. For comparison, a fully deliquesced SSS particle from a separate experiment at 235 K and its Raman spectra are reported (f). A separate particle is needed because SSS particles nucleate ice prior to deliquescence at 215 K. The dry lateral diameter of the particle (b−e) is approximately 15 μm.
particle has taken up a small layer of water, dampening light scattering, and producing shifts in the Raman spectra. We define the RH at the onset of these changes as the water uptake RH, 17% in this case. Quality Assurance/Quality Control. For each deliquescence, ice nucleation, and water uptake experiment, at least three experiments were conducted at each temperature. Further, at least two solutions and two separate fused silica discs were used at each temperature. Finally, each experiment was conducted on a freshly sprayed disc. Error bars are reported as one standard deviation.
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RESULTS AND DISCUSSION The DRH and heterogeneous ice nucleation onsets for effloresced SSS are shown in Figure 2. At temperatures warmer than ∼230 K, SSS particles fully deliquesce before they nucleate ice. The average DRH from 230 to 235 K was 80.8 ± 0.3%. These points are similar to the DRH values of pure NaCl from 236 to 240 K (78 ± 2%),10 suggesting that the final crystalline inclusions to deliquesce are NaCl. This is in agreement with the study by Koop et al.,25 who have shown that the DRH of SSS from 239 to 273 K is not significantly different from pure NaCl. At temperatures below ∼225 K, the SSS particles nucleate ice before they fully deliquesce. Also shown in Figure 2 are previous ice nucleation results from the same setup on effloresced NaCl and NaCl·2H2O.10 It can be seen that the heterogeneous ice nucleation behavior of SSS differs from effloresced NaCl particles. Both effloresced NaCl and NaCl· 29236
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Figure 2. Full DRH and onset ice saturation ratios for heterogeneous ice nucleation on effloresced SSS particles as a function of temperature. The solid line indicates water saturation (100% RH), and the dashed gray lines correspond to 90, 80, 70, and 60% RH. The dashed black line corresponds to the homogeneous ice nucleation limit.39 Also shown are ice nucleation data on effloresced NaCl and NaCl·2H2O.10 For clarity, two regions of different behavior have been shaded. In the green shaded region, the particles deliquesce before they can nucleate ice. In the purple shaded region, the particles nucleate ice heterogeneously before they fully deliquesce. Error bars are reported as one standard deviation.
Figure 3. Water uptake relative humidities for pure, effloresced SSS particles from 225 to 255 K. Also plotted are the deliquescence/ice nucleation results from Figure 2. The solid black line corresponds to Sice = 1, and the dotted gray lines correspond to deliquescence/ice nucleation experimental trajectories for constant frost points. As shown, the water uptake RH for all temperatures explored are lower than the ice saturation curve. Thus, the particles are in a mixed liquid−solid phase at the start of the deliquescence/ice nucleation experiments from 215 to 235 K.
2H2O nucleate ice before they deliquesce at 230 and 235 K, but the SSS particles deliquesce before they nucleate ice. Additionally, at 225 K, both particle types nucleate ice heterogeneously, but sea-salt particles require higher Sice values to nucleate ice than pure NaCl particles. Thus, the presence of additional inorganic ions seems to affect the hygroscopic and surface properties of sea-salt particles compared to pure NaCl particles. To investigate this further, we conducted water uptake experiments along each experimental trajectory (Figure 3), and we collected optical microscope images and Raman spectra of SSS particles through the course of a typical ice nucleation experiment (Figure 4). As shown in Figure 3, as the frost point is held constant and the temperature is decreased, the SSS particles take up water well below the ice saturation line for all experimental trajectories explored in this work. Further, there seems to be little temperature dependence in the water uptake RH for pure SSS particles; across all temperatures studied, the average water uptake RH was 17 ± 1%. This is corroborated by the findings in Figure 4. As shown, the SSS particle begins the experiment as a cuboid with a sulfate peak at 1010 cm−1, consistent with effloresced SSS particles (Figure 4a). By 37% RH, the particle is above the water uptake RH and has visually darkened, the sulfate band has shifted to 981 cm−1, and the area under the −OH peak has increased (Figure 4b). At this point, the particle is in an internally mixed liquid−solid phase and can only nucleate ice in the immersion mode. At Sice ∼ 1, the particle has picked up enough water that the outline of the particle is now spherical, although there are still crystalline inclusions inside available as immersion mode ice nuclei (Figure 4c). These inclusions are better visualized in the binary contrast image (threshold = 75) provided (Figure 4c, inset). Further, this morphological change is accompanied by a large increase in the area under the −OH stretch. In the final frame (Figure 4d), the particle has nucleated ice in the immersion
Figure 4. Images (50×) of an SSS particle dry (a), during a typical ice experiment (b), at Sice ∼ 1 (c), and after it has nucleated ice at 215 K (d). As shown, SSS particles take up an appreciable amount of water before the start of an ice experiment and, therefore, must be nucleating ice in the immersion mode. In the lower panel, Raman spectra taken at the center of the particle are shown. The size bar in the lower left hand corner of panel (a) corresponds to 10 μm.
mode; the crystalline inclusion that acted as the ice nucleus can be seen in the center of the particle. This is shown more clearly in the Raman line map (Figure 5). Here, the hydrate and sulfate stretches are correlated with the dimensions of the ice nucleus. The water−ice stretch, however, extends beyond the dimensions of the hydrate and sulfate stretches. 29237
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Figure 5. Line map (100×) of the SSS particle that has nucleated ice in Figure 4d. As shown, the edges of the ice particle exhibit only water−ice stretches, but the immersion ice nucleus in the center of the particle is correlated with the hydrate and sulfate peaks.
particles, suggesting that a glassy sucrose coating could be dictating the ice nucleation properties of SSS/sucrose particles below ∼220 K. To investigate this further, we have collected optical images and Raman spectra through the course of a typical ice nucleation experiment on a SSS/sucrose particle (Figure 7). As shown, when the particle is at Sice = 0.00, the
To investigate the effect of sugar-like molecules on sea-salt particles, we mixed Instant Ocean with equal parts sucrose, a disaccharide similar in composition to the sugar-like molecules found in the ocean. The DRH and heterogeneous ice nucleation onsets for these particles from 215 to 235 K are shown in Figure 6. At temperatures above ∼225 K, these mixed
Figure 6. Full DRH and onset ice saturation ratios for heterogeneous ice nucleation on effloresced SSS/sucrose particles as a function of temperature. The solid line indicates water saturation (100% RH), and the dashed gray lines correspond to 90, 80, 70, and 60% RH. The dashed black line corresponds to the homogeneous ice nucleation limit.39 Also shown are ice nucleation onsets for pure, amorphous sucrose.20 For clarity, two regions of different behavior have been shaded. In the green shaded region, the particles deliquesce before they can nucleate ice. In the purple shaded region, the particles nucleate ice heterogeneously before they fully deliquesce. Due to the kinetic effects associated with humidity-induced glass transitions, these shaded regions are relevant for the trajectories, humidification rate, and particle sizes used in this study. Error bars are reported as one standard deviation.
Figure 7. Images (50×) of an SSS/sucrose particle dry (a), during a typical ice experiment (b), at Sice ∼ 1 (c), and after it has nucleated ice at 215 K. As shown, SSS/sucrose does not uptake a quantifiable amount of water prior to ice nucleation and, therefore, could be nucleating ice in the depositional mode or in immersion mode with trace amounts of water. In the lower panel, the Raman spectra taken at the center of the particle are shown. The size bar in the lower left hand corner of panel (a) corresponds to 10 μm.
particle is a slightly irregular cuboid with a dark coating. This is consistent with an effloresced particle in an amorphous organic matrix.16,17 The absence of a sulfate peak at 981 cm−1 indicates that the aforementioned bound sulfate species are present in these mixed sea-salt organic particles. Although sucrose is a hygroscopic species,27 the particle does not visually take up water at Sice = 0.53 (Figure 7b) or near ice saturation at Sice = 0.97 (Figure 7c). We suggest that this is because the organic matrix is in a highly viscous (semi-)semisolid or glassy state under these temperature and RH conditions.31 This is in concurrence with the Raman spectral data, which verify that, within the limitations of the spectrometer, the particle does not take up water. This is in stark contrast with the results on pure
particles fully deliquesce before they nucleate ice. Further, in comparing of Figures 2 and 6, it can be seen that the addition of sucrose to SSS slightly decreases their DRH (76 ± 2%). This behavior has been noted previously for mixed inorganic− organic particles.30 At temperatures below ∼220 K, however, the SSS/sucrose particles nucleate ice before they fully deliquesce. Also shown in Figure 6 are the Sice values for the onset of heterogeneous ice nucleation on pure, glassy sucrose from 215 to 235 K from Baustian et al.20 As shown, the onset Sice values for the SSS/ sucrose particles are similar to the pure amorphous sucrose 29238
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Figure 8. Line map (100×) of the SSS/sucrose particle that has nucleated ice in Figure 7d. As shown, the ice particle has formed on the outside of the particle, and the organic stretch is not spatially correlated with the ice stretch.
heterogeneous ice nucleation mechanism. It is important to note that these cirrus clouds were associated with deep convection, and the in-cloud and near-cloud particle populations may be different; previous studies, however, have shown that even in-cloud interstitial aerosol is far depleted in sea-salt particles compared to ice residuals.9 Wise et al.10 proposed that the ice nuclei could be effloresced NaCl particles, and they also found that NaCl·2H2O particles are particularly efficient ice nuclei, with ice nucleating near Sice = 1.0. Follow up work using the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber and infrared spectroscopy confirmed that NaCl·2H2O particles can form below 227.1 K and are efficient depositional ice nuclei.11 The salt particles observed in the AIDA chamber study had slightly higher Sice values (1.15−1.25) but are still considered to have essentially the same ice nucleating ability as mineral dust under these conditions.11 It should be noted that that the onset Sice values found in these two laboratory experiments are not directly comparable due to differences in particle size, mass, and available surface area.35 Although pure NaCl is an efficient ice nucleus, sea-salt aerosol contains additional deliquescent inorganic salts that alter the chemical composition and can impact hygroscopic properties. Here, we have shown that dry SSS particles form internally mixed liquid−solid phases as the RH is increased under cirrus conditions. Consequently, these internally mixed liquid−solid SSS particles have different ice nucleation mechanisms and efficiencies than pure NaCl. We find that SSS particles deliquesce prior to nucleating ice above 230 K; however these particles undergo efficient immersion ice freezing from 215 to 225 K with Sice = 1.08−1.29. In addition, we have also shown SSS/sucrose particles may have a glassy organic coating readily available for depositional ice nucleation below 220 K at Sice = 1.12−1.16. Above 225 K, however, these particles deliquesce prior to ice nucleation. Thus, we suggest that sea-salt and mixed sea-salt/disaccharide particles can influence ice cloud formation in both the depositional and immersion mode below 225 K. The anvil cirrus encountered during the heterogeneous freezing cases during CRYSTAL-FACE were collected from 11.3 to 13.9 km; the campaign-average temperatures for these altitudes was approximately 204 to 224 K.36 Further, ice residuals were sampled during TC4 from convective anvil cirrus 8−13 km.8 This corresponds to campaign-mean temperatures as cold as 215 K.37 These temperatures overlapped with the regions of heterogeneous ice nucleation for the sea-salt and mixed sea-salt/organic particles in this study. Finally, the convective inflows during CRYSTAL-FACE and TC4 were
SSS (Figure 4). In Figure 7d, the mixed sea-salt/organic particle appears to have nucleated ice in the depositional mode. We cannot, however, discount the possibility that a thin, nanometer-scale liquid layer has formed on the surface of the organic matrix.32 Here, a thin, but fully encompassing aqueous organic layer may form on the surface of the glassy matrix if the RH surpasses the partial deliquescence glass transition RH. Under these conditions, the glassy matrix is in quasiequilibrium with the surrounding water vapor. Thus, immersion mode freezing of a nanometer-scale layer of water by the subsurface highly viscous organic matrix cannot be discounted; however, due to the limitations of our instrument, we will continue to refer to this as depositional freezing. Regardless of the freezing mode, however, these results suggest that the bulk of the organic matrix is viscous enough to serve as a heterogeneous IN under these conditions. To confirm this, we have taken a line map of the particle in Figure 7d (Figure 8). In previous studies, we have shown that low viscosity, liquid organics will coat growing ice particles to reduce the surface tension over the entire system.22 In Figure 8, the ice particle has formed on the outside of the ice nucleus and the organic stretch is not spatially correlated with the ice stretch, indicating that the organic did not spread out over the surface of the growing ice particle. Thus, the organic coating was in a nonflowing, likely glassy state and, therefore, was available as a surface for heterogeneous ice nucleation. Finally, the authors note that at 225 and 230 K pure amorphous sucrose nucleates ice depositionally at RH values similar to that observed for full deliquescence of mixed SSS/ sucrose particles. Thus, the inorganic ions in sea salt must act a plasticizing agent and lower the glass transition temperature and/or RH compared to the pure glass. This behavior has been noted before for organic glasses containing inorganic sulfuric acid,33 sulfate,21,34 and NaCl.28
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ATMOSPHERIC IMPLICATIONS
In a compilation of flight data from four different aircraft campaigns spanning almost 10 years and covering a range of geographic regions and seasons, it was shown that sea-salt particles were enhanced in heterogeneous ice residuals compared to background free tropospheric aerosol. In two of the flight missions, Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTALFACE) and Tropical Composition, Cloud, and Climate Coupling (TC4), sea-salt particles were found to be 19 and 31% of all ice residuals, respectively; these numbers are greatly enhanced from the abundance of sea-salt particles found in near cloud, free troposphere air (
