Continuous Flow Diffusion Chamber Research Articles

Kaolinite particles as ice nuclei: learning from the use of different kaolinite samples and different coatings

Abstract. Kaolinite particles from two different sources (Fluka and Clay Minerals Society (CMS)) were examined with respect to their ability to act as ice nuclei (IN). This was done in the water-subsaturated regime where often deposition ice nucleation is assumed to occur, and for water-supersaturated conditions, i.e., in the immersion freezing mode. Measurements were done using a flow tube (the Leipzig Aerosol Cloud Interaction Simulator, LACIS) and a continuous-flow diffusion chamber (CFDC). Pure and coated particles were used, with coating thicknesses of a few nanometers or less, where the coating consisted of levoglucosan, succinic acid or sulfuric acid. In general, it was found that the coatings strongly reduced deposition ice nucleation. Remaining ice formation in the water-subsaturated regime could be attributed to immersion freezing, with particles immersed in concentrated solutions formed by the coatings. In the immersion freezing mode, ice nucleation rate coefficients jhet from both instruments agreed well with each other, particularly when the residence times in the instruments were accounted for. Fluka kaolinite particles coated with either levoglucosan or succinic acid showed the same IN activity as pure Fluka kaolinite particles; i.e., it can be assumed that these two types of coating did not alter the ice-active surface chemically, and that the coatings were diluted enough in the droplets that were formed prior to the ice nucleation, so that freezing point depression was negligible. However, Fluka kaolinite particles, which were either coated with pure sulfuric acid or were first coated with the acid and then exposed to additional water vapor, both showed a reduced ability to nucleate ice compared to the pure particles. For the CMS kaolinite particles, the ability to nucleate ice in the immersion freezing mode was similar for all examined particles, i.e., for the pure ones and the ones with the different types of coating. Moreover, jhet derived for the CMS kaolinite particles was comparable to jhet derived for Fluka kaolinite particles coated with sulfuric acid. This is suggestive for the Fluka kaolinite possessing a type of ice-nucleating surface feature which is not present on the CMS kaolinite, and which can be destroyed by reaction with sulfuric acid. This might be potassium feldspar.

Feldspar minerals as efficient deposition ice nuclei

Abstract. Mineral dusts are well known to be efficient ice nuclei, where the source of this efficiency has typically been attributed to the presence of clay minerals such as illite and kaolinite. However, the ice nucleating abilities of the more minor mineralogical components have not been as extensively examined. As a result, the deposition ice nucleation abilities of 24 atmospherically relevant mineral samples have been studied, using a continuous flow diffusion chamber at −40.0 ± 0.3 °C and particles size-selected at 200 nm. By focussing on using the same experimental procedure for all experiments, a relative ranking of the ice nucleating abilities of the samples was achieved. In addition, the ice nucleation behaviour of the pure minerals is compared to that of complex mixtures, such as Arizona Test Dust (ATD) and Mojave Desert Dust (MDD), and to lead iodide, which has been previously proposed for cloud seeding. Lead iodide was the most efficient ice nucleus (IN), requiring a critical relative humidity with respect to ice (RHi) of 122.0 ± 2.0% to activate 0.1% of the particles. MDD (RHi) 126.3 ± 3.4%) and ATD (RHi 129.5 ± 5.1%) have lower but comparable activity. From a set of clay minerals (kaolinite, illite, montmorillonite), non-clay minerals (e.g. hematite, magnetite, calcite, cerussite, quartz), and feldspar minerals (orthoclase, plagioclase) present in the atmospheric dusts, it was found that the feldspar minerals (particularly orthoclase) and some clays (particularly kaolinite) were the most efficient ice nuclei. Orthoclase and plagioclase were found to have critical RHi values of 127.1 ± 6.3% and 136.2 ± 1.3%, respectively. The presence of feldspars (specifically orthoclase) may play a significant role in the IN behaviour of mineral dusts despite their lower percentage in composition relative to clay minerals.

Laboratory investigation of Martian water ice cloud formation using dust aerosol simulants

The ice nucleation abilities of submicron aerosol particles of two Martian regolith analogs, the Mojave Mars simulant and Johnson Space Center Mars‐1, were investigated with the University of Toronto continuous flow diffusion chamber. The temperature range studied (> 200 K) is relevant to low‐altitude water ice cloud formation in the Martian atmosphere and the aerosol particles were suspended in air, in contrast to previous experiments. Both simulants were found to be active ice nuclei in the deposition nucleation mode between 223 K and 203 K. The Mojave Mars simulant particles were found to be better ice nuclei than the Johnson Space Center Mars‐1 particles requiring lower supersaturations to nucleate ice. It was observed that the critical supersaturation (Scrit) to activate 1% of the particles increased with decreasing temperature in accord with previous low‐temperature studies, rising to a value of above 1.7 at 203 K. This corroborates literature results that there is a substantial barrier to ice nucleation at low temperatures, underlining the need for incorporating this effect in Martian cloud microphysical models. It was also found that Scrit did not change when the size of the Mojave Mars simulant particles was increased from 240 to 400 nm. Comparison of the Martian simulants with other mineral dusts shows that the Johnson Space Center Mars‐1 analog behaves similarly to the well‐known terrestrial ice nuclei such as kaolinite and Arizona test dust particles, whereas the Mojave Mars simulant behaves closer to another clay, montmorillonite. The wettability parameter, m, was calculated to range from 0.955 to 0.959.

Biogenic ice nuclei in boundary layer air over two U.S. High Plains agricultural regions

With 18% of the total U.S. landmass devoted to croplands, farmland and farming activities are potentially major sources of biogenic particles to the atmosphere. Farms harbor large populations of microbes both in the soil and on plant surfaces which, if injected into the atmosphere, may serve as nuclei for clouds. In this study, we investigated two farms as potential sources of biological ice nuclei (IN): an organic farm in Colorado and a cornfield in Nebraska. We used a continuous‐flow diffusion chamber (CFDC) to obtain real‐time measurements of IN at these farm sites. Total aerosol particles were also collected at the sites, and their temperature‐dependent ice nucleating activity was determined using the drop freezing method. Quantitative polymerase chain reaction and DNA sequencing of 16S rDNA clone libraries were used to test aerosols and washings of local vegetation for abundance of theinagene in ice nucleation active bacteria (from the well‐known group within theγ‐Proteobacteria) and to identify airborne primary biological aerosol particles. The vegetation in each of these farms contained 6 × 105 to 2 × 107 ina genes per gram vegetation. In contrast to the vegetation, airborne ina gene concentrations at the organic farm were typically below detectable limits, demonstrating a disconnect between local vegetative sources and the air above them. However, for measurements made during combine harvesting at the Nebraska corn field, ina gene concentrations were 19 L−1, with maximum IN concentrations of 50 L−1 determined from the CFDC at −20°C and above water saturation. At both farms, there was also an apparent biological contribution to the IN population which did not contain the ina gene.

Combustion particles as ice nuclei in an urban environment: Evidence from single-particle mass spectrometry

This paper presents measurements of the single-particle composition of ice nuclei (IN) in downtown Toronto, Canada, made at 239 ± 1 K, 134 ± 2% relative humidity with respect to ice (RHi). IN were activated within the University of Toronto Continuous-Flow Diffusion Chamber (UT-CFDC), separated from background aerosol using a pumped counterflow virtual impactor (PCVI), and analyzed using a dual-polarity single-particle mass spectrometer (ATOFMS). To account for particles leaked by the PCVI, the ratio of particles observed at high RHi to low RHi was calculated. This ratio was greater than unity for EC (elemental carbon), BB (biomass burning) and dust particles, however only the increase in EC was statistically significant (1σ level) due to a low number of detected particles. The remaining particle categories were: metal-rich organic carbon (M/OC), potassium-rich OC (K/OC), OC potentially mixed with EC (OC/EC) and “other”, none of which were enhanced in number at high RHi. To complement the direct PCVI observations and enhance the number of spectra available, a second study was performed where particle composition and IN concentrations (CIN) were measured in parallel. IN concentrations were regressed against the same categories as above, and dust, BB and EC particles were identified as the best predictors of CIN. Consistency between these two methods provides increased confidence in their individual results and encourages additional study of these potential IN.

Observations of ice nuclei and heterogeneous freezing in a Western Pacific extratropical storm

Abstract. In situ airborne sampling of refractory black carbon (rBC) particles and Ice Nuclei (IN) was conducted in and near an extratropical cyclonic storm in the western Pacific Ocean during the Pacific Dust Experiment, PACDEX, in the spring of 2007. Airmass origins were from Eastern Asia. Clouds associated primarily with the warm sector of the storm were sampled at various locations and altitudes. Cloud hydrometeors were evaporated by a counterflow virtual impactor (CVI) and the residuals were sampled by a single particle soot photometer (SP2) instrument, a continuous flow diffusion chamber ice nucleus detector (CFDC) and collected for electron microscope analysis. In clouds containing large ice particles, multiple residual particles were observed downstream of the CVI for each ice particle sampled on average. The fraction of rBC compared to total particles in the residual particles increased with decreasing condensed water content, while the fraction of IN compared to total particles did not, suggesting that the scavenging process for rBC is different than for IN. In the warm sector storm midlevels at temperatures where heterogeneous freezing is expected to be significant (here −24 to −29 °C), IN concentrations from ice particle residuals generally agreed with simultaneous measurements of total ice concentrations or were higher in regions where aggregates of crystals were found, suggesting heterogeneous freezing as the dominant ice formation process in the mid levels of these warm sector clouds. Lower in the storm, at warmer temperatures, ice concentrations were affected by aggregation and were somewhat less than measured IN concentrations at colder temperatures. The results are consistent with ice particles forming at storm mid-levels by heterogeneous freezing on IN, followed by aggregation and sedimentation to lower altitudes. Compositional analysis of the aerosol and back trajectories of the air in the warm sector suggested a possible biomass burning source for much of the aerosol. Comparison of the particles from the CFDC with the other aerosol in the residuals of ice particles suggested that the largest portion of IN had similar inferred origins (from biomass burning with minor amounts of rBC) as the other aerosol, but contained slightly elevated amounts of calcium and less influence from sea salt.

Manchester Ice Nucleus Counter (MINC) measurements from the 2007 International workshop on Comparing Ice nucleation Measuring Systems (ICIS-2007)

Abstract. An ice nucleus counter was developed and constructed to enable investigation of potential ice nucleating materials. The Manchester Ice Nucleus Chamber (MINC) is a concentric-cylinder continuous flow diffusion chamber (CFDC). A full explanation of the MINC instrument is given here, along with first results and a comparison to an established instrument of similar design (Colorado State University CFDC) during sampling of common ice nucleating aerosols at the 2007 International workshop on Comparing Ice nucleation Measuring Systems (ICIS-2007). MINC and CSU-CFDC detected the onset of ice nucleation under similar conditions of temperature and supersaturation for several different types of ice nuclei. Comparisons of the ratio of ice nuclei to total aerosol concentrations as a function of supersaturation with respect to water (SSw) showed agreement within one order of magnitude. Possible reasons for differences between the two instruments relating to differences in their design are discussed, along with suggestions to future improvements to the current design.

Results from the University of Toronto continuous flow diffusion chamber at ICIS 2007: instrument intercomparison and ice onsets for different aerosol types

Abstract. The University of Toronto continuous flow diffusion chamber (UT-CFDC) was used to study heterogeneous ice nucleation at the International Workshop on Comparing Ice Nucleation Measuring Systems (ICIS 2007) which also represented the 4-th ice nucleation workshop, on 14–28 September 2007. One goal of the workshop was to inter-compare different ice nucleation measurement techniques using the same aerosol sample source and preparation method. The aerosol samples included four types of desert mineral dust, graphite soot particles, and live and dead bacterial cells (Snomax®). This paper focuses on the UT-CFDC results, with a comparison to techniques of established heritage including the Colorado State CFDC and the AIDA expansion chamber. Good agreement was found between the different instruments with a few specific differences, especially at low temperatures, perhaps due to the variation in how onset of ice formation is defined between the instruments and the different inherent residence times. It was found that when efficiency of ice formation is based on the lowest onset relative humidity, Snomax® particles were most efficient followed by the desert dusts and then soot. For all aerosols, deposition mode freezing was only observed for T<45 K except for the dead bacteria where freezing occurred below water saturation as warm as 263 K.

Laboratory investigations of the impact of mineral dust aerosol on cold cloud formation

Abstract. Dust particles represent a dominant source of particulate matter (by mass) to the atmosphere, and their emission from some source regions has been shown to be transported on regional and hemispherical scales. Dust particles' potential to interact with water vapor in the atmosphere can lead to important radiative impacts on the climate system, both direct and indirect. We have investigated this interaction for several types of dust aerosol, collected from the Southwestern United States and the Saharan region. A continuous flow diffusion chamber was operated to measure the ice nucleation ability of the dust particles in the temperature range of relevance to cirrus and mixed-phase clouds (−65

Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation

Abstract. During the FROST-2 (FReezing Of duST) measurement campaign conducted at the Leipzig Aerosol Cloud Interaction Simulator (LACIS), we investigated changes in the ice nucleation properties of 300 nm Arizona Test Dust mineral particles following thermochemical processing by varying amounts and combinations of exposure to sulphuric acid vapour, ammonia gas, water vapour, and heat. The processed particles' heterogeneous ice nucleation properties were determined in both the water subsaturated and supersaturated humidity regimes at −30 °C and −25 °C using Colorado State University's continuous flow diffusion chamber. The amount of sulphuric acid coating material was estimated by an aerosol mass spectrometer and from CCN-derived hygroscopicity measurements. The condensation of sulphuric acid decreased the dust particles' ice nucleation ability in proportion to the amount of sulphuric acid added. Heating the coated particles in a thermodenuder at 250 °C – intended to evaporate the sulphuric acid coating – reduced their freezing ability even further. We attribute this behaviour to accelerated acid digestion of ice active surface sites by heat. Exposing sulphuric acid coated dust to ammonia gas produced particles with similarly poor freezing potential; however a portion of their ice nucleation ability could be restored after heating in the thermodenuder. In no case did any combination of thermochemical treatments increase the ice nucleation ability of the coated mineral dust particles compared to unprocessed dust. These first measurements of the effect of identical chemical processing of dust particles on their ice nucleation ability under both water subsaturated and mixed-phase supersaturated cloud conditions revealed that ice nucleation was more sensitive to all coating treatments in the water subsaturated regime. The results clearly indicate irreversible impairment of ice nucleation activity in both regimes after condensation of concentrated sulphuric acid. This implies that the sulphuric acid coating caused permanent chemical and/or physical modification of the ice active surface sites; the possible dissolution of the coating during droplet activation did not restore all immersion/condensation-freezing ability.

Chemical processing does not always impair heterogeneous ice nucleation of mineral dust particles

Exposing Arizona Test Dust (ATD) particles to nitric acid vapor in an aerosol flow tube impaired subsequent deposition ice nucleation below water‐saturation, but promoted condensation/immersion‐freezing on approach to water saturation and had no apparent impact on freezing of activated droplets above water saturation. The fraction of particles capable of nucleating ice at −30°C was determined using a continuous flow diffusion chamber. Exposure to HNO3 at 26% relative humidity with respect to water (RHw) reduced the fraction of particles subsequently nucleating ice to below our quantification limit in the deposition nucleation regime below 97% RHw, while leading to a sharper step‐wise increase in ice nucleation between 97–100% RHw compared to unreacted dust. These observations contrast with the effect of concentrated sulfuric acid condensation, which in most cases has been reported to reduce ice nucleation of ATD and other dusts both below and above water saturation.

A Classical-Theory-Based Parameterization of Heterogeneous Ice Nucleation by Mineral Dust, Soot, and Biological Particles in a Global Climate Model

AbstractAn ice nucleation parameterization based on classical nucleation theory, with aerosol-specific parameters derived from experiments, has been implemented into a global climate model—the Community Atmosphere Model (CAM)-Oslo. The parameterization treats immersion, contact, and deposition nucleation by mineral dust, soot, bacteria, fungal spores, and pollen in mixed-phase clouds at temperatures between 0° and −38°C. Immersion freezing is considered for insoluble particles that are activated to cloud droplets, and deposition and contact nucleation are only allowed for uncoated, unactivated aerosols. Immersion freezing by mineral dust is found to be the dominant ice formation process, followed by immersion and contact freezing by soot. The simulated biological aerosol contribution to global atmospheric ice formation is marginal, even with high estimates of their ice nucleation activity, because the number concentration of ice nucleation active biological particles in the atmosphere is low compared to other ice nucleating aerosols. Because of the dominance of mineral dust, the simulated ice nuclei concentrations at temperatures below −20°C are found to correlate with coarse-mode aerosol particle concentrations. The ice nuclei (IN) concentrations in the model agree well overall with in situ continuous flow diffusion chamber measurements. At individual locations, the model exhibits a stronger temperature dependence on IN concentrations than what is observed. The simulated IN composition (77% mineral dust, 23% soot, and 10−5% biological particles) lies in the range of observed ice nuclei and ice crystal residue compositions.

Observations of ice nucleation by ambient aerosol in the homogeneous freezing regime

We measured the freezing activation curves for ambient particles as a function of relative humidity with respect to water over the temperature range of −40° to −50°C using a continuous flow diffusion chamber, and compared the observations with those for ammonium sulfate particles and predictions from a parametric representation of homogeneous freezing of solution particles as a function of water activity and temperature. Since it has been proposed that the rate of homogeneous freezing depends on solution water activity, we made separate measurements of the hygroscopicity (κ) of the ambient aerosol. Observed κ ranged from 0.1 to 0.2, lower than that of ammonium sulfate (0.6) and representative of a continental aerosol. As predicted for this difference in κ, there was no significant difference between the homogeneous freezing conditions of size‐selected ammonium sulfate and the apparent homogeneous freezing conditions of same‐sized ambient aerosol. Further, the parameterization predicted freezing fraction‐relative humidity relationships for non size‐selected ambient aerosol that differed by only 0.5 to 1.5% relative humidity from observed relations at the tested temperatures, well within experimental uncertainty. Our findings confirm that the tested ambient aerosols, with hygroscopicities typical of continental regions, freeze homogeneously as expected based on present understanding for single component solution drops in the laboratory. Results also confirm that freezing is more sensitive to particle size than to composition, for particles containing at least a few percent by volume of hygroscopic species.

Ice Nucleation onto Arizona Test Dust at Cirrus Temperatures: Effect of Temperature and Aerosol Size on Onset Relative Humidity

The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC) was used to study ice formation onto monodisperse Arizona Test Dust (ATD) particles. The onset relative humidity with respect to ice (RH(i)) was measured as a function of temperature in the range 251-223 K for 100 nm ATD particles. It was found that for 0.1% of the particles to freeze, water saturation was required at all temperatures except 223 K where particles activated at RH(i) below water saturation. At this temperature, where deposition mode freezing is occurring, we find that the larger the particle size, the lower the onset RH(i). We also demonstrate that the total number of particles present may influence the onset RH(i) observed. The surface area for ice activation, aerosol size, and temperature must all be considered when reporting onset values of ice formation onto ATD mineral dust particles. In addition, we calculate nucleation rates and contact angles of ice germs with ATD aerosols which indicate that there exists a range of active sites on the surface with different efficiencies for activating ice formation.

Ice nucleation behavior of biomass combustion particles at cirrus temperatures

Measurements of ice formation by biomass combustion particles generated in controlled laboratory burns were made using a continuous flow diffusion chamber operating in the temperature range between −45 and −60°C. These measurements are the first of their kind to investigate the role that such particles may play in ice formation in upper tropospheric clouds. Measurements of aerosol water uptake were used to examine the role of hygroscopicity in low‐temperature ice nucleation by biomass combustion particles. Ice formation by the smoke particles at below −45°C and below water saturation followed the temperature and relative humidity dependencies predicted for water activity‐based homogeneous freezing within measurement uncertainties. As predicted, relative humidity dependence of homogeneous freezing fractions on hygroscopicity was weak in comparison with the temperature dependence for biomass particles of typical atmospheric sizes. The action of heterogeneous ice nucleation in or on these small smoke particles in the haze particle regime could not be distinguished within measurement uncertainties. Impedance of homogeneous freezing below −50°C was inferred for freezing higher number fractions of predominantly organic smoke particles separately reported to contain an abundance of large organic molecules, consistent with the recently predicted onset conditions for and impact of viscous or glassy aerosol phase states. Finally, we note that precision and accuracy requirements of instrumentation for resolving modest impacts (a few degrees less supercooling or a few percent lower relative humidity) of heterogeneous freezing, as compared with homogeneous freezing in the upper tropospheric temperature regime present difficulties for validating aerosol impacts on cirrus.

The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC): A Simple Design for Ice Nucleation Studies

A new instrument, the University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC), has been designed to study ice nucleation at low temperatures. Based on previous continuous flow instruments, it is a parallel plate model that minimizes convective instabilities by operating horizontally with the warmer plate on top. A variable position sample injector can account for effects arising from gravitational settling of ice particles that form. The residence time in the chamber can vary between 2.6 to 25 s and ice particle formation is monitored with a two-channel optical particle counter. Observation of homogeneous freezing of 100 nm sulfuric acid aerosols was used to verify the accuracy of the calculated relative humidities (RHs) in the chamber to be ±4%, where we report onset RHs for 0.1% of the particles freezing in the temperature range of 218 to 243 K. We also show that the chamber accurately establishes conditions of water saturation by conducting water uptake studies onto sulfuric acid aerosol at 2...

Ice nuclei emissions from biomass burning

Biomass burning is a significant source of carbonaceous aerosol in many regions of the world. When present, biomass burning particles may affect the microphysical properties of clouds through their ability to function as cloud condensation nuclei or ice nuclei. We report on measurements of the ice nucleation ability of biomass burning particles performed on laboratory‐generated aerosols at the second Fire Lab at Missoula Experiment. During the experiment we generated smoke through controlled burns of 21 biomass fuels from the United States and Asia. Using a Colorado State University continuous flow diffusion chamber, we measured the condensation/immersion freezing potential at temperatures relevant to cold cumulus clouds (−30°C). Smokes from 9 of the 21 fuels acted as ice nuclei at fractions of 1:10,000 to 1:100 particles in at least one burn of each fuel; emissions from the remaining fuels were below the ice nuclei detection limit for all burns of each fuel. Using a bottom‐up emission model, we estimate that smokes that emit ice nuclei fractions exceeding 1:10,000 particles can perturb ice nuclei concentrations on a regional scale.

Correction to “African dust aerosols as atmospheric ice nuclei”

[1] In the paper “African dust aerosols as atmospheric ice nuclei” by P. J. DeMott et al. (Geophysical Research Letters, 30(14), 1732, doi:10.1029/2003GL017410, 2003), it was found that a standard processing procedure was incorrectly performed. The consequence of that error is a reduction in the previously reported ice nuclei (IN) number concentrations associated with aerosol measurements within the Saharan Aerosol Layer (SAL). While these results do not affect the basic conclusions of the study, the data have been frequently cited and used as a basis for numerical simulations of dust impacts on ice phase formation in Florida storms during the NASA CRYSTAL-FACE project [Phillips et al., 2008; van den Heever et al., 2006]. We therefore provide some corrected data and Figures 2 and 4 in this note. [2] The continuous flow diffusion chamber (CFDC) used for the measurements reported by DeMott et al. [2003] uses the growth in optical size of ice crystals nucleated on ambient aerosol particles as the basis for determining ice nuclei concentrations, following exposure to ice supersaturated conditions at one processing temperature. Standard procedure also involves use of an aerosol impactor upstream of the CFDC to limit aerosol aerodynamic sizes introduced into the diffusion chamber to a size that is approximately one-half of the size used to determine the presence of ice crystals nucleated and grown in the CFDC. The size separation of liquid aerosol or activated cloud droplets from grown ice crystals is assured by passing the flow through an ice saturated section in the lower one-third of the CFDC. For CRYSTAL-FACE, the inlet aerosol impactor had a cut-size of 1 μm for the volumetric flow used. The intended threshold (lower) ice crystal size limit for this project was 2 μm. [3] Although the optical particle counter used for sizing particles exiting the CFDC was calibrated before and after the field deployment to assure correct size attribution of nucleated ice crystals versus aerosols, during processing, the calibration data were not applied correctly. Instead of setting the lower size limit for ice crystals to 2 μm, it was set to 1 μm or the same value as the 50% cut-point of the inlet aerosol impactor. Since a substantial number of supermicron particles were present in the SAL during CRYSTAL-FACE (Figure 2, revised herein), the IN concentrations reported were rather strongly “contaminated” by aerosol influences. Reprocessed data using the correct lower size limit for nucleated ice crystal attribution are now shown in revised Figure 2, and in revised Figure 4, which compares the IN number concentration vertical profiles on three days during CRYSTAL-FACE. Peak IN number concentrations at −36°C and 123% ice relative humidity (RHice) on July 28 and July 29 are lowered from originally reported values of 1.3 cm−3 to approximately 0.3 cm−3, about a factor of four times lower. Corrected data for the July 29 case are applicable to the profile shown in Figure 2 of Sassen et al. [2003], but have no impact on the conclusions of that paper, or the still strong contribution of dust particles as ice nuclei compared to the background atmosphere. Nevertheless, corrections impact the entire archived data set from the field study. Corrected data corrections will be re-archived. An additional brief correction will be published regarding the cirrus anvil ice nuclei data presented by Prenni et al. [2007]. Revised data affects only the magnitude of dust impacts noted or implemented in models and should not affect the conclusions for any of these studies. Corrections were applied already for CRYSTAL-FACE data used for parameterization development in the recent paper of Phillips et al. [2008].

Laboratory study of heterogeneous ice nucleation in deposition mode of montmorillonite mineral dust particles aged with ammonia, sulfur dioxide, and ozone at polluted atmospheric concentrations

Heterogeneous ice nucleation in deposition mode of montmorillonite mineral dust aerosol particles exposed to atmospheric trace gases (ammonia, sulfur dioxide, and ozone) was studied at temperatures warmer than −40°C with a continuous flow diffusion chamber. Pure and typical polluted atmospheric concentrations of ammonia, sulfur dioxide, and ozone gases were used to age montmorillonite mineral dust aerosol particles at room temperature and atmospheric pressure in a stainless steel chamber. Ammonia-, sulfur dioxide-, and ozone-exposed montmorillonite mineral dust aerosols act as ice nuclei in heterogeneous deposition freezing at warmer temperatures than required for homogeneous freezing. The ice nucleation efficiency of montmorillonite mineral dust aerosols increased about two times due to exposure to ammonia at a typical atmospheric concentration of about 100 ppt. This is the first experimental evidence for the enhancement of the ice nucleation efficiency of montmorillonite mineral dust aerosols by ammonia gas at typical atmospheric concentrations. Montmorillonite exposure to either pure (100%) or 45 ppm sulfur dioxide or to ozone at 200 ppb shows no clear evidence for changing the ice nucleation efficiency of montmorillonite mineral dust particles. Thus, we conclude that enhancements atmospheric trace gases (e.g., sulfur dioxide and ozone) due to anthropogenic activities have no significant impact on the heterogeneous ice nucleation of montmorillonite mineral dust particles.

An Empirical Parameterization of Heterogeneous Ice Nucleation for Multiple Chemical Species of Aerosol

Abstract A novel, flexible framework is proposed for parameterizing the heterogeneous nucleation of ice within clouds. It has empirically derived dependencies on the chemistry and surface area of multiple species of ice nucleus (IN) aerosols. Effects from variability in mean size, spectral width, and mass loading of aerosols are represented via their influences on surface area. The parameterization is intended for application in large-scale atmospheric and cloud models that can predict 1) the supersaturation of water vapor, which requires a representation of vertical velocity on the cloud scale, and 2) concentrations of a variety of insoluble aerosol species. Observational data constraining the parameterization are principally from coincident field studies of IN activity and insoluble aerosol in the troposphere. The continuous flow diffusion chamber (CFDC) was deployed. Aerosol species are grouped by the parameterization into three basic types: dust and metallic compounds, inorganic black carbon, and insoluble organic aerosols. Further field observations inform the partitioning of measured IN concentrations among these basic groups of aerosol. The scarcity of heterogeneous nucleation, observed at humidities well below water saturation for warm subzero temperatures, is represented. Conventional and inside-out contact nucleation by IN is treated with a constant shift of their freezing temperatures. The empirical parameterization is described and compared with available field and laboratory observations and other schemes. Alternative schemes differ by up to five orders of magnitude in their freezing fractions (−30°C). New knowledge from future observational advances may be easily assimilated into the scheme’s framework. The essence of this versatile framework is the use of data concerning atmospheric IN sampled directly from the troposphere.