Gourihar Kulkarni

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

Ice nucleation properties of atmospherically relevant dust minerals coated with soluble materials are not yet well understood. We determined ice nucleation ability of bare and sulfuric acid-coated mineral dust particles as a function of temperature (−25 to −35°C) and relative humidity with respect to water (RHw; 75 to 110%) for five different mineral dust types: (1) Arizona test dust, (2) illite, (3) montmorillonite, (4) K-feldspar, and (5) quartz. The particles were dry dispersed and size selected at 200 nm, and we determined the fraction of dust particles nucleating ice at various temperatures and RHw. Under water-subsaturated conditions, compared to bare dust particles, we found that coated particles showed a reduction in their ice nucleation ability. Under water-supersaturated conditions, however, we did not observe a significant coating effect (i.e., the bare and coated dust particles had nearly similar nucleating properties). X-ray diffraction patterns of the coated particles indicated that acid treatment altered the crystalline nature of the surface and caused structural disorder; thus, we concluded that the lack of such structured order reduced the ice nucleation efficiency of the coated particles in deposition ice nucleation mode. In addition, our single column model results show that coated particles significantly modify cloud properties such as ice crystal number concentration and ice water content compared to bare particles in water-subsaturated conditions. However, in water-supersaturated conditions, cloud properties differ only at warmer temperatures. These modeling results imply that future aged dust particle simulations should implement coating parameterizations to accurately predict cloud properties.

Comparison of Experimental and Numerical Studies of the Performance Characteristics of a Pumped Counterflow Virtual Impactor

Experiments and Computational Fluid Dynamic (CFD) simulations were performed to evaluate the performance characteristics of a Pumped Counterflow Virtual Impactor (PCVI). The diameter at which 50% of the particles were transmitted was determined for various flow configurations. Experimentally determined 50% cut sizes varied from 2.2 to 4.8 micrometers and CFD predicted diameters agreed within ±0.4 micrometers. Both experimental and CFD results showed similar transmission efficiency (TE) curves. CFD TE was always greater than experimental results, most likely due to impaction losses in fittings not included in the simulations. Ideal transmission, corresponding to 100% TE, was never realized in either case due to impaction losses and small-scale flow features such as eddies. Areas where CFD simulations showed such flow recirculation zones were also found to be the locations where particulate residue was deposited during experiments. CFD parametric tests showed that PCVI performance can be affected by the nozzle geometry and misalignment between the nozzle and collector orifice. We conclude that CFD can be used with confidence for counterflow virtual impactor (CVI) design. Modifications to improve the performance characteristics of the PCVI are suggested.

Ice nucleation activity of diesel soot particles at cirrus relevant temperature conditions: Effects of hydration, secondary organics coating, soot morphology, and coagulation: Ice Nucleation of Soot Particles

Ice formation by diesel soot particles was investigated at temperatures ranging from −40 to −50°C. Size-selected soot particles were physically and chemically aged in an environmental chamber, and their ice nucleating properties were determined using a continuous flow diffusion type ice nucleation chamber. Bare (freshly formed), hydrated, and compacted soot particles, as well as α-pinene secondary organic aerosol (SOA)-coated soot particles at high relative humidity conditions, showed ice formation activity at subsaturation conditions with respect to water but below the homogeneous freezing threshold conditions. However, SOA-coated soot particles at dry conditions were observed to freeze at homogeneous freezing threshold conditions. Overall, our results suggest that heterogeneous ice nucleation activity of freshly emitted diesel soot particles are sensitive to some of the aging processes that soot can undergo in the atmosphere.

Ice nucleation activity of diesel soot particles at Cirrus relevant conditions: Effects of hydration, secondary organics coating, hydration, soot morphology, and coagulation

Ice formation by diesel soot particles was investigated at temperatures ranging from −40 to −50°C. Size-selected soot particles were physically and chemically aged in an environmental chamber, and their ice nucleating properties were determined using a continuous flow diffusion type ice nucleation chamber. Bare (freshly formed), hydrated, and compacted soot particles, as well as α-pinene secondary organic aerosol (SOA)-coated soot particles at high relative humidity conditions, showed ice formation activity at subsaturation conditions with respect to water but below the homogeneous freezing threshold conditions. However, SOA-coated soot particles at dry conditions were observed to freeze at homogeneous freezing threshold conditions. Overall, our results suggest that heterogeneous ice nucleation activity of freshly emitted diesel soot particles are sensitive to some of the aging processes that soot can undergo in the atmosphere.

Effects of crystallographic properties on the ice nucleation properties of volcanic ash particles

Specific chemical and physical properties of volcanic ash particles that could affect their ability to induce ice formation are poorly understood. In this study, the ice nucleating properties of size-selected volcanic ash and mineral dust particles in relation to their surface chemistry and crystalline structure at temperatures ranging from –30 to –38 °C were investigated in deposition mode. Ice nucleation efficiency of dust particles was higher compared to ash particles at all temperature and relative humidity conditions. Particle characterization analysis shows that surface elemental composition of ash and dust particles was similar; however, the structural properties of ash samples were different.

Ice formation on nitric acid‐coated dust particles: Laboratory and modeling studies

Changes in the ice nucleation characteristics of atmospherically relevant mineral dust particles caused by a coating of nitric acid are not well understood. Further, the atmospheric implications of dust coatings on ice-cloud properties under different assumptions of primary ice nucleation mechanisms are unknown. We investigated the ice nucleation ability of Arizona Test Dust, illite, K-feldspar, and quartz as a function of temperature (−25°C to −30°C) and relative humidity with respect to water (75% to 110%). The particles (bare or nitric acid coated) were size selected at 250 nm, and the fraction of particles nucleating ice at various temperature and saturation conditions was determined. All of the dust species nucleated ice at subsaturated conditions, although the coated particles (except quartz) showed a reduction in their ice nucleation ability relative to bare particles. However, at supersaturated conditions, bare and coated particles had nearly equivalent ice nucleation characteristics. The results of a single-column model showed that simulated ice crystal number concentrations are mostly dependent upon the coated particle fraction, primary ice nucleation mechanisms, and competition among ice nucleation mechanisms to nucleate ice. In general, coatings were observed to modify ice-cloud properties, and the complexity of ice-cloud and mixed-phase-cloud evolution when different primary ice nucleation mechanisms compete for fixed water vapor budgets was supported.

Development of a Mobile Ice Nucleus Counter

An ice nucleus counter has been constructed. The instrument uses built-in refrigeration systems for wall cooling. A cascade refrigeration system will allow the cold wall to operate as low as -70 deg C, and a single stage system can operate the warm wall at -45 deg C. A unique optical particle counter has been constructed using polarization detection of the scattered light. This allows differentiation of the particles exiting the chamber to determine if they are ice or liquid.