10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Immersion Freezing Efficiency of Airborne Mineral Dust at Various Particle Size-classes
NAAMA REICHER, Shira Raveh-Rubin, Yinon Rudich, The Weizmann Institute of Science
Abstract Number: 1062 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Mineral dust is a prominent atmospheric aerosol, affecting clouds microphysics and climate by its ability to act as cloud condensation nuclei (CCN) or as ice nucleating particles (INP). Mixed-phase clouds, which contain both supercooled droplets and ice crystals, are highly sensitive to the presence of INPs since they can trigger instant glaciation. It is still not well understood how nucleation is facilitated by the dust surface, but it was shown that mineralogy plays a crucial role in determining the efficiency of the ice nucleation process. Feldspars, for instance, were identified recently as highly efficient INPs, compared with other abundant mineral phases, such as clay minerals (illite, montmorillonite and kaolinite), calcite or quartz. Moreover, different types of feldspars also differ in their ice nucleation efficiency and active temperature regions. Therefore, the prediction of ice nucleation efficiency of different dust events depends on the mineral composition of the dust, which can vary according to the geographic source of the dust, the particle size and changes in its mineralogy during atmospheric transport. In order to examine the effect of the particle size on ice nucleation efficiency in ambient samples, airborne mineral dust was sampled during dust events from two source regions, the Sahara Desert and the Syrian Desert. The particles were collected and size-segregated using micro-orifice uniform deposit impactor (MOUDI) during five dust storm events. The collected particles were immersed in nL-size water droplets using microfluidics technique, and ice nucleation properties were studied using the WeIzmann Supercooled Droplets Observation on Microarray (WISDOM). We found that ice nucleation efficiency was similar for the different dust sources, but varied with particle size. In all but one event, the freezing efficiency of larger particles was higher, and supermicron particles exhibited higher ice nucleation efficiency in comparison with the submicron particles. Moreover, active site density spectra (ns) of supermicron particles were similar to the spectra obtained for standards of feldspar minerals, suggesting that this size fraction is more characterized by feldspars. In the event where particles size did not affect the ice nucleation efficiency, the total efficiency was lower than that measured in the other dust events, and this was attributed to a nearby biomass-burning event, which contributed biomass-burning aerosols to the sample or interacted with the mineral dust surface. These results suggest that mineralogy change with the particle size and that further characterization of the mineralogy difference with atmospheric transport is needed to better establish our prediction of ice nucleation efficiency of atmospheric dust.