Characterization of Aerosols Sources Based on Chemical Analysis and Examines Their Effects on Glaciation Temperature in Rain Clouds

Different aerosols can interact in different ways with clouds and change clouds physical properties. The type of these interactions depends on aerosol size, chemical composition and concentration in the air mass. It is well established, that the ability of aerosols to serve as cloud condensation nuclei (CCN) and as ice nuclei (IN) increases with aerosol size, but the influence of aerosol chemical composition is poorly understood. In this study we investigate the influence of different aerosol sources on rain clouds in Israel. During five winters (2007-2011, 2013), rain samples were collected in the north of Israel for chemical analysis. The rain samples were analyzed for 23 metal concentrations. Through the chemical composition of the rain water, we managed to identify four different types of air masses that arrive to Israel during rain events: (1) Saharan air mass containing mainly desert dust, (2) Marine air mass containing mainly sea salt and some anthropogenic contribution from Europe, (3) Mixed air mass containing a mixture of marine aerosols, desert dust and air pollution and (4) anthropogenic air mass containing mainly air pollution from Europe and/or local contribution mixed with marine aerosols. The influence of each type of aerosols on ice initiation in clouds was examined. Our results indicate that glaciation temperature (Tg) in clouds increase with increasing dust concentration, despite droplet size decreasing with greater dust concentrations. This indicates that the freezing mechanism of cloud droplets in this case is due to high concentration of effective IN. We also show that Tg increases as the air mass becomes more marine. But in this case, the increase in Tg is due to lower aerosol concentrations that leads to larger cloud droplets which freeze faster and prone to ice multiplication. The anthropogenic aerosols were found to suppress freezing in the cloud as indicated by lowering Tg. This effect is stronger when pollution had been mixed with marine aerosols than with desert dust.