A decrease in precipitation during winter allows polluted air parcels from mid-latitudes to reach the Arctic. Low vertical mixing in the region concentrates aerosols and decreases scavenging. Aerosol impacts on cloud microphysical parameters remain poorly understood. However, cloud properties and pollution concentrations also vary with meteorological state, which poses the challenge of how to disentangle the impact of aerosols on clouds from that of natural thermodynamic variability. In this study we combine measurements from satellite instruments POLDER-3 and MODIS to temporally and spatially co-locate cloud properties over 65o in latitude with carbon monoxide concentrations, passive tracer of aerosol content, from GEOS-Chem between 2005 and 2010. We also add ERA-I reanalysis of meteorological parameters to stratify meteorological parameters, such as specific humidity and lower tropospheric stability. The goal is to determine the extent to which differences in cloud phase can be attributed to differences in aerosol content and not in meteorological parameters. We evaluated the degree of supercooling ΔT<sub>50</sub> that is required for 50% of a chosen ensemble of low- level clouds to be in the ice phase. Consistent with Rangno & Hobbs (2001), our results suggest that small droplet effective radii are related to high values of ΔT<sub>50</sub> . Also, anthropogenic pollution plumes lower the degree of supercooling by approximately 4°C, independent of the decrease in effective radius and change of meteorological regime. This effect of anthropogenic aerosol on the transition temperature to freezing has not been reported before to our knowledge and lacks clear explanation. Rangno, Arthur L., and Peter V. Hobbs. "Ice particles in stratiform clouds in the Arctic and possible mechanisms for the production of high ice concentrations." Journal of geophysical research 106 (2001): 15.
Air Pollution in the Arctic: Climate, Environment and Societies