We are addressing these issues by combining measurements from satellite instruments POLDER-3 and MODIS totemporally and spatially co-localate cloud microphysical properties with carbon monoxide concentrations, which serves as a passive tracer of pollution plumes, from GEOS-Chem and FLEXPART. We
also include ERA-Interim reanalysis of meteorological parameters to stratify the dataset by meteorological state, looking specifically at specific humidity and lower tropospheric stability, two primary variables controlling cloud behaviors. Thus, observed differences in cloud microphysical parameters can be attributed to the presence of pollution plumes rather than meteorological variability.
We define a net Aerosol-Cloud Interaction parameter (ACInet) as a measure of the sensitivity of a cloud at any given location to pollution plumes from distant sources, allowing for the possibility of wet scavenging of aerosols en route. ACInet provides a measure of the impact on liquid-cloud microphysical properties in Arctic of pollution plumes from anthropogenic and biomass burning sources originating at mid-latitudes. For a time period between 2005 and 2010, we find that, controlling for meteorological state, the effect of biomass pollution plumes on clouds is smaller (ACInet ~ 0) than that for anthropogenic pollution plumes (ACInet ~ 0.30).
We also investigate the impacts of anthropogenic aerosol on thermodynamic phase transitions. We present a novel technique for using satellite datasets to study freezing nucleation rates in clouds. Clouds with small cloud droplet effective radii are observed to require a greater degree of supercooling to freeze. Clouds that are exposed to high pollution concentrations require a lower degree of supercooling. In the latter case, the estimated decrease in the energy barrier for freezing due to an increase in aerosol concentration can be up to 48%, suggesting a factor of four increase in the freezing nucleation rate.