Impacts of Aerosols from Biomass Burning and Anthropogenic Pollution Sources on Low-Level Arctic Clouds

The Arctic region is warming particularly rapidly compared to the planet as a whole. An increasing number of studies indicate that aerosol-cloud interactions (ACI) may be playing an important role, although the nature of the interactions is still poorly understood. A particular challenge is obtaining independent datasets for cloud microphysical parameters and aerosol content so that the interactions can be expressed as dependent and independent variables. Clouds affect aerosols through, e.g. wet scavenging as much as aerosols affect clouds, so decoupling one from the other is not obvious. Further, aerosol concentrations and cloud properties are both a function of meteorological state. Correlations between aerosols and clouds could be entirely a coincidence.

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.