Modification of Arctic clouds by long-range aerosol transport

Abstract

The Arctic region is warming particularly rapidly. Aerosol impacts on cloud microphysical parameters are still poorly understood. Aerosol-cloud interactions (ACI) play an important role for cloud radiative properties and climate change. A challenge in the study of ACI is the use of independent datasets for cloud microphysical parameters and aerosol content so they cannot influence one another. In this study, we combine measurements from satellite instruments POLDER-3 and MODIS to temporally and spatially colocate cloud microphysical properties with carbon monoxide concentrations from GEOS-Chem and FLEXPART, serving as a passive tracer of aerosol content. We also add ERA-I reanalysis of meteorological parameters to stratify me- teorological parameters such as specific humidity and lower tropospheric stability. Thus, observed differences in cloud microphysical parameters can be attributed to differences in aerosol content rather than meteorological variability. We define a net aerosol-cloud interaction parameter (ACInet) which can be interpreted as a measure of the sensitivity of a cloud at any given location to pollution plumes from distant sources. We use this parameter to study the impact of aerosols from anthropogenic and biomass burning sources from midlatitudes on liquid-cloud microphysical properties in Arctic, for a time period between 2005 and 2010, above ocean, and for controlled meteorological regimes. Our results suggest that the effect of biomass pollution plumes on clouds is smaller (ACInet close to 0) than that for anthropogenic pollution plumes (ACInet close to 0.30). Meteorological parameters can inhibit the aerosol-cloud interaction or favor the aerosol-cloud interaction. The impact of anthropogenic aerosol on thermodynamic phase transition are analyzed. The smaller the effective radius, the higher the supercooling temperature whereas the greater the aerosol concentration, the lower the supercooling temperature. Independently of changes in effective radius, decrease in energy barrier due to an increase in aerosol concentration can be up to 48%