Evaluation of the aerosol indirect effect using satellite, chemical transport model, and aircraft data from ICARTT

Satellite data has been used by recent studies to assess the magnitude of the effect of pollution aerosols on clouds. However, satellite pollution measurements must be made in clear air adjacent to clouds. Since cloud properties depend foremost on meteorology, almost by definition, the clouds and aerosols lie in different meteorological regimes. Our method to avoid these concerns is to collocate Aqua and Terra MODIS cloud retrievals with output from the high spatial and temporal resolution FLEXPART chemical transport model, which uses ECMWF fields to describe the diffusion and transport of atmospheric tracers. Anthropogenic CO and SO₂ are used as pollution tracers because their concentrations are tied primarily to mixing and are not affected by precipitation and oxidation. We focus on low-level clouds in three 4° x 4° latitude - longitude boxes over the northwest Atlantic Ocean downwind of the northeastern U.S. megacity during the summer 2004 ICARTT mission. Preliminary results show a strong correlation between pollution and reduced cloud droplet effective radius in each of the three 4° x 4° boxes.