The impact of mesoscale convective systems on regional visibility and oxidant distributions during persistent elevated pollution episodes

Abstract

It is generally accepted that atmospheric pollutants can be transported over distances described as synoptic in scale. However, the redistribution and removal of the various pollutant species may be significantly influenced, often in a highly episodic way, by phenomena which are inherently mesoscale in nature. Detailed mesoanalyses over a 31 hour period reveal the dramatic influence of several types of mesoscale convective systems on regional ozone and visibility. In the impacted air mass, afternoon ozone was significantly depleted (20-40 ppb vs 100-120 ppb in the polluted regional air mass). Visibilities, which prior to the wind shift/rain had typically been 4-13 km rose in a matter of a few hours to 27-80 km at many locations. This convective aerosol removal event covered Virginia, northern North Carolina, Delmarva, and more than 500 km into the Atlantic, as clearly evidenced by GOES satellite imagery. Assuming a relationship between visibility and sulfate aerosol mass concentrations, it is speculated that approximately 30 x 106 kg of sulfate were removed from the planetary boundary layer (PBL). The removal was not due solely to wet deposition. Rather, consideration of thunderstorm dynamics suggests that perhaps two-thirds of the removal from the PBL could have been due to convective displacement into the free atmosphere, perhaps to altitudes as high as the tropopause.