Precipitation Scavenging of Urban Pollutants by Convective Storm Systems

Abstract

During the summer periods of 1972 and 1973 a precipitation chemistry network for the analysis of rainborne urban pollutants was operated in a region surrounding the St. Louis metropolitan area. The purposes of this network were to assess the effectiveness of convective storms in removing urban pollutants and to provide a data base for scavenging model development. Designed on the basis of a material balance of pollutant over the city, this network concentrated on elucidation of the scavenging behavior of the inorganic, nonmetallic species NH₄⁺, NO₃⁻, NO₂⁻, SO₂, SO₄⁼ and H⁺. Quantities of rainborne material deposited on the network downwind of the city, comparable to the urban pollution burden, indicated precipitation scavenging to be a highly efficient removal mechanism. Much of the observed rainborne sulfate and nitrate appears to have been incorporated into the rain by scavenging of gaseous precursors. This finding implies strongly that a rapid oxidation of SO₂ to sulfate occurs in cloud systems in warm, polluted environments and leads to a possible explanation for observed seasonal trends in sulfate levels.