ROLE OF SULFATES IN REGIONAL CLOUD-CLIMATE INTERACTIONS

During the summers of 1993-1996, field experiments were conducted at M t. Mitchell, NC, in the southeastern US. The effect of anthropogenic p ollution on cloud microphysical properties such as liquid water conten t(omega), cloud droplet number concentration (N), effective radii (R-e ff), cloud condensation nuclei (CCN) activation spectrum and cloud ref lectivity were investigated. Cloud water sulfate concentration was use d as a measure of anthropogenic pollution. Back trajectory analysis wa s used to categorize the cloud forming air masses that arrived at the site. The sulfates and nitrates emission inventories of the U.S. Envir onmental Protection Agency (EPA) were used to classify air masses as p olluted marine (PM), polluted continental (PC) or as highly polluted ( HP). Empirical values for the relationships between CCN-N, CCN-sulfate , N-sulfate and R-eff-N for different air masses have been obtained. A quantitative nonlinear relationship between CCN and N was obtained. T he sublinear relationship between CCN and sulfate mass indicated the s usceptibility of the different air masses to the sulfate content. The lesser polluted air masses exhibited greater changes in CCN concentrat ions for smaller increases in sulfate mass concentrations as compared to the more polluted air masses. The relationship between R-eff and N for different air masses, as well as the N-sulfate mass relationship, suggested that the counteracting effect of sulfates on greenhouse warm ing for the southeastern US would be of magnitude greater than -4.0 W m(-2) obtained by modeling studies. The nonlinear relationships betwee n the cloud microphysical/optical properties and the sulfate content o f the air mass implies the existence of an optimum level for the sulfa te concentration beyond which cloud reflectivity stays unaffected. Ana lysis of 3 years of observational data established this level at 400-5 00 mu eq l(-1) of sulfate in the cloud water. (C) 1998 Elsevier Scienc e B.V. All rights reserved.