Role of sulfates in regional cloud–climate interactions1

During the summers of 1993–1996, field experiments were conducted at Mt. Mitchell, NC, in the southeastern US. The effect of anthropogenic pollution on cloud microphysical properties such as liquid water content (ω), cloud droplet number concentration (N), effective radii (Reff), cloud condensation nuclei (CCN) activation spectrum and cloud reflectivity were investigated. Cloud water sulfate concentration was used as a measure of anthropogenic pollution. Back trajectory analysis was used to categorize the cloud forming air masses that arrived at the site. The sulfates and nitrates emission inventories of the U.S. Environmental Protection Agency (EPA) were used to classify air masses as polluted marine (PM), polluted continental (PC) or as highly polluted (HP). Empirical values for the relationships between CCN–N, CCN–sulfate, N–sulfate and Reff–N for different air masses have been obtained. A quantitative nonlinear relationship between CCN and N was obtained. The sublinear relationship between CCN and sulfate mass indicated the susceptibility of the different air masses to the sulfate content. The lesser polluted air masses exhibited greater changes in CCN concentrations for smaller increases in sulfate mass concentrations as compared to the more polluted air masses. The relationship between Reff and N for different air masses, as well as the N–sulfate mass relationship, suggested that the counteracting effect of sulfates on greenhouse warming for the southeastern US would be of magnitude greater than −4.0 W m−2 obtained by modeling studies. The nonlinear relationships between the cloud microphysical/optical properties and the sulfate content of the air mass implies the existence of an optimum level for the sulfate concentration beyond which cloud reflectivity stays unaffected. Analysis of 3 years of observational data established this level at 400–500 μeq l−1 of sulfate in the cloud water.