Sulfate-induced cooling in the southeastern US: An observational assessment

The effect of sulfates on cloud microphysical and optical properties have b een studied using data from a rural mountaintop location in the Mt. Mitchel l Slate Park, North Carolina. Although the amended Clean Air Act has limite d the sulfur emissions beginning 1990, we found the sulfate concentrations greater during 1993-97 compared to that during 1986-89. Cloud albedo inferr ed from satellite data and calculated from surface observations does not in dicate a monotonic increase with increasing sulfates. The direct and indire ct (cloud-mediated) sulfate forcing are assessed to be 4.8 and -4 W m(-2). These values exceed current model predictions as evaluated by other investi gators. Surface temperature records of the region during 1949-94 indicate a cooling trend tacitly supporting our assessment.