GENERAL-CIRCULATION MODEL ASSESSMENT OF THE SENSITIVITY OF DIRECT CLIMATE FORCING BY ANTHROPOGENIC SULFATE AEROSOLS TO AEROSOL-SIZE AND CHEMISTRY

Climate response to atmospheric changes brought about by human activit y may depend strongly on the geographical and temporal pattern df radi ative forcing [Taylor and Penner, 1904]. In the case of aerosols stemm ing from anthropogenic sulfur emissions, geographical and temporal var iations are certainly caused by variations in local mass concentration [Charlson et al., 1991; Kiehl and Briegleb, 1993], but could also ari se from variations in the optical properties of sulfate aerosols. Sinc e optical properties (including their relative humidity (RH) variation ) depend fundamentally on aerosol size and chemical form and since siz e and chemical form are features of the aerosol which are not likely t o be modeled on the global scale in the near future, geographical and temporal variations in optical properties could represent a stumbling block to accurate climate change forecasts. While extensive measuremen ts of aerosol optical properties are needed to fully assess this probl em, a preliminary assessment call be gained by considering the sensiti vity of climate forcing to realistic variations in sulfate aerosol siz e and chemical form. Within a plausible set of assumptions (sulfate ae rosol resides in the accumulation mode size range and only interacts w ith water vapor and ammonia vapor), we show that this sensitivity is f airly small (+/-20%). This low sensitivity derives from a number of co mpensating factors linking the three optical parameters identified by Charlson et al. [1991]. By implication; these optical parameters, low RH scattering efficiency, the ratio of hemispheric backscatter to tota l scatter, and the RH dependence of scattering efficiency, should not be treated independently in either theoretical or experimental investi gations of direct climate forcing. A suggested logical focus for such investigations is the backscatter efficiency at high RH. If borne out by future research, low sensitivity to sulfate aerosol size and chemis try would mean that direct sulfate climate forcing can be incorporated in global climate models with only a knowledge of sulfate mass concen tration. We emphasize, therefore, the need to study the extent to whic h our assumptions break down, in particular, the fraction of anthropog enic sulfate that forms on coarse mode particles (i.e., those with dia meters > 1 mu m) and the extent and effects of sulfate interactions wi th other accumulation mode components. Finally, we find that a signifi cant fraction of direct aerosol forcing occurs in cloud-covered region s, according to a simple bulk parameterization.