DIRECT SHORTWAVE FORCING OF CLIMATE BY THE ANTHROPOGENIC SULFATE AEROSOL - SENSITIVITY TO PARTICLE-SIZE, COMPOSITION, AND RELATIVE-HUMIDITY

Recent estimates of global or hemispheric average forcing of climate b y anthropogenic sulfate aerosol caused by scattering of shortwave radi ation (''direct'' effect) are uncertain by somewhat more than a factor of 2. The principal sources of this uncertainty are atmospheric chemi stry properties (yield, residence time), and microphysical properties (scattering efficiency, upscatter fraction, and the dependence of thes e properties on particle size, composition, and relative humidity, (RH )). This paper examines the sensitivity of forcing to these microphysi cal properties to identify and improve understanding of the properties required to reduce the uncertainty in the forcing. The relations betw een aerosol loading and forcing developed here are suitable for compar ing modeled and measured aerosol forcing at specific locations and for use in climate models, provided aerosol composition and microphysical properties are known, calculated, or assumed. Results are presented s howing the dependence of scattering efficiency, upscatter fraction, an d normalized forcing (W m(-2)/g(SO42-) m(-2) or W g(SO42-)(-1)) on dry particle size (expressed as mole(sulfate) per particle), composition ((NH4)(2)SO4, NH4HSO4, H2SO4), solar zenith angle, latitude, and seaso n. Forcing is strongly dependent on dry particle size and RH but is re latively insensitive to composition. The normalized forcing can be int egrated over a known or assumed size distribution to evaluate the sulf ate aerosol forcing. Global and annual average values of the normalize d forcing are evaluated as a function of particle size and RH. Dependi ng on values of these variables, normalized forcing may be less than, intermediate to, or greater than the range of previous estimates of su lfate aerosol forcing.