Recent estimates of global or hemispheric average forcing of climate by anthropogenic sulfate aerosol caused by scattering of shortwave radiation ("direct" effect) are uncertain by somewhat more than a facotr of 2. The principal sources of this uncertainty are atmospheric chemistry properties (yield, residence time), and microphysical properties (scattering efficiency, upscatter fraction, and the dependence of these properties on particle size, composition, and relative humidity, (RH)). This paper examines the sensitivity of forcing to these microphysical properties to identify and improve understanding of the properties required to reduce the uncertainty in the forcing. The relations between aerosol composition and microphysical properties are known, calculated, or assumed. Results are presented showing the dependence of scattering efficiency, upscatter fraction, and 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)2SO4, NH4HSO4, H2SO4), solar zenith angle, latitude, and season. Forcing is strongly dependent on dry particle size and RH but is relatively insensitive to composition. The normalized forcing can be integrated over a known or assumed size distribution to evaluate the sulfate aerosol forcing. Global and annual average values of the normalized forcing are evaluated as a function of particle size and RH. Depending on values of these variables, normalized forcing may be less than, intermediate to, or greater than the range of previous estimates of sulfate aerosol forcing.
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