The critical role of sulfate aerosol in climate is described by
Charlson et al. [1991] and Charlson et al. [1992]. Their
estimate of the averaged global mean forcing since pre-industrial
times (-0.6 W m
) was derived using anthropogenic sulfate mass
distributions computed from a three-dimensional model of the global
sulfur cycle ( Langner and Rodhe [1991]). The magnitude of the
average forcing is close to that estimated for greenhouse gases. The
cooling and warming effects will not ``cancel'', however, since the
distribution of anthropogenic aerosols is markedly different than that
of greenhouse gases, and since aerosol abundance and physical and
chemical properties---all of which strongly influence radiative
transfer---are subject to significant spatial and temporal
variability.
The effects of parameters such as the aerosol composition and size distribution were demonstrated by Kiehl and Briegleb [1993]. In their radiative transfer calculation, they used the same sulfate mass distributions as Charlson et al. [1991], but applied an assumed aerosol size distribution and composition (sulfuric acid or ammonium sulfate) to estimate water uptake as a function of local relative humidity. These assumptions resulted in smaller computed forcings (by about a factor of two) than the previous estimates. This study demonstrates that the effects upon optical properties of variations in the mean particle size and the degree of neutralization of the aerosol can be significant. Ammonia is an important neutralizing species for sulfate, but much is still unknown about its global budget and the factors that control the degree to which ammonia is in multiphase equilibrium ( Quinn et al. [1992]).
It is often assumed that contributions of various species to scattering or absorption is additive, and aerosol optical properties are specified as mass scattering and absorption coefficients which are applied to total mass of the species present (e.g., Malm et al. [1994]). Based on their observations of marine sulfate aerosol, Hegg et al. [1993] argue that commonly-employed values of the sulfate mass scattering efficiency are too large; differences may be due to the presence of other species and to the assumed size distributions. The basis for assignment of mass scattering or absorption efficiencies is a critical issue in both visibility and climate studies. White [1986] presents a review of the assumptions inherent in various approaches, and implications thereof to the apportionment of extinction into contributions by individual aerosol chemical components.