SENSITIVITY OF DIRECT CLIMATE FORCING BY ATMOSPHERIC AEROSOLS TO AEROSOL-SIZE AND COMPOSITION

We evaluate, using a box model, the sensitivity of direct climate forc ing by atmospheric aerosols for a ''global mean'' aerosol that consist s of fine and coarse modes to aerosol composition, aerosol size distri bution, relative humidity (RH), aerosol mixing state (internal versus external mixture), deliquescence/crystallization hysteresis, and solar zenith angle. We also examine the dependence of aerosol upscatter fra ction on aerosol size, solar zenith angle, and wavelength and the depe ndence of single scatter albedo on wavelength and aerosol composition. The single most important parameter in determining direct aerosol for cing is relative humidity, and the most important process is the incre ase of the aerosol mass as a result of water uptake. An increase of th e relative humidity from 40 to 80% is estimated for the global mean ae rosol considered to result in an increase of the radiative forcing by a factor of 2.1. Forcing is relatively insensitive to the fine mode di ameter increase due to hygroscopic growth, as long as this mode remain s inside the efficient scattering size region. The hysteresis/deliques cence region introduces additional uncertainty but, in general, errors less than 20% result by the use of the average of the two curves to p redict forcing. For fine aerosol mode mean diameters in the 0.2-0.5 mu m range direct aerosol forcing is relatively insensitive (errors less than 20%) to variations of the mean diameter. Estimation of the coars e mode diameter within a factor of 2 is generally Sufficient for the e stimation of the total aerosol radiative forcing within 20%. Moreover, the coarse mode, which represents the nonanthropogenic fraction of th e aerosol, is estimated to contribute less than 10% of the total radia tive forcing for all RHs of interest. Aerosol chemical composition is important to direct radiative forcing as it determines (1) water uptak e with RH, and (2) optical properties. The effect of absorption by aer osol components on forcing is found to be significant even for single Scatter albedo values of omega=0.93-0.97. The absorbing aerosol compon ent reduces the aerosol forcing from that in its absence by roughly 30 % at 60% RH and 20% at 90% RH. The mixing state of the aerosol (intern al versus external) for the particular aerosol considered here is foun d to be of secondary importance, While sulfate mass scattering efficie ncy (m(2) (g SO42-)(-1)) and the normalized sulfate forcing(W (g SO42- )(-1))increase strongly with RH, total mass scattering efficiency (m(2 ) g(-1))and normalized forcing (W g(-1)) are relatively insensitive to RH, wherein the mass of all species, including water, are accounted f or. Following S. Nemesure et al. (Direct shortwave forcing of climate by;anthropogenic sulfate aerosol: sensitivity to particle size, compos ition, and relative humidity, submitted to Journal of Geophysical Rese arch, 1995), we find that aerosol forcing achieves a maximum at a part icular solar zenith angle, reflecting a balance between increasing ups catter fraction with increasing solar zenith angle and decreasing sola r flux (from Rayleigh scattering) with increasing solar zenith angle.