MARGINAL DIRECT CLIMATE FORCING BY ATMOSPHERIC AEROSOLS

Citation
Jj. West et al., MARGINAL DIRECT CLIMATE FORCING BY ATMOSPHERIC AEROSOLS, Atmospheric environment, 32(14-15), 1998, pp. 2531-2542
Citations number
21
Categorie Soggetti
Environmental Sciences","Metereology & Atmospheric Sciences
Journal title
ISSN journal
13522310
Volume
32
Issue
14-15
Year of publication
1998
Pages
2531 - 2542
Database
ISI
SICI code
1352-2310(1998)32:14-15<2531:MDCFBA>2.0.ZU;2-V
Abstract
Previous research on the direct effect of atmospheric aerosols on clim ate has estimated the average radiative forcing per unit sulfate mass, and has used this average to calculate the magnitude and spatial dist ribution of sulfate forcing. In this paper, we posit that radiative fo rcing is often a nonlinear function of sulfate mass concentration. In contrast to measures of average forcing, we introduce the concept of ' 'marginal forcing'', which is defined as the change in radiative forci ng for an incremental change in sulfate concentration. A multi-compone nt, size-resolved aerosol box model is used, which couples an aerosol chemical equilibrium model with a model for calculating radiative forc ing based on Mie theory. The results for a typical nonurban continenta l aerosol show that total aerosol mass and radiative forcing are nonli near functions of sulfate concentration. This nonlinearity is mainly d ue to the chemical interaction of sulfate with volatile inorganic comp onents of the aerosol (ammonium, nitrate, and water). As a result, the marginal forcing varies significantly as a function of sulfate concen tration, from - 550 to + 20 W (g SO42-)(-1) at a relative humidity (RH ) of 80%. Estimates of marginal forcing are strongly sensitive to RH. Absolute marginal forcing also decreases significantly with total nitr ate concentration, increases with total ammonia concentration, and gen erally increases with temperature. We estimate hat the bias in assumin g a constant average forcing may cause overestimates in local continen tal aerosol radiative forcing by up to 50%, and in the marginal forcin g by a factor of two or more.This bias is greatest at intermediate sul fate concentration, high RH, high total nitrate concentration,low tota l ammonia concentration( greater than or equal to 2 mu g m(-3)), and l ow temperature. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.