GENERAL-CIRCULATION MODEL-CALCULATIONS OF THE DIRECT RADIATIVE FORCING BY ANTHROPOGENIC SULFATE AND FOSSIL-FUEL SOOT AEROSOL

A new radiation code within a general circulation model is used to ass ess the direct solar and thermal radiative forcing by sulfate aerosol of anthropogenic origin and soot aerosol from fossil-fuel burning. The radiative effects of different aerosol profiles, relative humidity pa rameterizations, chemical compositions, and internal and external mixt ures of the two aerosol types are investigated. The contribution to th e radiative forcing from cloudy sky regions is found to be negligible for sulfate aerosol: this is in contrast to recent studies where the c loudy sky contribution was estimated using a method in which the spati al correlation between cloud amount and sulfate burden was ignored. Ho wever, the radiative forcing due to fossil-fuel soot aerosol is enhanc ed in cloudy regions if soot aerosol exists within or above the cloud. The global solar radiative forcing due to sulfate aerosol is estimate d to be -0.38 W m(-2) and the global thermal radiative forcing is esti mated to be +0.01 W m(-2). The hemispheric mean radiative forcings var y by only about 10% for reasonable assumptions about the chemical form of the sulfate aerosol and the relative humidity dependence; the unce rtainties in the aerosol loading are far more significant. If a soot/s ulfate mass ratio of 0.075 is assumed, then the global solar radiative forcing weakens to -0.18 W m(-2) far an external mixture and weakens further for an internal mixture. Additionally, the spatial distributio n of the radiative forcing shows strong negative/positive forcing cont rasts that may influence the dynamical response of the atmosphere. Alt hough these results are extremely sensitive to the adopted soot/sulfat e ratio and the assumed vertical profile, they indicate that fossil fu el soot aerosol map exert a nonnegligible radiative forcing and emphas ize the need to consider each anthropogenic aerosol species.