THE EFFECT OF ANTHROPOGENIC SULFATE AND SOOT AEROSOL ON THE CLEAR-SKYPLANETARY RADIATION BUDGET

Carbonaceous soot within the troposphere can significantly modify the clear-sky radiative forcing. Using an extension to a simple radiation calculation and two model-derived sulfate aerosol data sets, the impac t of an assumed soot/sulfate mass ratio of between 0.05 and 0.1 is exa mined. Fossil. fuel derived soot causes a positive global-mean radiati ve forcing which for one data set ranges from +0.03 to +0.24W m(-2) th e lower estimate is for an external mixture with a soot/sulfate ratio of 0.05 and the upper estimate is for an internal mixture and a soot/s ulfate ratio of 0.10. These values compare to a global-mean radiative forcing of -0.34W m(-2) due to sulfate aerosol. Soot also significantl y reduces the interhemispherical difference in the radiative forcing d ue to sulfate aerosol. The nature and amount of soot must be well esta blished if the climatic role of tropospheric aerosols is to be fully u nderstood.