The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary 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 impact of an assumed soot/sulfate mass ratio of between 0.05 and 0.1 is examined. Fossil fuel derived soot causes a positive global‐mean radiative forcing which for one data set ranges from +0.03 to +0.24 Wm −2 ; the 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/sulfate ratio of 0.10. These values compare to a global‐mean radiative forcing of −0.34 Wm −2 due to sulfate aerosol. Soot also significantly reduces the interhemispherical difference in the radiative forcing due to sulfate aerosol. The nature and amount of soot must be well established if the climatic role of tropospheric aerosols is to be fully understood.