AN ASSESSMENT OF THE RADIATIVE EFFECTS OF ANTHROPOGENIC SULFATE

We use a coupled climate/chemistry model with cloud nucleation process es parameterized in terms of local aerosol number, anthropogenic sulfa te mass concentration, and updraft velocity to investigate both direct and indirect anthropogenic sulfate radiative forcings. We estimate th at the global direct radiative forcing is about -0.4 W m(-2) with a ma ximum over Europe where the strongest anthropogenic sulfur emissions o ccur. With different approaches for the formation of anthropogenic sul fate and its relation to aerosol size distribution, we estimate that t he indirect forcing may range from -0.6 to -1.6 W m(-2). This range re duces to -0.4 to -1.1 W m(-2) if a prescribed marine background partic le number concentration is universally applied over the ocean. Contrar y to the direct effect which is mote significant over continents, the calculated maximum of indirect forcing is located over the Atlantic Oc ean near the coastline of North America. Our simulations indicate that anthropogenic sulfate may result in important increases in reflected Solar radiation, which would mask locally the warming from increased g reenhouse gases. We also compare the simulated cloud drop effective ra dii with those retrieved from satellite data to validate the accuracy of our cloud drop parameterization.