AEROSOLS FROM BIOMASS BURNING OVER THE TROPICAL SOUTH-ATLANTIC REGION- DISTRIBUTIONS AND IMPACTS

The NASA Global Tropospheric Experiment (GTE) Transport and Atmospheri c Chemistry Near the Equator-Atlantic (TRACE A) expedition was conduct ed September 21 through October 26, 1992 to investigate factors respon sible for creating the seasonal South Atlantic tropospheric ozone maxi mum. During these flights, fine aerosol(0.1-3.0 mu m) number densities were observed to be enhanced roughly tenfold over remote regions of t he tropical South Atlantic and greater over adjacent continental areas , relative to northern hemisphere observations and to measurements rec orded in the same area during the wet season. Chemical and meteorologi cal analyses as well as visual observations indicate that the primary source of these enhancements was biomass burning occurring within gras sland regions of north central Brazil and southeastern Africa. These f ires exhibited fine aerosol (N) emission ratios relative to CO (dN/dCO ) of 22.5 +/- 9.7 and 23.6 +/- 15.1 cm(-3) parts per billion by volume (ppbv)(-1) over Brazil and Africa, respectively. Convection coupled w ith counterclockwise now around the South Atlantic subtropical anticyc lone subsequently distributed these aerosols throughout the remote Sou th Atlantic troposphere. We calculate that dilute smoke from biomass b urning produced an average tenfold enhancement in optical depth over t he continental regions as well as a 50% increase in this parameter ove r the middle South Atlantic Ocean; these changes correspond to an esti mated net cooling of up to 25 W m(-2) and 2.4 W m(-2) during clear-sky conditions over savannas and ocean respectively. Over the ocean our a nalyses suggest that modification of CCN concentrations within the per sistent eastern Atlantic marine stratocumulus clouds by entrainment of subsiding haze layers could significantly increase cloud albedo resul ting in an additional surface radiative cooling potentially greater in magnitude than that caused by direct extinction of solar radiation by the aerosol particles themselves.