Evidence for large-scale transport of biomass burning aerosols from sunphotometry at a remote South African site

We present the results of sunphotometry measurements at De Aar, a remote site on the central South African plateau, during and after the intensive dry season field campaign of SAFARI 2000. We determine a 6-month-long time series of aerosol optical depths over the site. Twelve haze events are identified, for which we derive Angström exponents and their derivatives, and, through cross-plots of these parameters, typical aerosol sizes and levels of hydration. These results, in conjunction with meteorological data and air trajectory calculations, show biomass burning to be the main aerosol generating source for 8 of the 12 events, and responsible for the 5 cases with the highest turbidity. While the bulk of the biomass emission is clearly of African origin, we identify several possible South Atlantic crossings of aged smoke from fires in the Amazon basin. We define the southern edge of the main aerosol transport route over southern Africa during the austral winter. We estimate that, for the half-year investigated, 84% of the losses of visible solar irradiation over our experimental location are caused by biomass burning haze, and conclude that these types of aerosols have the most critical impact on solar irradiation and atmospheric albedo over the entire southern Africa.