Lower tropospheric aerosol loadings over South Africa: The relative contribution of aeolian dust, industrial emissions, and biomass burning

The southern African haze layer is a ubiquitous subcontinental-scale featur e of the lower atmosphere that extends to a depth of similar to 5 km(simila r to 500 hPa level) on non rain days, particularly in winter. Aerosols deri ved from biomass burning are commonly thought to contribute substantially t o the total background aerosol loading within the layer. It is shown that i n both summer and winter this supposition is without foundation over South Africa. Summer and winter aerosol loadings are derived from gravimetric ana lysis of stacked filter units and from proton-induced X ray emission (PIXE) analysis of one to four hourly resolved streaker samples. From concentrati ons of eleven inorganic elements, apportionment into four primary sources, biomass burning particulates, aeolian dust, industrial sulphur aerosols, an d marine aerosols, has been effected. It is shown that the background bioma ss burning component of the total aerosol loading over South Africa in gene ral, and within the plume of material being recirculated over South Africa and from there exported from the subcontinent south of 22 degrees S to the Indian Ocean in particular, is minimal in both summer and winter. Except ov er coastal and adjacent inland areas, marine aerosols likewise make up a sm all fraction of the total loading. This is particularly so over the inland plateau areas. Crustally -derived aeolian dust and industrially -produced s ulphur aerosols are demonstrated to be the major summer and winter constitu ents of the haze layer over South Africa and the particulate material being transported to the Indian Ocean region. Sulphur is transported within the aerosol plume exiting southern Africa to the Indian Ocean as agglomerates o n aeolian dust nuclei.