Simulation of biomass burning aerosols mass distributions and their direct and semi-direct effects over South Africa using a regional climate model

In this study, we examine the mass distributions, direct and semi-direct effects of different biomass burning aerosols (BBAs) over South Africa using the 12-year runs of the Regional Climate Model (RegCM4). The results were analyzed and presented for the main BB season (July-October). The results show that Mpumalanga, KwaZulu Natal and the eastern parts of Limpopo are the main local source areas of BBAs in South Africa. In comparison to carbonaceous aerosols, BB-induced sulfate aerosol mass loading and climatic effects were found to be negligible. All carbonaceous aerosols reduce solar radiation at the surface by enhancing local atmospheric radiative heating. The climatic feedback caused by BBAs, resulted in changes in background aerosol concentrations. Thus, on a regional scale, climatic effects of BBAs were also found in areas far away from the BBA loading zones. The feedback mechanisms of the climate system to the aerosol radiative effects resulted in both positive and negative changes to the low-level columnar averaged net atmospheric radiative heating rate (NAHR). Areas that experienced an NAHR reduction showed an increase in cloud cover (CC). During the NAHR enhancement, CC over arid areas decreased; whereas CC over the wet/semi-wet regions increased. The changes in surface temperature (ST) and surface sensible heat flux are more closely correlated with BBA semi-direct effects induced CC alteration than their direct radiative forcing. Furthermore, decreases (or increases) in ST, respectively, lead to the reductions (and enhancements) in boundary layer height and the vice versa on surface pressure. The direct and semi-direct effects of BBAs also jointly promoted a reduction and rise in surface wind speed that was spatially highly variable. Overall, the results suggest that the CC change induced by the presence of radiatively interactive BBAs is important to determine alterations in other climatic variables.