Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 3 2009 10.5194/acpd-9-13439-2009 http://www.atmos-chem-phys-discuss.net/9/13439/2009/ http://www.atmos-chem-phys-discuss.net/9/13439/2009/acpd-9-13439-2009.html http://www.atmos-chem-phys-discuss.net/9/13439/2009/acpd-9-13439-2009.pdf 13439 13474 2009-06-18 Chemical apportionment of southern African aerosol mass and optical depth B. I. Magi brian.magi@noaa.gov Princeton University, Princeton, New Jersey, USA We investigate the aerosol mass apportionment and derive aerosol optical properties that characterize the aerosol over extratropical and tropical southern Africa during the biomass burning season. We find that 54% and 83% of the extratropical and tropical aerosol mass, respectively, is composed of carbonaceous species, consistent with the fact that the major source of particulate matter in southern Africa is biomass burning. This mass apportionment implies that carbonaceous species in the form of organic carbon (OC) and black carbon (BC) play a critical role in the aerosol optical properties. By combining the in situ measurements of aerosol mass concentrations with concurrent measurements of aerosol optical properties at a wavelength of 550 nm, we find that 80–90% of the aerosol scattering is due to carbonaceous aerosol, where our derived mass scattering cross sections (MSC) for OC and BC are 3.9±0.6 m²/g and 1.6±0.2 m²/g, respectively. Our derived values of mass absorption cross sections (MAC) for OC and BC are 0.7±0.6 m²/g and 8.2±1.1 m²/g, respectively. The values of MAC imply that 26–27% of the aerosol absorption in southern Africa is due to OC, with the remainder due to BC. Our results provide important constraints for aerosol properties in a region dominated by biomass burning and should be integrated into climate models to improve aerosol simulations. 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