Unexpected biomass burning aerosol absorption enhancement explained by black carbon mixing state

Direct and semi‐direct radiative effects of biomass burning aerosols (BBA) from southern and central African fires are still widely debated, in particular because climate models have been unsuccessful in reproducing the low single scattering albedo in BBA over the eastern Atlantic Ocean. Using state‐of‐the‐art airborne in‐situ measurements and Mie scattering simulations, we demonstrate that low single scattering albedo in well‐aged BBA plumes over southern West Africa results from the presence of strongly absorbing refractory black carbon (rBC), whereas the brown carbon contribution to the BBA absorption is negligible. Coatings enhance light absorption by rBC‐containing particles by up to 210%. Our results show that accounting for the diversity in black carbon mixing state by combining internal and external configurations is needed to accurately estimate the optical properties, and henceforth the shortwave direct radiative effect and heating rate of BBA over southern West Africa.