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. Plain Language Summary Extensive seasonal fires over southern and central Africa result in the transport of massive amounts of biomass burning aerosols over huge areas of the eastern Atlantic Ocean. Recent field observations highlight that biomass burning aerosols transported from the coast of southern Africa to the far north over southern West Africa were characterized by low single scattering albedo. This finding is of paramount interest, because radiative heating within the absorbing aerosol layer is hypothesized to affect the low cloud deck over this specific region and may ultimately influence the large-scale circulation. However, debate remains about the causes of the low single scattering albedo by biomass burning aerosols, causing ambiguous parameterizations of their optical properties in climate models. Here we present simultaneous airborne measurements of the composition and optical properties of biomass burning aerosols transported over southern West Africa. We show that black carbon particles dominated the light absorption by biomass burning aerosols at mid-visible wavelengths. Our findings indicate that the black carbon mixing state plays a significant role in the aerosol optical properties and may be an important modulator to be considered in climate models for simulating direct and semi-direct radiative effects of biomass burning aerosol over southern West Africa.