An attribution of the low single-scattering albedo of biomass-burning aerosol over the southeast Atlantic

Abstract. Aerosol over the remote southeast Atlantic is some of the most sunlight-absorbing aerosol on the planet: the in-situ free-tropospheric single-scattering albedo at the 530 nm wavelength (SSA_530nm) ranges from 0.83 to 0.89 within ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) aircraft flights from late August–September. Here we seek to explain the low SSA. The SSA depends strongly on the black carbon (BC) number fraction, which ranges from 0.15 to 0.4. Organic aerosol (OA) to BC mass ratios of 8–14 and modified combustion efficiency values > 0.975 point indirectly to the dry, flame-efficient combustion of primarily grass fuels, with back trajectories ending in the miombo woodlands of Angola. The youngest aerosol plume, aged 4–5 days since emission and sampled directly west of Angola, broadly consisted of two plumes, with the higher, thicker plume transported more quickly off of the continent by stronger winds. The particle size and fraction of BC-containing particles increased with chemical age, consistent with vapor condensation and coagulation. The particle volume and OA : BC mass ratio reduced simultaneously, attributed primarily to evaporation through photochemistry rather than dilution or thermodynamics. The CLARIFY (CLoud-Aerosol-Radiation Interaction and Forcing: Year-2017) aircraft campaign held near the more remote Ascension Island in August–September 2017 report higher BC number fractions, lower OA : BC mass ratios, lower SSA yet larger mass absorption coefficients compared to this study's. Values from the one analyzed ORACLES-2017 flight, held midway to Ascension Island, are intermediate, confirming the long-range changes. Inorganic ammonium nitrate, thought responsible for the vertical structure in SSA at Ascension Island through thermodynamic gas-particle partitioning, increases from ~20 % of the total nitrate in the ORACLES September flights, to 50 % for the August 2017 ORACLES flight midway to Ascension. Overall the data are consistent with continuing oxidation through fragmentation releasing aerosols that subsequently enter the gas phase, reducing the OA mass, rather than evaporation through dilution or thermodynamics. The data support the following best-fit: SSA_530nm=0.801+0055*(OA : BC) (r = 0.84). The fires of southern Africa emit approximately one-third of the world's carbon; the emitted aerosols are distinct from other regional BBAs and their aerosol composition also needs to be represented appropriately to realistically depict regional aerosol radiative effects.