Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

Recent studies highlight that biomass-burning aerosol over the remote southeast Atlantic is some of the most sunlight-absorbing aerosol on the planet. In-situ measurements of single-scattering albedo at the 530 nm wavelength (SSA_530nm) range from 0.83 to 0.89 within six flights (five in September, 2016 and one in late August, 2017) of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) aircraft campaign, increasing with the organic aerosol to black carbon (OA:BC) mass ratio. OA:BC mass ratios of 10 to 14 are lower than some model values and consistent with BC-enriched source emissions, based on indirect inferences of fuel type (savannah grasslands) and dry, flame-efficient combustion conditions. These primarily explain the low single-scattering albedos. We investigate whether continued chemical aging of aerosol plumes of intermediate age (4-7 days after emission, as determined from model tracers) within the free troposphere can further lower the SSA_530nm.  A mean OA to organic carbon mass ratio of 2.2 indicates highly oxygenated aerosol with the chemical marker f44 indicating the free-tropospheric aerosol continues to oxidize after advecting offshore of continental Africa. Two flights, for which BC to carbon monoxide (CO) ratios remain constant with increasing chemical age, are analyzed further. In both flights, the OA:BC mass ratios decrease while BC:CO remains constant, indicating continuing net aerosol loss. One flight sampled younger (~ 4 days) aerosol within the strong zonal outflow of the 4-6 km altitude African Easterly Jet-South. This possessed the highest OA:BC mass ratio of the 2016 campaign and overlaid slightly older aerosol with proportionately less OA, although the age difference of one day is not enough to attribute to a large-scale recirculation and subsidence pattern. The other flight sampled aerosol constrained closer to the coast by a mid-latitude disturbance and found older aerosol aloft overlying younger aerosol. Its vertical increase in OA:BC and nitrate to BC was less pronounced than when younger aerosol overlaid older aerosol, consistent with compensation between a net aerosol loss through aging and a thermodynamical partitioning. Organic nitrate provided 68% on average of the total nitrate for the 6 flights, in contrast to measurements made at Ascension Island that only found inorganic nitrate. Some evidence for the thermodynamical partitioning to the particle phase at higher altitudes with higher relative humidities for nitrate is still found. The 470-660 nm absorption Angstrom exponent is slightly higher for the youngest aerosol near the African coast than further offshore (approximately 1.2 versus 1.0-1.1), indicating some brown carbon may be present. The data support the following parameterization: SSA_530nm=0.80+0056*(OA:BC). This indicates a 20% decrease in SSA_530nm can be attributed to chemical aging, or the net 25% reduction in OA:BC documented for constant BC:CO ratios. This work is under review within https://acp.copernicus.org/preprints/acp-2021-1081/; we welcome relevant comments submitted under the ACPD site.