Vertical variability of the properties of highly aged biomass burning aerosol transported over the southeast Atlantic during CLARIFY-2017

Abstract. Seasonal biomass burning (BB) from June to October in central and southern Africa leads to absorbing aerosols being transported over the south Atlantic Ocean every year, and contributes significantly to the regional climate forcing. The vertical distribution of submicron aerosols and their properties were characterized over the remote southeast Atlantic for the first time, using airborne in-situ measurements made during the CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY-2017) campaign. BB aerosols were intensively observed in the region surrounding Ascension Island, in the marine boundary layer (MBL) and free troposphere (FT) up to 5 km. We show that the aerosols had undergone a significant aging process during > 7 days transit from source, as indicated by highly oxidized organic aerosol and thickly coated black carbon (BC). The highly aged BB aerosols in the CLARIFY region were also especially rich in BC compared with those from other regions. We also found significant vertical variation in the single scattering albedos (SSA) of these aerosols, as a function of relative chemical composition and size. The lowest SSA was generally in the low FT layer around 2000 m altitude (medians: 0.83 at 405 nm and 0.80 at 658 nm). This finding is important since it means that BB aerosols across the east Atlantic region are more absorbing than is currently represented in climate models. Furthermore, in the FT, we show that SSA increased with altitude and this was associated with an enhanced inorganic nitrate mass fraction and aerosol size. This likely results from increased partitioning to the existing particles at higher altitude with lower temperature and higher relative humidity. After entrainment into the BL, aerosols were generally smaller in size than were observed in the FT, and had a larger fraction of scattering material with resultant higher average dry SSA, mostly due to marine emissions and aerosol removal by drizzle. Our results provide unique observational constraints on aerosol parameterizations used in modelling regional radiation interactions over this important region. We recommend that future work should consider the impact of this vertical variability on climate models.