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. Seasonal biomass burning (BB) from June to October incentral and southern Africa leads to absorbing aerosols being transportedover the South Atlantic Ocean every year and contributes significantly tothe regional climate forcing. The vertical distribution of submicronaerosols and their properties were characterized over the remote southeastAtlantic, using airborne in situ measurements made during theCLoud-Aerosol-Radiation Interactions and Forcing for Year 2017(CLARIFY-2017) campaign. BB aerosols emitted from flaming-controlled fireswere intensively observed in the region surrounding Ascension Island, in themarine boundary layer (MBL) and free troposphere (FT) up to 5 km. We showthat the aerosols had undergone a significant ageing process during&gt; 7 d transit from source, as indicated by the highly oxidizedorganic aerosol. The highly aged BB aerosols in the far-field CLARIFY regionwere also especially rich in black carbon (BC), with relatively low single-scattering albedos (SSAs), compared with those from other BB transportedregions. The column-weighted dry SSAs during CLARIFY were observed to be 0.85, 0.84and 0.83 at 405, 550 and 658 nm respectively. We also found significantvertical variation in the dry SSA, as a function of relative chemicalcomposition and size. The lowest SSA in the column was generally in the lowFT layer around 2000 m altitude (averages: 0.82, 0.81 and 0.79 at 405, 550and 658 nm). This finding is important since it means that BB aerosolsacross the southeast Atlantic region are more absorbing than currentlyrepresented in climate models, implying that the radiative forcing from BBmay be more strongly positive than previously thought. Furthermore, in theFT, average SSAs at 405, 550 and 658 nm increased to 0.87, 0.86 and 0.85with altitude up to 5 km. This was associated with an enhanced inorganicnitrate mass fraction and aerosol size, likely resulting from increasedpartitioning of ammonium nitrate to the existing particles at higheraltitude with lower temperature and higher relative humidity. Afterentrainment into the boundary layer (BL), aerosols were generally smaller in dry size than inthe FT and had a larger fraction of scattering material with resultanthigher average dry SSA, mostly due to marine emissions and aerosol removalby drizzle. In the BL, the SSA decreased from the surface to the BL top,with the highest SSA in the column observed near the surface. Our resultsprovide unique observational constraints on aerosol parameterizations usedin modelling regional radiation interactions over this important region. Werecommend that future work should consider the impact of this verticalvariability on climate models.

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