Seasonal variations in fire conditions are important drivers in the trend of aerosol optical properties over the south-eastern Atlantic

From June to October, southern Africa produces one-third of the global biomass burning (BB) emissions by widespread fires. BB aerosols are transported westward over the south-eastern Atlantic with the midtropospheric winds, resulting in significant radiative effects. Ascension Island (ASI) is located midway between Africa and South America. From June 2016 to October 2017, a 17-month in situ observation campaign on ASI found a low single-scattering albedo (SSA) as well as a high mass absorption cross-section of black carbon (MAC(BC)), demonstrating the strong absorbing marine boundary layer in the south-eastern Atlantic. Here we investigate the monthly variations of critical optical properties of BB aerosols, i.e. SSA and MAC(BC), during the BB seasons and the driving factors behind these variations. Both SSA and MAC(BC) increase from June to August and decrease in September and October. The average SSA during the BB seasons is 0.81 at 529 nm wavelength, with the highest mean similar to 0.85 in October and the lowest similar to 0.78 in August. The absorption enhancement (E-abs) derived from the MAC(BC) shows similar trends with SSA, with the average during the whole of the BB seasons at similar to 1.96 and similar to 2.07 in 2016 and 2017, respectively. As the E-abs is higher than the similar to 1.5 commonly adopted value by climate models, this result suggests the marine boundary layer in the south-eastern Atlantic is more absorbing than model simulations. We find the enhanced ratio of BC to CO (Delta BC/Delta CO, equal to BC/Delta CO as the BC background concentration is considered to be 0) is well correlated with SSA and MAC(BC), providing a simple way to estimate the aerosol optical characteristics in the south-eastern Atlantic. The exponential function we proposed can approximate SSA and MAC(BC) with BC/Delta CO, and when BC/Delta CO is small it can capture the rapid growth of SSA as BC/Delta CO decreases. BC/Delta CO is influenced by combustion conditions and aerosol scavenging. From the analysis of the location of BB, the primary source fuel, the water content in the fuel, combined with the mean cloud cover and precipitation in the transport areas of the BB plume, we conclude that the increase in BC/Delta CO from June to August is likely to be caused by burning becoming more flaming. The reduction in the water content of fuels may be responsible for the change in the burning conditions from June to August. The decrease in BC /Delta CO in September and October may be caused by two factors, one being a lower proportion of flaming conditions, possibly associated with a decrease in mean surface wind speed in the burning area, and the other being an increase in precipitation in the BB transport pathway, leading to enhanced aerosol scavenging, which ultimately results in an increase in SSA and MAC(BC).