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

15 From June to October, southern Africa produces one-third of the global biomass burning (BB) emissions by widespread fires. BB aerosols are westward transported over the south-eastern Atlantic with the mid-tropospheric winds, resulting in significant radiative effects. Ascension Island (ASI) is located midway between Africa and South America. A 17-month in-situ 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 20 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 ~0.85 in October and the lowest ~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 BB seasons at ~ 1.96 and ~ 2.07 in 2016 and 2017. As the E abs is higher than the ~1.5 commonly adopted 25 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 (BC/∆CO) is well correlated with SSA and MAC BC , providing a simple way to estimate those aerosol optical characteristics in the south-eastern Atlantic. The exponential function we proposed can approximate SSA and MAC BC with BC/∆CO, and can better capture the growth of SSA as the decrease of BC/∆CO, especially when BC/∆CO is small. BC/∆CO is influenced by combustion conditions and aerosol scavenging . From the analysis 30 of the location of BB burning, the primary source fuel, the water content in the fuel, combined with the mean cloud cover and precipitation in the transport areas of BB plume, we conclude that the increase in BC/∆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 changing et al., 2016, p.6), which provide monthly mean fire radiative power at the centre of a 1 km pixel. To identify the main areas of combustion, the accumulated fire counts were calculated each month with the confidence greater than or equal to nominal for each pixel, then areas with the fire count >500 are considered the main burning areas. The land cover data was taken from the MODIS version 6, which provide annual land cover types derived from six different classification schemes 145 (Sulla-Menashe et al., 2019). Here we used the Land Cover Type 3 product with the annual leaf area index (LAI) classification, to show the land cover types on the African continent. Normalized Difference Moisture Index (NDMI) is used to determine vegetation moisture content. It is derived from the Landsat 8 Operational Land Imager (OLI)/Thermal Infrared Sensor (TIRS surface reflectance (Wilson and Sader, 2002), and can be used to monitor fuel levels in fire-prone areas. the more flaming the combustion is, the lower the SSA. For the MAC BC , smouldering combustion can result in a smaller BC core and thicker coating, which could lead to a higher shell-core ratio of BC particles and thus a higher MAC BC . We further propose SSA and MAC BC can be approximated by either linear or exponential functions with BC/∆CO. Both functions are able to represent the denser data in general, while the exponential function can better capture the rapid growth of SSA as the decrease of BC/∆CO when BC/∆CO < 0.003. These functions provide us with a simple way to estimate the parameters of the aerosol optical characteristics in the south-eastern Atlantic, and can help to improve the parameterisations used in climate models.