Aerosol hygroscopicity over the South-East Atlantic Ocean during the biomass burning season: Part II – Influence of burning conditions on CCN hygroscopicity

Abstract. Biomass burning (BB) is an important source of cloud condensation nuclei in the Southeast Atlantic (SEA). The formation of cloud droplets depends on aerosol hygroscopicity (κ), but its variations during the BB season are poorly understood. In this study, we investigate κ during the 2016 and 2017 BB seasons using 18 months of in situ observations on Ascension Island. The results show that κ varied monthly, reaching a low in August and increasing from September to October. The mean κ was 0.33 in 2016 and 0.55 in 2017, showing a significant difference. The changes in κ were mainly associated with differences in the mass fraction of sulfate aerosol. Source attribution showed that about 67 % of the sulfate-containing particles originated from BB, suggesting that BB is the main driver of the changes in sulfate aerosol. The ratio of black carbon to excess carbon monoxide (BC/ΔCO), used to indicate BB combustion conditions, correlated well with κ and the sulfate mass fraction: higher ratios (more flaming combustion) reduced the sulfate mass fraction and thus a lower κ. Therefore, the observed lower BC/ΔCO ratios in 2017 explain the higher κ values, suggesting less flaming combustion in that year. Meteorological changes in 2017, including lower wind speeds and higher relative humidity in Africa, may contributed to the altered combustion conditions, explaining the lower BC/ΔCO and higher κ in 2017. Overall, this study highlights the critical role of BB in understanding the sulfate budget, aerosol hygroscopicity, and CCN concentration in the marine boundary of the SEA.