Aerosol hygroscopicity over the southeast Atlantic Ocean during the biomass burning season - Part 1: From the perspective of scattering enhancement

Aerosol hygroscopicity plays a vital role in aerosol radiative forcing. One key parameter describing hygroscopicity is the scattering enhancement factor, f(RH), defined as the ratio of the scattering coefficient at humidified relative humidity (RH) to its dry value. Here, we utilize the f(80 %) from ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) 2016 and 2018 airborne measurements to investigate the hygroscopicity of aerosols, its vertical distribution, its relationship with chemical composition, and its sensitivity to organic aerosol (OA) hygroscopicity over the southeast Atlantic (SEA) Ocean during the biomass burning (BB) season.We found that aerosol hygroscopicity remains steady above 2 km, with a mean f(80 %) of 1.40 +/- 0.17. Below 2 km, aerosol hygroscopicity increases with decreasing altitude, with a mean f(80 %) of 1.51 +/- 0.22, consistent with higher values of BB aerosol hygroscopicity found in the literature. The hygroscopicity parameter of OA (kappa OA) is retrieved from the Mie model with a mean value of 0.11 +/- 0.08, which is in the middle to upper range compared to the literature. Higher OA hygroscopicity is related to aerosols that are more aged, oxidized, and present at lower altitudes. The enhanced biomass burning aerosol (BBA) hygroscopicity at lower altitudes is mainly due to a lower OA fraction, increased sulfate fraction, and greater kappa OA at lower altitudes.We propose a parameterization that quantifies f(RH) with chemical composition and kappa OA based on Mie simulation of internally mixed OA-(NH4)2SO4-BC mixtures. The good agreement between the predictions and the ORACLES measurements implies that the aerosols in the SEA during the BB season can be largely represented by the OA-(NH4)2SO4-BC internal mixture with respect to the f(RH) prediction. The sensitivity of f(RH) to kappa OA indicates that applying a constant kappa OA is only suitable when the OA fraction is low and kappa OA shows limited variation. However, in situations deviating these two criteria, kappa OA can notably impact scattering coefficients and aerosol radiative effect; therefore, accounting for kappa OA variability is recommended.