Prominent role of organics in aerosol liquid water content over the south-eastern Atlantic during biomass burning season

Abstract. The interaction between atmospheric aerosols and moisture is crucial for aerosol properties and their climate effects. In this study, thanks to the rich measurements of aerosol properties during the 2016 and 2018 ORACLES campaigns, we investigate the aerosol liquid water content (ALWC) over the south-eastern Atlantic Ocean during the biomass burning (BB) season, as well as the seldom-reported ALWC associated with organic aerosols (OA) (ALWCOA) (OA). ALWCOA is determined using the OA hygroscopicity parameter κOA, derived from in-situ hygroscopicity measurements, particle number size distribution, and chemical composition. The ALWC can be determined either with the overall hygroscopic parameter κf(RH) or from the sum of ALWCOA and the ALWC simulated from ISORROPIA-II, a thermodynamic equilibrium model for inorganic aerosol. The ALWC from both methods is highly correlated at all RHs with an R2 of 0.99. The ALWC increases with aerosol loading and ambient relative humidity (RH). Due to the lower RH and higher aerosol loading in the 2016 campaign, the ALWC for both campaigns are generally consistent. ALWCOA accounts for 38±16 % of the total ALWC during both campaigns. Notably, the contribution of ALWCOA is greater than commonly reported in the literature, highlighting the significance of OA in ALWC and therefore the aerosol direct radiative forcing in this climatically significant region. The strong correlation between κOA and ALWCOA/ALWC, as indicated by an R2 value of 0.72, underscores the importance of a good estimation of κOA in the ALWC estimation. Additionally, the significant difference between ALWCOA values calculated using real-time κOA and those calculated with the campaign mean κOA, highlights the limitation of using a constant κOA value, a practice commonly adopted in climate models.