Aerosol acidity controls methanesulfonic acid evaporation from aerosols during Antarctic katabatic outflow

Abstract Methanesulfonic acid, a key oxidation product of dimethyl sulfide, plays a crucial role in the atmospheric sulfur cycle and is an important contributor to aerosol growth and cloud condensation nuclei formation in remote marine and polar atmospheres. Its role is particularly significant in the Southern Ocean, where particle concentrations are low and biogenic sulfur emissions help shape cloud properties. Here, we focus on an eight-day period during the austral summer along coastal East Antarctica marked by elevated gaseous MSA that coincided with persistent katabatic outflow. By combining our ship-based observations, back-trajectory analysis and thermodynamic modelling, we show that this outflow brings biogenically dominated, highly acidic aerosols with elevated gaseous methanesulfonic acid produced by evaporation from aerosol particles. Although a decrease in relative humidity can enhance this process, aerosol acidity is identified as the primary driver. Our findings highlight the strongly acidic nature of Southern Ocean aerosol and provide new insights into processes involved in the marine sulfur cycle. Acidity-driven MSA evaporation should be considered when using dimethyl sulfide oxidation products to evaluate and improve atmospheric chemistry and climate models.