Oxidation mechanisms for volatile methylated sulfur compounds and the major contribution of methanesulfonic acid to Southern Ocean aerosol particles

Abstract. Volatile methylated sulfur compounds (VMS), particularly dimethyl sulfide (DMS) and methanethiol (MeSH), are important natural sources of atmospheric sulfur. Their oxidation pathways and contribution to aerosols and cloud condensation nuclei (CCN) remain uncertain. Here, we investigate four gas-phase chemical mechanisms of increasing complexity for VMS oxidation using the global chemistry-climate model EMAC, and evaluate the results against shipborne and ground-based observations of DMS, sulfuric acid (SA), and methanesulfonic acid (MSA) between 2016 and 2019. In the marine boundary layer, DMS mixing ratios are largely insensitive to the choice of mechanism and agree well with observations, whereas simulated SA and MSA differ markedly between mechanisms. Notably, oxidation by bromine monoxide (BrO) is the dominant process controlling the DMS loss rates and concentrations in the Southern Ocean. We also evaluate the contribution of MSA to global new particle formation in the marine boundary layer, based on recent measurements of (SA+MSA)-NH₃ -H₂O nucleation at the CERN CLOUD chamber. Our simulations show that, under the cold and humid conditions of the Southern Ocean and Antarctic, MSA-induced nucleation rates become comparable to those of SA, with MSA accounting for around 25 % of CCN_0.4 over the Southern Ocean and up to 40 % over the Antarctic. MSA is therefore a key trace gas in the sulfur budget of these regions and a substantial source of CCN. This is particularly relevant for the Southern Ocean, where climate models exhibit a large positive shortwave radiation bias that has been linked to underestimated CCN concentrations.