Methanesulfonic acid revealed as major driver of particle formation over polar regions and the Southern Ocean: a global EMAC study

Methanesulfonic acid (MSA) has recently been identified as an efficient driver of new particle formation and growth under cold atmospheric conditions, exhibiting ultra low volatility the same way sulfuric acid (SA) does. Both MSA and SA originate from the oxidation of volatile methylated sulfur compounds (VMS), particularly dimethyl sulfide (DMS) and methyl mercaptan (MeSH), with MeSH acting as a significant but previously overlooked source of these compounds, which constitute a major natural source of atmospheric sulfur. In cold regions, oxidation pathways favour MSA over SA production, leading to elevated MSA-to-SA ratios over the polar regions and the Southern Ocean.

In this study, the representation of marine sulfur was revised in the global chemistry–climate model EMAC by updating DMS emissions, explicitly including MeSH, and extending the associated gas-phase, multiphase, and aerosol chemistry of SA and MSA. The model is evaluated against observations from four ship campaigns and nine ground-based stations in oceanic regions, spanning four years and covering diverse latitudes and longitudes. MSA condensation onto particles, its aqueous-phase processing in aerosols and clouds, and its contribution to particle growth are treated explicitly. A volatility-dependent MSA nucleation parameterization is implemented to capture efficient particle formation in cold, MSA-rich environments.

Including MSA-driven particle formation and growth in EMAC leads to an increase of at least 50% in cloud condensation nuclei (CCN) concentrations over the Antarctic and Southern Ocean. This demonstrates that MSA is a major driver of particle formation and growth in these climate-critical regions, which have traditionally been associated with large uncertainties in CCN abundance and associated aerosol–cloud–climate interactions in global climate models.