Comment on egusphere-2023-1056

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Abstract
Marine emissions of dimethyl sulfide (DMS) and the subsequent formation of its oxidation products methane sulfonic acid (MSA) and sulfuric acid (H2SO4) are well-known natural precursors of atmospheric aerosols, contributing to particle mass and cloud formation over ocean and coastal regions. Despite a long-recognized and well-studied role in the marine troposphere, DMS oxidation chemistry remains a work in progress within many current air quality and climate models, with recent advances exploring heterogeneous chemistry and uncovering previously unknown intermediate species. With the identification of additional DMS oxidation pathways and intermediate species influencing its eventual fate, it is important to understand the impact of these pathways on the overall sulfate aerosol budget and aerosol size distribution. In this work, we update and evaluate the DMS oxidation mechanism of the chemical transport model GEOS-Chem by implementing expanded DMS oxidation pathways into the model. These updates include gas- and aqueous-phase reactions, the formation of the intermediates dimethyl sulfoxide (DMSO) and methane sulphinic acid (MSIA), as well as cloud loss and aerosol uptake of the recently quantified intermediate hydroperoxymethyl thioformate (HPMTF). We find that this updated mechanism collectively decreases the global mean surface-layer gas-phase sulfur dioxide (SO2) mixing ratio by 38 % and enhances sulfate aerosol (SO42-) mixing ratio by 16 %. We further perform sensitivity analyses exploring the contribution of cloud loss and aerosol uptake of HPMTF to the overall sulfur budget. Comparing modeled concentrations to available observations we find improved biases relative to previous studies. To quantify impacts of these chemistry updates on global particle size distributions and mass concentration we use the TOMAS aerosol microphysics module, finding changes in particle formation and growth affect the size distribution of aerosol. With this new DMS-oxidation scheme the global annual mean surface layer number concentration of particles with diameters smaller than 80 nm decreases by 12 %, with cloud loss processes related to HPMTF mostly responsible for this reduction. However, global annual mean number of particles larger than 80 nm increases by 4.5 %, suggesting that the new scheme promotes seasonal particle growth to these sizes capable of acting as cloud condensation nuclei (CCN).

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