| 18 Aug 2026
Reduction and evaluation of a near-explicit dimethyl sulfide and methanethiol oxidation mechanism
Abstract. Marine sulfur species play a key role in the Earth system, but simulating their complex oxidation mechanisms is a significant challenge for Earth system models. We have taken a comprehensive gas-phase mechanism of the OH and NO3-initiated oxidation of dimethyl sulfide and methanethiol (156 reactions and 55 sulfur species) and generated a series of reduced complexity mechanisms using an automated rates-based approach for future incorporation into global models. Four reduced mechanisms were produced, ranging from 38–91 reactions. While the error in the reduced mechanisms typically increased with fewer reactions, the smallest mechanism, which included 38 reactions and 18 species, had an average error of less than 15 % in the concentrations of key sulfur species compared to the full mechanism. This reduced mechanism was compared to a published mechanism of similar size (38 reactions) used in global modelling, which showed average concentrations of methanesulfonic acid and OCS differed by factors of 15 and 3.6, respectively, across all marine box models. While the most reduced mechanism constitutes a large increase in the number of reactions and species typically used in Earth system models for dimethyl sulfide and methanethiol, it provides a traceable link to the most recent experimental work on the subject. To assess its performance in a global modelling framework, an adapted reduced mechanism from this work was implemented into UKCA/UKESM, where it outperformed the highly simplified StratTrop dimethyl sulfide oxidation mechanism when evaluated against ATom-3 and ATom-4 measurements.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Jacob et al. provide several reduced mechanisms for DMS oxidation based on a mechanism presented in previous papers (Jacob et al., 2024; 2026). The methods are clear and the reduced mechanisms seem to perform reasonably well compared to UKCA. In general, there is a need to update DMS chemistry in global models, and the authors are trying to address that need while recognizing that most global models are unable to include hundreds of reactions and intermediates. I have three main questions and then minor questions and suggestions below.
First, the authors focus exclusively on gas-phase chemistry in this mechanism development, neglecting the condensed phase (e.g., MSIA + O3/OH in cloud and aerosol to form MSA). They might have made this choice because UKCA includes much of the condensed phase chemistry already (e.g., they discuss in-cloud HPMTF oxidation to form sulfate), but it is not clear from the text. If they intend to have this mechanism to be used in models other than UKCA, the authors should provide a recommendation on condensed phase chemistry as well. Should global models include in-cloud oxidation of DMSO, MSIA, HPMTF, etc. to form MSA and sulfate? If so, how? If not, why not?
My second question is regarding halogen + DMS chemistry. The authors omit DMS + BrO and DMS + Cl because UKCA does not include treatment of halogen chemistry. However, DMS + BrO is a potentially major sink of DMS and influences the amount of climate-relevant aerosols (MSA and sulfate) produced from DMS. Would this analysis be better performed with a model that includes halogen chemistry such as CAM6-Chem or GEOS-Chem? Or is this a minor omission compared to the current state of DMS chemistry in most global models?
My third question is regarding the overestimate of DMS compared to observations. UKCA overestimates DMS in the lowest 2km of the atmosphere, indicating overestimated emissions by maybe an order of magnitude or more. First, the authors should say where emissions come from in this model, i.e. the DMS climatology and transfer velocity parameterization. Is this overestimate in DMS consistent across other DMS obs-model comparisons? I believe that it is, and the authors should cite some examples. Second, if DMS is overestimated by a large factor, shouldn’t SO2 and HPMTF also be overpredicted by a similar factor? If DMS emissions are too high, but SO2 and HPMTF are in reasonable agreement, then wouldn’t it follow that the chemical mechanism may not be producing enough SO2 and HPMTF from DMS? It might be helpful to plot ratios (e.g. SO2/HPMTF, MSA/SO2, MSA/SO4) to better evaluate the chemistry given that DMS emissions are likely too high.
Minor questions and suggestions: