the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4710', Anonymous Referee #1, 28 Sep 2026
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RC2: 'Comment on egusphere-2026-4710', Anonymous Referee #2, 26 Oct 2026
This manuscript provides a framework for oxidation mechanism reduction, and evaluates a reduced DMS oxidation mechanism relative to an existing evaluated mechanism and 1 set of in-situ observations (AToM 3 and 4). The manuscript is well-written, clear and timely, and provides a much-needed reduced DMS oxidation mechanism that could be implemented into Earth System models. This work is certainly suitable for publication is Atmospheric Chemistry and Physics, once the following major questions and minor comments have been addressed.
Major Comments.
(1) Related to the overestimation of DMS and reasonable model-measurement agreement with HPMTF and SO2: Which emissions parameterization is used (e.g., linear relationship between transfer velocity and wind speed, or a high-order relationship that is known to over predict emissions)? Which seawater climatology is used? While the hypothesis that the DMS overestimate is caused by the emissions parameterization is reasonable, could the exclusion of halogen-driven oxidation not also contribute to this overestimation? Why does a large overestimate in DMS emission not result in overestimates in SO2 and HPMTF? Does overestimate of DMS and reasonable agreement in some oxidation products suggest that the mechanism is not producing enough products (i.e., if the DMS emission overestimate was mitigated, the model would under predict oxidation products by a significant amount)?
(2) The authors neglect both halogen chemistry and aqueous phase reactions because the UKCA data does not include the needed oxidant concentrations (L275-278). While the authors discuss the potential importance of halogen chemistry in a few places throughout the manuscript, aqueous or multiphase processes are largely neglected from the discussion. Given several of the papers cited by this work show the importance of multiphase processes for DMS oxidation and subsequent impacts on aerosol microphysics and sulfate abundance, can the authors comment on how including multiphase processes may impact their results? Notably, how might multiphase processes impact the model-measurement agreement for HPMTF and SO2, both of which have important aqueous phase sinks.
Minor Comments.L18: The authors should cite and include global estimates from more recent climatologies and seawater DMS estimation products.
L27: I encourage the authors to cite Novak et al 2022 (https://doi.org/10.5194/acp-22-6309-2022), for the only currently published direct measurements of MeSH emissions, because Wohl 2024 relies on empirical estimates.
L29: Include Tashmim et al, 2025 for inclusion of MeSH emission and chemistry.
L31-32: While I agree that comparison to observations and assessment of uncertainty is key, this sentence is perhaps self-serving. Many model studies evaluate their outputs against field observations. If the authors have specific recommendations on how these comparisons should be done, and/or how uncertainty assessment should be included and designed, then I encourage the authors to articulate this.
L36: Another study that compared mechanisms, and demonstrated the importance of the oxidation scheme: https://doi.org/10.1029/2023JD040635
Figure 1: red points are difficult to distinguish from orange
Figure 2: Add a colour scale?
L174: Are these oxidation percentages based on the pseudo-first order rate constants for DMS oxidation, or on the rate of DMS oxidation for each pathway (i.e., accounting for global variation and latitudinal trends in DMS mixing ratios)?
Figure 3: Give conditions in this randomly selected box model, such as T?
Figure 7: What do the box models assume about RO2 concentrations (presumably the major RO2 of relevance is CH3O2)? What is known about molecular structure effect on the CH3SO2OO radical reaction with other RO2 of varying structure? Figure 7: For clarity, please show the relevant pathway(s) for CH2SO2O radical production in the sources approach (red species/arrows at right side).
L279: Observational evidence has also shown the important of halogen chemistry: https://doi.org/10.1021/acs.est.6c06288
L291: Clarify which threshold is meant here
L301: TPA is not defined
L370: Given the shape of the MSA distribution, is the average a useful, representative quantity?
L371-373: How do the authors deal with the scaling of the equilibrium constant for CH3SO2 + O2 done in Chen et al 2023 (see their SI)? Is this scaling adopted directly? This presumably adds considerable uncertainty to the mechanism, and the formation yields of MSA. The authors may wish to comment on this in the main text or supplement.
L389-391: While I appreciate what the authors are trying to do here, isn't a recommendation for a specific mechanism over another premature without evaluation against observations (which the authors do in the next section)?
L405-410: Should also include CH3SH + Br With k_Br = 9.2 x 10^-12 exp(-390/T) from Burkholder et al. 2019 At typical Br mixing ratios, this reactions is likely competitive.
L410: Mechanisms of DMS + Cl reaction is uncertain and depends strongly on temperature; the DMS-Cl adduct can react with O2 to form DMSO, which may be an important pathway for this reaction. See Barnes et al., 2006 for some discussion on this reaction; I would advise against using Hoffmann 2016 as a sole reference for the DMS + Cl reaction pathway.
L428-429: What is the TOGA detection limit? How are below DL values treated in the TOGA dataset used here?
L435-438: Have the authors contacted the authors of Veres et al., 2021 to determine how they prefer their data be represented in light of the correction factor and the unavailability of the corrected data at present? If not, I would strongly encourage the authors to do so and determine whether the corrected data should be obtained from Veres et al directly, or if the current publicly available version should be used. The HPMTF data version, and correction (or lack thereof) should also be noted in the caption to Figures 13 and 14.
L442-445: Can the authors provide some context for these numbers? Is this a relatively large or small increase? Is it prohibitive for including in simulations?
L454-455: find this an interesting place to use passive voice. The authors recommend their oxidation mechanism based on the comparisons described in this manuscript. It is not recommended by some outside body, which the construction of this sentence suggests.
Citation: https://doi.org/10.5194/egusphere-2026-4710-RC2
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- 1
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: