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https://doi.org/10.5194/egusphere-2026-4020
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-4020
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
21 Jul 2026
| 21 Jul 2026
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
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 CCN0.4 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.
How to cite. Ruhl, S., Kohl, M., Baalbaki, R., Xenofontos, C., Vella, R., Gromov, S., He, X.-C., Shen, J., Alfaouri, D., Atabakhsh, S., Dada, L., DeVivo, J., Duplissy, J., El Haddad, I., Harder, H., Jokinen, T., Junninen, H., Kanawade, V. P., Sebastian, M. K., Klebach, H., Kulmala, M., Kunkler, F., Lelieveld, J., Lehtipalo, K., Lietzke, C. J., Liu, L., Mauldin, R., Mentler, B., Möhler, O., Petäjä, T., Russell, D. M., Simon, M., Thakur, R. C., Yu, W., Zhang, J., Zheng, Z., Kirkby, J., Tost, H., Christoudias, T., Sander, R., and Pozzer, A.: Oxidation mechanisms for volatile methylated sulfur compounds and the major contribution of methanesulfonic acid to Southern Ocean aerosol particles, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-4020, 2026.
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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Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Matthias Kohl
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Rima Baalbaki
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Christos Xenofontos
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Ryan Vella
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
now at: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
Sergey Gromov
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Xu-Cheng He
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Jiali Shen
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland
Dina Alfaouri
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Samira Atabakhsh
Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany
Lubna Dada
PSI Center for Energy and Environmental Sciences, Paul Scherrer Institute, Villigen, Switzerland
Jenna DeVivo
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, USA
Jonathan Duplissy
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Imad El Haddad
PSI Center for Energy and Environmental Sciences, Paul Scherrer Institute, Villigen, Switzerland
Hartwig Harder
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Tuija Jokinen
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Heikki Junninen
Institute of Physics, University of Tartu, Tartu, Estonia
Vijay P. Kanawade
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Global Center for Clean Air Research (GCARE), School of Engineering, Civil and Environmental Engineering, University of Surrey, Guildford, Surrey, United Kingdom
Milin K. Sebastian
Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research Atmospheric Aerosol Research, Karlsruhe, Germany
Hannah Klebach
Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, Frankfurt am Main, Germany
Markku Kulmala
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Felix Kunkler
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Jos Lelieveld
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Katrianne Lehtipalo
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Atmospheric composition research, Finnish Meteorological Institute, Helsinki, Finland
Clara J. Lietzke
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, USA
PSI Center for Energy and Environmental Sciences, Paul Scherrer Institute, Villigen, Switzerland
Roy Mauldin
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, USA
Bernhard Mentler
Ion Molecule Reactions and Environmental Physics Group, Institute of Ion Physics and Applied Physics, Leopold-Franzens University Innsbruck, Innsbruck, Austria
Ottmar Möhler
Global Center for Clean Air Research (GCARE), School of Engineering, Civil and Environmental Engineering, University of Surrey, Guildford, Surrey, United Kingdom
Tuukka Petäjä
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Douglas M. Russell
Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, Frankfurt am Main, Germany
Mario Simon
Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, Frankfurt am Main, Germany
Roseline C. Thakur
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Wenjuan Yu
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Jiangyi Zhang
Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Zhensen Zheng
Ion Molecule Reactions and Environmental Physics Group, Institute of Ion Physics and Applied Physics, Leopold-Franzens University Innsbruck, Innsbruck, Austria
IONICON Analytik GmbH, 6020 Innsbruck, Austria
Jasper Kirkby
Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, Frankfurt am Main, Germany
CERN, European Organization for Nuclear Research, 1211 Geneva, Switzerland
Holger Tost
Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany
Theodoros Christoudias
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Rolf Sander
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Andrea Pozzer
Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
Climate and Atmosphere Research Center (CARE-C), The Cyprus Institute, Nicosia, Cyprus
Short summary
Marine microbes release sulfur gases that form tiny particles able to seed clouds. Using a global chemistry-climate model, we compare four schemes for how these gases oxidise and test them against ship and ground measurements. We find that methanesulfonic acid can drive new particle formation as strongly as sulfuric acid over the cold Southern Ocean, supplying around 25 % of the cloud-forming particles there, a natural, climate-relevant source that models often overlook.
Marine microbes release sulfur gases that form tiny particles able to seed clouds. Using a...
General comments
This paper addresses the atmospheric fate of VMS compounds, namely DMS and MeSH, and their contribution to aerosols and CCN using various complexity mechanisms combined with the chemistry-climate model EMAC.
It highlights the challenges of simulating atmospheric sulfur species as no single mechanism reproduces all three species (i.e. DMS, MSA and SA) simultaneously in a satisfactory way. The sensitivity experiments identify halogen and MeSH chemistry as the two dominant uncertainties on the marine VMS budget, particularly bromine (BrO) oxidation in the Southern Ocean. This study also identifies the importance of MSA to NPF in colder, polar regions. Models need to address these uncertainties to better simulate sulfur-driven CCN estimates.
It is a very well written paper and the conclusion are clear. I would highly support publication of this manuscript with the following modifications.
Major comments
Introduction:
Great introduction but would like to see a paragraph on the specific SO influences e.g. high wind speeds which increase flux and hence ocean emissions and emission of halogen compounds from sea ice and snow-covered land (which could be moved from the paragraph starting on Page 4, Line 82 in the ‘Model Configuration’ section).
We think MSA should be introduced in the first paragraph, along with SA, and their important roles in influencing climate should be emphasised. Therefore, Page 2, Line 25 and the paragraph starting on Page 3, Line 52 could be moved to paragraph 1.
Page 4, Line 78 and Page 15, Line 243: The climatologies are the correct/up-to-date ones to use. However, an inappropriate K parameterisation is used for DMS and MeSH. These gases are more soluble than CO2, and more appropriate parameterisations exist (the most recent was presented in Dong et al., 2026). More soluble gases are less sensitive to bubble-mediated gas exchange so wind speed parameterisations are more linear (i.e. there is less enhancement of gas transfer at high winds). Note also that, even when calculating the air-sea CO2 flux, the Wanninkhof 1992 relationship has been revisited and improved, reducing flux estimates at intermediate-high wind speeds (Wanninkhof, 2014)). All of the above means that the model fluxes are overestimated. It is unlikely that the authors will be able to correct their results at this point, but they should acknowledge the impact that this will have had throughout the manuscript (e.g. Table 7). For example, it seems incorrect to focus on the AIRSEA flux formulation as the main cause of difference with other flux estimates because the choice of K parameterisation will have a much bigger impact.
Dong, Y., Jähne, B., Woolf, D. K., Krall, K. E., Yang, M., Czerski, H., Liang, J., Brooks, I. M., McNeil, C. L., Wanninkhof, R., Ho, D. T., Atamanchuk, D., and Marandino, C. A.: The role of bubbles in air-sea gas exchange: A critical review, Reviews of Geophysics, 64, e2025RG000903, https://doi.org/10.1029/2025RG000903, 2026
Wanninkhof, R.: Relationship between wind speed and gas exchange over the ocean revisited, Limnology and Oceanography-Methods, 12, 351-362, 10.4319/lom.2014.12.351, 2014
Page 17, Line 292 and Page 28, Line 464: This statement is unsupported by the reference provided. More importantly, while changes in the ratio of DMS:MeSH are likely to occur in the future, the community does not have sufficient data to be sure of the direction of change. We suggest that these statements are tempered to reflect the current level of understanding.
Minor comments
Page 2, Line 11: Please define CCN0.4 here as well as on Page 25, Line 410.
Page 2, Line 21: “Estimates of total VMS emissions…” should specify that these are seawater emissions so change to “estimates of total VMS ocean emissions”.
Page 2, Line 29: Could add the following reference about the importance of MeSH:
Mynard, C., Franklin, E. B., Alroe, J., Somerville, N., Patti, A., Siems, S. T., et al. (2025). Constraining atmospheric methanethiol estimates over the Southern Ocean. Geophysical Research Letters, 52, e2025GL116470. https://doi.org/10.1029/2025GL116470
Page 3, Line 45: Split into two sentences and it should be ‘account’, not ‘accounts’. Like this: “For instance, none of the CMIP6 models account for DMS oxidation explicitly. Instead, they treat DMS as a prescribed, non-reactive tracer (Thornhill et al., 2021).”
Page 4, Section 2.1 (Line 90): It is not explicitly clear that MeSH is included in all mechanisms apart from MECCA. Could this information be added to Table 1? More generally, could more information be included in Table 1, reducing the amount of text is needed in the paragraphs introducing each mechanism?
Page 4, Line 95: Split into two sentences – “Table 1 gives an overview of the four VMS chemical schemes, which are described in more detail below. The full lists of reactions are provided as supplement material.”
Page 5, Line 106: Why were different temperature-dependent reaction rates used? Please state where the differences are documented.
Page 5, Line 117: Change ‘water-phase’ to ‘aqueous-phase’ to keep it consistent.
Page 6, Line 151: This paragraph should come before the description of volatile, semi-volatile and negligible-volatility, which starts on Line 141.
Page 9, Line 186: A sentence saying why the specific simulation period (2016 – 2019) was chosen would be useful, especially since the authors explain that records from outside this window were used. Is the period chosen to deliberately target before the change in international shipping emission regulations?
Page 10, Line 198: Please could the authors improve their distinction between the summary in Table 6 (which is an average of all sites but still comparing mechanisms) and that presented in Figure 2.
Page 10, Line 203: Do you mean the NAAMES-4 leg?
Page 13, Line 223: Re-phrase. Suggest “With both fast oxidants scarce, BrO becomes the leading DMS oxidant over the Southern Ocean (Figure 3c) due to its abundance, despite the relatively slow BrO+DMS reaction rate (which is slower than those of OH and NO3).”
Page 15, Line 254: What did Wohl report the MeSH lifetime to be? Please state here.
Page 16, Figure 4a: Please be consistent with the labelling of the colour bar i.e. ‘MeSH fluxes’ as in Figure 3a, rather than ‘MeSH emissions’.
Page 20, Line 337: “This campaign-to-campaign spread…” This is unclear, please re-phrase.
Page 23, Line 365: Is there a reason why MSA mixing ratios hug the coast of Antarctica (Figure 8b)? Could it be due to melting ice causing phytoplankton blooms? Please suggest possible reasons.
Page 24, Line 387: Change ‘grwoth’ to ‘growth’.
Page 25, Line 400: Could include the following very recent reference about the importance of iodine oxoacids in NPF:
Du, M., Brean, J., Worsnop, D.R. et al. Arctic cloud condensation nuclei enhanced by iodine, sulfur and organic precursors. Nat. Geosci. 19, 1039–1046 (2026). https://doi.org/10.1038/s41561-026-02062-6
Page 25, Line 406: Put the reference to Miljevic et al. (2025) in brackets.
Page 28, Line 487-490: This is a very long sentence and hard to follow. Suggest it is split into two.