Journal Article

Methanethiol Abundance and Oxidation in a Polluted Marine Atmosphere

Wohl, Charel, Suntharalingam, Parvadha, Oram, David E., Edwards, Pete M., Forster, Grant L.

2025

Overview

Biological activity in the surface ocean leads to emissions of methanethiol (MeSH) and dimethyl sulfide (DMS). Measurements of MeSH in the marine atmosphere are sparse and the impact of NOx pollution on MeSH oxidation remains unexplored. We present measurements of MeSH and DMS at a coastal site with NOx up to 24.3 ppb in the United Kingdom during May and June. Winds coming from the seaward (northerly) direction showed a median (25th quantiles) MeSH mixing ratio of 15.7 (7.9–26.9)  ppt. The measurements reveal significantly lower MeSH during daytime. Atmospheric box model calculations suggest that ∼25% of the MeSH oxidation is initiated by NO3 at this site and that NOx pollution can reduce the SO2 yield from MeSH. This work is further evidence for the prevalence of MeSH and illustrates the impact of NOx pollution on MeSH oxidation with associated implications for its role in aerosol‐cloud processes, and climate. Plain Language Summary The oceans emit substantial amounts of volatile, gaseous sulfur in the form of methanethiol and DMS. Methanethiol measurements in marine air are very sparse, partly because it is hard to measure. Methanethiol is of interest, because it very efficiently reacts in the atmosphere to form SO2 at a close to 100% yield. SO2 is a particle forming sulfur gas, cooling the climate. We measured methanethiol in air on the UK coast and found it to be present at 10–30 ppt, a tiny fraction of the molecules in air. We find higher mixing ratios when the winds are from the sea, likely because the oceans are emitting this compound. We also find higher mixing ratios at night, probably due to removal processes initiated by sunlight and physical processes in the atmosphere. Using a computer model, we calculate that nitrogen oxides from shipping exhausts and terrestrial combustion sources can react with methanethiol at night. They have the potential to decrease the efficiency of SO2 production from methanethiol down to a yield of less than 50%. This case study gives a better appreciation of methanethiol's climatic impact and how this might be different in a polluted marine atmosphere. Key Points Measurements confirm that methanethiol is prevalent at typically 5–25 ppt at a coastal site in UK Mixing ratios are higher when winds are from the ocean and at night, likely due to ocean emissions and daytime OH oxidation Box modeling highlights NO3 as an important oxidant for MeSH and that NOx pollution has the potential to decrease the SO2 yield from MeSH

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Air / Air pollution / Air-sea interaction / Atmosphere / Biological activity / Climate / Dimethyl sulfide / Exhausts / Marine biology / measurement / Methanethiol / Mixing ratio / Night / Nitrogen compounds / Nitrogen oxides / NOx pollution / Oceans / Oxidation / Photochemicals / Pollution control / Shipping / Sulfur / Sulfur dioxide / Sulphides / Sulphur / Sunlight / Vocus‐PTR‐ToF / Winds