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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

Biogenic volatile organic compounds (BVOCs) are known to strongly influence the global climate by affecting various atmospheric constituents such as oxidants and aerosols. Among the several BVOCs that are emitted continuously into the atmosphere, studies have shown that up to 96% of the emissions have been missed out by current analytical techniques. In this study, we used a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus) to characterize and quantify emissions from a branch of a downy birch tree at a boreal forest site in Hyytiälä, Finland in August 2019. During the measurement period, we were able to observe real-time emissions of hydrocarbons with up to 20 carbon atoms and oxygenated compounds (OVOCs) with up to 4 oxygen atoms. OVOCs accounted for around 90% of the total observed emissions with the largest contribution from C 8 H 8 O 3 (0.37 μgg –1 h –1 ; ∼60% of total). For the first time, emissions of diterpenes (C 20 H 32 , C 20 H 36 , and C 20 H 38 ) were observed from downy birch tree, although in minor quantities (0.1% of total emissions). During this late growing season, C 10 H 16 and C 10 H 14 contributed ∼7% in total emissions, while the sum of C 5 H 8 , C 15 H 22 , and C 15 H 24 contributed around ∼3%. The branch experienced abiotic stress during the measurement period, which might explain the unusually high emissions of C 8 H 8 O 3 . Standardized emission potentials are reported for all compounds using two Guenther algorithms. While emissions of most compounds fit well with either of the two algorithms, emissions of certain compounds like C 8 H 8 O 3 could not be explained by either suggesting the influence of other factors besides temperature and light. Vocus PTR-TOF-MS can help identify a diverse range of molecules even if emitted in minute quantities. The BVOCs detected from birch emissions may be important in the formation of secondary organic aerosols but their implications in the atmosphere need to be verified with further studies.