Atmospheric Gas Phase Formation Mechanism of Methanesulfonic Acid

The oxidation of dimethyl sulfide (DMS) leads to the formation of sulfuric acid (SA) and methane sulfonic acid (MSA), which has a great impact on atmospheric aerosol and cloud formation (Barnes, Hjorth et al. 2006). Despite the great importance, the formation mechanism of MSA from DMS has remained unclear for decades (Shen, Scholz et al. 2022).

           The reaction of DMS with OH radical forms methanesulfinic acid (MSIA), methane sulfenic acid (MSEA), methylation radical (CH₃S) radical, and hydroperoxymethyl thioformate (HPMTF)(Berndt, Scholz et al. 2019, Shen, Scholz et al. 2022). Among them, the oxidation of the first three all undergoes either the CH₃SO radical or the CH₃SO₂ radical as intermediates(Kukui, Borissenko et al. 2003, Berndt, Chen et al. 2020, Chen, Berndt et al. 2021).

           We theoretically investigated the atmospheric fate of the CH₃SO and CH₃SO₂ radicals. The results suggest that CH₃SO radical mainly reacts bimolecularly forming CH₃SO₂, and the CH₃SO₂ radical either decomposes forming SO₂ or adds O₂ forming the peroxy radical CH₃S(O)₂OO in the atmosphere. We show that the branching ratio of SO₂ and CH₃S(O)₂OO formation from CH₃SO₂ is temperature sensitive, and the ratio of SO₂ to CH₃S(O)₂OO decreases from about 99:1 to about 95:5 when reducing the temperature from 300 K to 260K. The peroxy radical CH₃S(O)₂OO can react bimolecularly forming the CH₃SO₃ intermediate, which can abstract an H from HO2 forming MSA. In addition, we show that MSA can also form directly via the reaction of MSIA and OH followed by O₂ addition (Chen, Lane et al. 2023).

           Our study indicates that temperature may play a crucial role in explaining atmosphere MSA formation. SO₂ is likely the dominant product from DMS + OH in the tropics and warm regions, while in the colder and polar regions, large amounts of MSA can be formed in the gas phase by DMS reacting with OH. Global modeling indicates that the proposed temperature-sensitive MSA formation mechanism leads to a substantial increase in the simulated global atmospheric MSA formation and burden (Chen, Lane et al. 2023).