Dimethyl sulfide chemistry over the industrial era: comparison of key oxidation mechanisms and long-term observations

Abstract. Dimethyl sulfide (DMS) is primarily emitted by marine phytoplankton and oxidized in the atmosphere to form methanesulfonic acid (MSA) and sulfate aerosols, which affect climate by influencing radiation and cloud properties. Ice cores in regions affected by pollution show an industrial-era decline in MSA, which has previously been interpreted to indicate a decline in phytoplankton abundance. However, a simultaneous increase in DMS-derived sulfate (bioSO₄) in a Greenland ice core suggests that pollution-driven oxidant changes caused the decline in MSA by influencing the relative production of MSA versus bioSO₄. Here we use GEOS-Chem, a global chemical transport model, over three time periods (preindustrial, peak North Atlantic NO_x pollution, and 21^st century) to investigate the chemical drivers of the industrial-era changes in MSA and bioSO₄, and examine whether four DMS oxidation mechanisms reproduce trends and seasonality in DMS, MSA, and bioSO₄ observations. We find that GEOS-Chem and box model simulations can reproduce ice core trends in MSA and bioSO₄, but model results are sensitive to both DMS oxidation mechanism and oxidant concentrations. Our simulations support the hypothesized nitrate-radical driven decline in MSA over the industrial era, but none of the GEOS-Chem simulations can capture the seasonality of in situ DMS observations while also reproducing ice core trends in MSA and bioSO₄. To reduce uncertainty in modeling DMS-derived aerosols, future work should investigate aqueous-phase chemistry, which produces 82–99 % of MSA and bioSO₄ in our simulations, and constrain atmospheric oxidant concentrations, including the nitrate radical, hydroxyl radical, and reactive halogens.