Efficient Production of Carbonyl Sulfide in the Low-NOx Oxidation of Dimethyl Sulfide

The oxidation of carbonyl sulfide (OCS) is the primary, continuous source of stratospheric sulfate aerosol particles, which can scatter shortwave radiation and catalyze heterogeneous reactions in the stratosphere. While it has been estimated that the oxidation of dimethyl sulfide (DMS), emitted from the surface ocean accounts for 8%-20% of the global OCS source, there is no existing DMS oxidation mechanism relevant to the marine atmosphere that is consistent with an OCS source of this magnitude. We describe new laboratory measurements and theoretical analyses of DMS oxidation that provide a mechanistic description for OCS production from hydroperoxymethyl thioformate, a ubiquitous, soluble DMS oxidation product. We incorporate this chemical mechanism into a global chemical transport model, showing that OCS production from DMS is a factor of 3 smaller than current estimates, displays a maximum in the tropics consistent with field observations and is sensitive to multiphase cloud chemistry. Plain Language Summary Accurate estimates of marine carbonyl sulfide (OCS) sources are critical for both modeling stratospheric aerosol concentrations, as OCS is an important precursor to stratospheric sulphate aerosol particles, and for estimating gross primary production, as OCS is readily consumed by the terrestrial biosphere. Despite the importance of OCS to both stratospheric aerosol chemistry and as an effective proxy for CO2 plant uptake, considerable uncertainty remains in the sources and sinks of OCS. A large source of this uncertainty arises in the marine sources, dominated by the oxidation of marine sulfur gases. Here, we examine the global production of OCS from the oxidation of a marine biologically produced molecule, dimethyl sulfide (DMS). We show that the multi-generational production of OCS proceeds through the oxidation of stable, water-soluble intermediates. Using a global chemical transport model, we find that OCS production is largest in the tropics, where cloud loss of hydroperoxymethyl thioformate, the primary precursor to OCS in the DMS oxidation mechanism, is at a minimum.