Heterogeneous Oxidation of Particulate Methanesulfonic Acid by the Hydroxyl Radical: Kinetics and Atmospheric Implications

Dimethyl sulfide (DMS) is a major source of sulfur to the marine boundary layer (MBL), and methanesulfonic acid (MSA) is one of its two main final oxidation products. MSA can participate in the nucleation and growth of aerosol particles, thereby affecting clouds and climate, and is used as a tracer of biological sulfur inputs. Unlike MSA, the other major oxidation product of DMS, sulfate, has several other sources, including volcanic and anthropogenic inputs. As a result, MSA to non-sea-salt sulfate (nss-sulfate) ratios are often used as proxies for biological activity; i.e., the MSA to nss-sulfate ratio in aerosol particles is used to estimate the marine biological contribution to nss-sulfate in the MBL. We present here a determination of the reactive uptake coefficient, y, for the heterogeneous oxidation of MSA by hydroxyl radicals within deliquesced ammonium sulfate aerosol particles with an MSA mass fraction of 0.16 (a typical marine value) at room temperature. We find gamma = 0.05 +/- 0.03. For high ambient gas-phase concentrations of the hydroxyl radical, this uptake coefficient corresponds to an estimated lifetime against heterogeneous oxidation of only a few days for MSA in MBL aerosol particles. Significantly, for typical gas-phase and condensed concentrations of the hydroxyl radical, the lifetime estimated here for heterogeneous oxidation is shorter than that for condensed-phase oxidation of MSA. This finding should be taken into consideration when using MSA to nss-sulfate ratios as tracers for DMS and biological activity, especially for air masses that have been exposed to considerable photochemical oxidation.