Hypobromous Acid as an Unaccounted Sink for Marine Dimethyl Sulfide?

Marine emissions of dimethyl sulfide (DMS) to the atmosphere play a fundamental role in the global sulfur (S) cycle and have important consequences for the Earth’s radiative balance. In the ocean, DMS is mainly produced by marine algae and bacteria via cleavage of the precursor compound dimethylsulfoniopropionate (DMSP). Here, we studied the reaction between DMS and the strong oxidant hypobromous acid (HOBr), which is also produced by marine algae. Further, reactions between DMS oxidation products and HOBr were studied. The second-order rate constants were determined in competition kinetic experiments using sulfite as a competitor. In addition, we developed a new HPLC-ICP-MS/MS method to identify and quantify the oxidation products of DMS and related compounds. We found that HOBr reacts very fast with DMS to dimethyl sulfoxide (DMSO), with a second-order rate constant of 1.6 × 109 M–1 s–1, while the subsequent oxidation of DMSO to dimethyl sulfone (DMSO2) is much slower (0.4 M–1 s–1). Concentrations of DMSP, DMSO2, and methanesulfonic acid (MSA) did not decrease when exposed to excess concentrations of HOBr, implying that these S-containing compounds are not or only slightly reactive toward HOBr. A quantitative comparison of known DMS sinks shows that HOBr may be an important, hitherto neglected sink for marine DMS that needs to be considered in ocean–atmosphere chemistry models.