Photodegradation of Dimethyl Sulfide (DMS) in Natural Waters: Laboratory Assessment of the Nitrate-Photolysis-Induced DMS Oxidation RENE Ä -CHRISTIAN BOUILLON* ,† AND WILLIAM L. MILLER ‡ Dalhousie University, Department of Oceanography, Halifax, Nova Scotia, Canada, B3H 4J1 The interaction of sunlight and dissolved chromophoric matter produces reactive chemical species that are significant in the removal of dimethyl sulfide (DMS) in the surface ocean. Using artificial solar radiation, we examined the role of several inorganic components of seawater on the kinetics of NO 3 - -photolysis-induced DMS removal in aqueous solution. This study strongly suggests that NO 3 - photolysis products react significantly with DMS in aqueous solution possibly via an electrophilic attack on the electron-rich sulfur atom. This supports previous field observations that indicate that NO 3 - photolysis has a substantial control on DMS photochemistry in nutrient-rich waters. A key finding of this research is that the oxidation rate of DMS induced by NO 3 - photolysis is dramatically enhanced in the presence of bromide ion. Moreover, our results suggest that bicarbonate/carbonate ions are involved in free radical production/scavenging processes important for DMS photochemistry. These reactions are pH dependent. We propose that DMS removal by some selective free radicals derived from bromide and bicarbonate/carbonate ion oxidation is a potentially important and previously unrecognized pathway for DMS photodegradation in marine waters. Introduction DMS biogeochemistry has received considerable attention because of the potential of DMS in the atmosphere to partially counteract the warming effect of greenhouse gases (1). Biogenic DMS from the ocean is the most important source of reduced sulfur compounds to the atmosphere in remote ocean locations (2). Atmospheric oxidation of DMS is a major source of sulfate aerosols and cloud condensation nuclei (CCN) that are thought to be involved in Earth’s climate regulation by influencing scattering, absorption, and reflec- tion of solar radiation (1). To assess the DMS flux to the atmosphere, it is crucial to understand the processes controlling DMS distribution in the surface ocean. However, DMS production and removal processes are only partially characterized. Dimethylsulfoniopropionate (DMSP), which is produced by some marine phytoplankton species, is the major source of oceanic DMS (3). Removal of DMS from surface waters involves mainly three processes: sea-air ventilation, bacterial transformation, and photodegradation (2-5). Until recently, DMS photochemical degradation has received little quantitative attention even though it may account for between 7% and 40% of the total loss of DMS in the surface ocean (5-8). Photodegradation of DMS in aqueous solutions proceeds via an indirect pathway that is initiated by the photochemical production of oxidants (Ox) (5, 9, 10). Previous studies have shown that the photoexcitation of chromophoric dissolved organic matter (CDOM) in natural waters is a major source of oxidants, such as singlet oxygen ( 1 O2), hydroxyl radical (OH • ), hydrogen peroxide (H2O2), and photoactivated CDOM, and these have been suggested as potential reactants in the photosensitized removal of DMS (11-13). On the basis of field investigations, an alternative source of photooxidants for DMS has recently been proposed (14, 15). A strong positive correlation between the quantum efficiency of DMS pho- tochemical removal and NO3 - concentrations in the northeast Pacific Ocean waters suggests that NO3 - photolysis also induces DMS degradation in natural waters (14). In addition, Toole et al. (15) observed that the photodegradation rates of DMS increased linearly with addition of NO3 - to seawater samples collected from the Southern Ocean. The purpose of this study was to investigate the kinetics and the possible mechanisms of DMS oxidation initiated by NO3 - photolysis in aqueous solution, a process that we previously suggested can efficiently remove DMS from seawater. In previous studies, free radical reactivity toward organic sulfur compounds has been mainly determined via flash photolysis and pulse radiolysis experiments (16-18). In this study, however, free radicals were generated using irradiation of an aqueous NO3 - solution with simulated solar radiation, an approach that closely replicates photochemical processes in the natural environment. Background Photolysis of NO3 - in seawater generates a series of free radicals (19). In UV-irradiated aqueous solution NO3 - is photodecomposed into a variety of intermediates which include NO2 - , NO2 • , O( 3 P), and O - (20). Warneck and Wurzinger (21) reported that pathway 2 is about 10 times more efficient than pathway 1 during irradiation at 305 nm. O - radicals are readily protonated to form OH • . In aqueous solution, OH • is a highly reactive short-lived oxidant (E )+1.83 V) that reacts with most inorganic and organic compounds at rates close to diffusion-controlled (i.e., with rate constants for a bimolecular reaction on the order of 10 9 to 10 10 L mol -1 s -1 ;(22)). O( 3 P) is believed to react with O2 producing O3 or to react with NO3 - forming NO2 - and O2 (23). In seawater, Zafiriou et al. (24) determined that OH • reacts almost exclusively with bromide ion (97%) and, to a lesser extent, with the carbonate system (1.8%) and dissolved organic matter (DOM) (0.2%). These secondary photochemi- cal reactions produce some longer-lived but less reactive free radicals such as Br2 -• , and CO3 -• . Br2 -• is produced via * Corresponding author phone: (910)962-3458; fax: (910)962-3013; e-mail: bouillonr@uncw.edu. † Current address: Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, NC, 28403. ‡ Current address: Department of Marine Science, Room 211, Marine Sciences Bldg., University of Georgia, Athens, GA, 30602. NO 3 - + hν f NO 2 - + O( 3 P) (1) NO 3 - + hν f NO 2 • + O - (2) Environ. Sci. Technol. 2005, 39, 9471-9477 10.1021/es048022z CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 9471 Published on Web 11/08/2005