An investigation on the photochemistry of dimethylsulfide in marine waters

Dimethylsulfide (DMS) is a climatically active biogenic trace gas emitted from the oceans. Photochemical reactions in the ocean provide a major sink for DMS. However, details of the oceanic photochemical removal processes of DMS are not yet fully understood. The goal of this thesis was to investigate the kinetic and mechanistic aspects of the photo-oxidation of DMS in marine waters.; A new theoretical approach has been developed for determining the apparent quantum yield for indirect photochemical removal reactions such as DMS photo-oxidation. Applying this theoretical approach, the temporal and spatial variation of the apparent quantum yield spectra for DMS photo-oxidation was determined for samples collected in the northeast Pacific Ocean and the northwest Atlantic Ocean. In July 2002, a positive strong correlation was found between apparent quantum yield values for DMS photo-oxidation and nitrate concentrations during the mesoscale iron-fertilization experiment SERIES in the northeast subarctic Pacific Ocean. In spring 2003, a tenfold variation in apparent quantum yield values was observed in surface waters from the northwest Atlantic Ocean. Results from both of these oceanic field expeditions suggested that the apparent quantum yield for DMS photo-oxidation is controlled by the concentration of nitrate and nitrite as well as the photochemical reactivity of the chromophoric dissolved organic matter.; The role of several inorganic components in seawater on the kinetics of nitrate-photolysis induced DMS removal was examined in aqueous solution. The rates of nitrate-photolysis induced oxidation of DMS were dramatically affected by addition of bromide and bicarbonate/carbonate ions. DMS removal by some selective free radicals derived from bromide and bicarbonate/carbonate ion oxidation represents potentially important and previously unrecognized pathways for DMS photo-oxidation in marine waters.; In situ photochemical removal rates of DMS were calculated during SERIES. It has been observed that the ultraviolet portion of the solar spectrum was most important in inducing DMS photo-oxidation. DMS photochemical removal was an important and, on some occasions, the dominant removal pathway for DMS during SERIES. Results from this thesis suggest that marine phytoplankton could indirectly influence the photo-oxidation of DMS by regulating the concentrations of nitrate/nitrite and DMS.