Photochemical and non-photochemical formation and destruction of carbonyl sulfide and methyl mercaptan in ocean waters

Abstract We measured the concentration of dissolved carbonyl sulfide (COS) in the water column of the Northeast Atlantic (49 °N, 12 °W), on a cruise of the RV “Meteor” in January 1994 using a purge-and-trap GC/FPD method ( G as C hromatograph/ F lame P hotometric D etection). Our results show that parts of the ocean can be undersaturated with respect to the overlying atmosphere and that COS is not always vertically distributed homogeneously within the mixed layer. Generally, the COS concentration was correlated with the under-water light intensity and decreased with depth. Deep water contained measurable amounts of COS, suggesting the existence of a non-photochemical production mechanism in addition to the photochemical mechanism operating in surface waters. During daytime, pronounced vertical COS concentration gradients developed. These gradients leveled off during the night probably because of homogenization due to mixing, hydrolytic decomposition, and the absence of photochemical production. Increased mixed layer depths tended to decrease the COS concentration due to downward mixing out of the zone of photochemical production (~ 25 m). Incubation experiments showed that COS was formed both photochemically and non-photochemically while methyl mercaptan (MeSH) was produced non-photochemically and destroyed photochemically. We calculated non-photochemical and photochemical production and destruction rate constants for MeSH and COS. The correlation of the photo-production of COS and the photo-destruction of MeSH suggests that there is a link between both reactions. Models are presented which describe the COS and MeSH concentrations during the incubation experiments. Laboratory incubation experiments also showed photochemical formation of COS from MeSH.