PHOTOCHEMICAL AND NONPHOTOCHEMICAL FORMATION AND DESTRUCTION OF CARBONYL SULFIDE AND METHYL MERCAPTAN IN OCEAN WATERS

We measured the concentration of dissolved carbonyl sulfide (COS) in t he water column of the Northeast Atlantic (49 degrees N, 12 degrees W) , on a cruise of the RV ''Meteor'' in January 1994 using a purge-and-t rap GC/FPD method (Gas Chromatograph/Flame Photometric Detection). Our results show that parts of the ocean can be undersaturated with respe ct to the overlying atmosphere and that COS is not always vertically d istributed homogeneously within the mixed layer. Generally, the COS co ncentration was correlated with the under-water light intensity and de creased with depth. Deep water contained measurable amounts of COS, su ggesting the existence of a non-photochemical production mechanism in addition to the photochemical mechanism operating in surface waters. D uring daytime, pronounced vertical COS concentration gradients develop ed. These gradients leveled off during the night probably because of h omogenization due to mixing, hydrolytic decomposition, and the absence of photochemical production. Increased mixed layer depths tended to d ecrease the COS concentration due to downward mixing out of the zone o f photochemical production (similar to 25 m). Incubation experiments s howed that COS was formed both photochemically and non-photochemically while methyl mercaptan (MeSH) was produced non-photochemically and de stroyed photochemically. We calculated non-photochemical and photochem ical production and destruction rate constants for MeSH and COS. The c orrelation 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 i ncubation experiments. Laboratory incubation experiments also showed p hotochemical formation of COS from MeSH.