The ocean as source

Recent reassessment has shown (Rhee et al. 2009) global oceanic emission estimates of CH4 to be 10 times lower and of N2O to be 3 times lower than the established emission values used in the recent IPCC report (Denman et al. 2007). The reasons for the discrepancies are unknown, but three studies (Conrad and Seiler 1988, Bates et al. 1996, Rhee et al. 2009) indicate that the open ocean is supersaturated with respect to atmospheric CH4 at a level of 0.04, about an order of magnitude lower than the CH4 value of 0.3 suggested by Ehhalt (1974) and used by others (e.g. Watts 2000 and the IPCC reports 1995, 2001, 2007). The much lower N2O estimate implies (Rhee et al. 2009) that upwelling activities and/or the amount of dissolved N2O in upwelling subsurface waters of the Atlantic are weaker than in the other oceans. 

Emissions of organohalogen (Br or I) gases (one to two carbon atoms) from the oceans can have significant effects on the oxidant chemistry of the atmosphere, and in some cases in particle formation. There is a growing database of shipboard measurements from which sea-to-air fluxes can be calculated (e. g. Chuck et al. 2005). However, in contrast to DMS, there is little understanding of how these compounds are formed, with both direct biological production and indirect photo- 

The most significant emissions (in Tg element yr ) from the oceans are summarized here (cf also Table 2.33 with older data)  

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Georgii HW, Gravenhorst G. 1977. The ocean as source or sink of reactive trace-gases. Pure Appl Geophys 115 503-511. 

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