Ocean emissions

Legrand, M., Feniet-Saigne, C., Saltzman, E. S. et al. (1991). Ice core record of oceanic emissions of dimethyl sulphide during the last climate cycle. Nature 350,144-146. 

Current research on the atmospheric cycling of sulfur compounds involves the experimental determination of reaction rates and pathways (see Plane review, this volume) and the field measurement of ambient concentrations of oceanic emissions and their oxidation products. Photochemical models of tropospheric chemistry can predict the lifetime of DMS and H2S in marine air however there is considerable uncertainty in both the concentrations and perhaps in the identity of the oxidants involved. The ability of such models to simulate observed variations in ambient concentrations of sulfur gases is thus a valuable test of our assumptions regarding the rates and mechanisms of sulfur cycling through the marine atmosphere. 

Methanesulfonic acid, although it comprises a relatively small fraction of total non sea-salt aerosol sulfur, has been shown (2) to be a ubiquitous component of marine aerosols. Its occurrence and distribution have been suggested as of use as an in situ tracer (3.4) for oceanic emissions and subsequent reaction and deposition pathways of organosulfur compounds and dimethyl sulfide in particular. 

Cutter and Church (13) have determined selenium species in western Atlantic precipitation so that the emission sources using this sulfur analogue, which is enriched in fossil fuels (primarily coal), can be more exactly identified. The results show a correlation of both total Se and the Se IV Se VI ratio with increasing protons and excess sulfur in precipitation from Lewes, Delaware, and on Bermuda. Their hypothesis is that, although some reduced forms (1 nM/kg) may come from background oceanic emissions, most oxidized Se is a reflection of fossil-fuel emissions from North America. 

Fig. 1.2), most of which escapes to the atmosphere (see Fig. 1.1 as well as Chapter 2 by Bange, this volume). Nitrous oxide acts as a greenhouse gas that is more than 200 times more potent than CO2 (Ramaswamy et ah, 2001). Therefore variations of this gas in the atmosphere can lead to changes in Earth s temperature and cHmate. Since the oceanic emission of N2O constitutes a substantial fraction to the total emission of N2O into the atmosphere, N2O provides for a direct potential link between the ocean nitrogen cycle and Earth s climate. 

The role of oceanic emissions in the global budget of atmospheric NHj 6. Outlook... 

Another interesting example of the biological influence on atmospheric chemistry is provided by sulfur. Under natural conditions, sulfur compounds in the atmosphere are provided by the oceanic emission of dimethyl disulfide (DMS). This biogenic emission results from the breakdown of sulfoniopropionate (DMSP), which is thought to be used by marine phytoplankton to control their osmotic pre.ssure. The oxidation of DMS leads to the formation of sulfur dioxide, which is further converted to sulfate particles. As indicated above, these particles, by scattering back to space some of the incoming solar radiation, tend to cool the earth s surface. Their presence also affects the optical properties of the clouds, which introduces an indirect climatic effect. 

It is chemically quite inert, having a higher ionization potential than other electropositive elements. Natural sources include ocean emission, degassing of the earth s crust, weathering, emission from volcanoes, geothermal zones, and Hg mineralized areas. 

Recent estimates indicate that of the approximately 200000 tons of mercury emitted to the atmosphere since 1890, about 95% resides in terrestrial soils, about 3% in the ocean surface waters, and 2% in the atmosphere (Expert Panel on Mercury Atmospheric Processes 1994). Some 20-30% of the current oceanic emissions are from mercury originally mobilized by natural sources (Eitzgerald and Mason 1996). Similarly, a potentially large fraction of terrestrial and vegetative emissions consists of recycled mercury from previously deposited anthropogenic and natural emissions (Expert Panel on Mercury Atmospheric Processes... 

Trace gas Atmospheric concentration (ppbv) Atmospheric iifetime (years) Major impact in atmosphere Increase in atmosphere (1980-90) Oceanic emission as % of total global emissions... 

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. 

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