Volatile oceanic emission

The oceanic burden in December 2004 shows the contamination of the ocean after 50 years of PFOA emissions (Figure 3.14). Highest PFOA burden is located in the northern Atlantic, Mediterranean, and the Arctic ocean. Contaminations of the Atlantic, Mediterranean and Pacific can be related to the vicinity to the oceanic source. PFOA in remote regions, however, such as in the Arctic must have been transported via atmosphere or ocean. MPI-MTCM does not simulate degradation of PFOA from volatile, highly mobile precursor substances, that contribute to the ocean burden in the Arctic by deposition. Then annual dry and wet deposition rates of PFOA in the model are small compared to the mass emitted directly to the ocean. This implies that the burden in the Arctic is results mainly from oceanic long-range transport. 

Model results in seawater are in good agreement with observational data of PFOA. Most differences can be attpageributed to deficiencies of the emission scenario. Despite this fact, the difference between model results and observational data are due to the limited horizontal and process resolution and the fact that the physical parameters of the model (temperature, surface pressure, vorticity or divergence of the wind velocity field) were not relaxed to observational data. Regarding these limitations, in particular individual vertical profiles compare quite well with observations. This study underlines the importance of the ocean as a transport medium of PFOA. The contribution of volatile precursor substances to long-range transport needs to be assessed. 

Little snlfnr is re-emitted from wetlands into the atmosphere. Table 8.7 gives estimates of global emissions of volatile sulfur compounds from different sources. Total emissions are in the range 98 to 120 Tg (S) year 75 % is anthropogenic, mainly from fossil fnel combustion in the northern hemisphere. The main natural sources are the oceans and volcanoes. Wetlands and soils contribnte less than 3 % of the total emission. 

Of all the metals in the periodic table, mercury, Hg (atomic number 80), is the only one to exist as a liquid at ambient temperatures. Mercury is also volatile, which means that uncontained mercury atoms evaporate into the atmosphere. Today, the atmosphere carries a load of about 5000 tons of mercury. Of this amount, about 2900 tons are from current human activities, such as the burning of coal, and 2100 tons appear to be from natural sources, such as outgassing from Earth s crust and oceans. Since the mid-igth century, however, humans have emitted an estimated 200,000 tons of mercury into the atmosphere, most of which has since subsided onto the land and sea. It is probable, therefore, that a large portion of the mercury emitted from "natural" sources is actually the re-emission of mercury originally put there by humans over the last 150 years. 

Over the past few years we have been studying the waters around the United Kingdom, including the North Sea, Irish Sea and N.E. Atlantic, in order to characterise dimethyl sulphide (DMS) emissions and assess the significance of this natural contribution to acidity of rainfall and the sulphur cycle. Biogenic DMS concentrations in seawater vary considerably both temporally and spatially and coastal and shelf water systems often contain higher concentrations of volatile sulphur than the open oceans (1.2). 

Typical concentrations of selenium in seawater are —0.1-0.2 p,g (Table 9) with an estimated mean residence time of 70 yr in the mixed layer and 1,100 yr in the deep ocean. The oceans are, therefore, an important sink for selenium (Haygarth, 1994 Jacobs, 1989). Biogenic volatilization of selenium from seawater to the atmosphere is estimated to be 5,000-8,0001 yr. Amouroux et al. (2001) have demonstrated that biotransformation of dissolved selenium in seawater by blooms of phytoplankton in the spring is a major pathway for the emission of gaseous selenium to the atmosphere. Hence, oceans are an important part of the selenium cycle. 

There are three prominent processes that release mercury of mixed natural and anthropogenic origin to the atmosphere. These three include biomass burning (deliberate and natural) and the evasion of mercury from soils and the ocean. The general factors controlling emission of mercury from soils have been discussed in the section on low-temperature volatilization. The mercury... 

The Global Mercury Cycle A recent review (Mason et al., 1994) on the global Hg cycling is diagrammed in Figure 10.24. The evasion of Hg from the ocean is balanced by the total oceanic deposition of Hg(II) from the atmosphere. The mechanisms, whereby reactive Hg species are reduced to volatile Hg in the oceans, are poorly known, but reduction appears to be biologically mediated. Deposition on land is the dominant sink for atmospheric Hg. Mason et al. (1994) estimate that over the last century anthropogenic emissions have tripled the concentration of Hg in the atmosphere and in the surface ocean. 

Natural and anthropogenic sulfur aerosols play a major role in atmospheric chemistry and potentially in modulating global climate. One theory holds that a negative feedback links the emission of volatile organic sulfur (mostly as DMS) from the ocean with the formation of cloud condensation nuclei, thereby... 

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