Hydrogen oceanic emission

Emission of hydrogen chloride is the third most important contribution to the global acidification from human activities. The two first are SO2 and NO, The HCI is a local pollutant, contrary to the other two. It is soluble in water and easily dissolved in rain droplets and, therefore, usually falls down near the emission source. The hydrogen chlorine emissions from combustion and gasification processes has been calculated to 3,5 Mt./year, The major part of the estimated global contribution of HCI to the atmosphere is evaporation from the oceans. Even with a redeponation of 90-% HCI to the oceans, the estimated emission will reach approximately 120 Mt./year, The majority of the emitted chlorine from a combustion process will leave as HCI in the gas phase which may cause problems like corrosion and formation of dioxins. 

Alternatives to fossil fuels, such as hydrogen, are explored in Box 6.2 and Section 14.3. Coal, which is mostly carbon, can be converted into fuels with a lower proportion of carbon. Its conversion into methane, CH4, for instance, would reduce C02 emissions per unit of energy. We can also work with nature by accelerating the uptake of carbon by the natural processes of the carbon cycle. For example, one proposed solution is to pump C02 exhaust deep into the ocean, where it would dissolve to form carbonic acid and bicarbonate ions. Carbon dioxide can also be removed from power plant exhaust gases by passing the exhaust through an aqueous slurry of calcium silicate to produce harmless solid products ... 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

Trace metals are introduced to the ocean by atmospheric feUout, river runoff, and hydrothermal activity. The latter two are sources of soluble metals, which are primarily reduced species. Upon introduction into seawater, these metals react with O2 and are converted to insoluble oxides. Some of these precipitates settle to the seafloor to become part of the sediments others adsorb onto surfaces of sinking and sedimentary particles to form crusts, nodules, and thin coatings. Since reaction rates are slow, the metals can be transported considerable distances before becoming part of the sediments. In the case of the metals carried into the ocean by river runoff, a significant fraction is deposited on the outer continental shelf and slope. Hydrothermal emissions constitute most of the somce of the metals in the hydrogenous precipitates that form in the open ocean. 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

Conventional processes for hydrogen production are among major producers of C02 emissions. It has been proposed recently that C02 produced in steam reforming or partial oxidation processes could be captured and sequestrated in the ocean or underground. In our work we estimated that the total energy consumption for C02 sequestration (C02 capture, pressurization, transportation and injection), will most likely exceed 5,000 kJ per kg of sequestrated C02. Since about 80% of world energy production is based on fossil fuels, this could potentially result in the production of 0.20-0.25 kg of C02 per kg of sequestrated C02. 

Figure 1 A generalized geochemieal cycle for sulfur of the early 1970s. Note the large emissions of hydrogen sulfide from the land and oceans and that volcanic sulfur emissions are neglected (units Tg (s) a ...

Although little water manages to pass through the cold trap, there are other sources of hydrogen in the upper air. As mentioned above, methane emissions from the surface mix upwards from the surface, as methane has no cold trap, and in the upper air photolysis leads eventually to release of H. In addition, there is a small emission of H2 from the surface, some of which will reach the upper part of the atmosphere however. Earth also sweeps up H and H2 from space. Over time, net hydrogen loss must have been limited we have kept the oceans. 

The anthropogenic H2 is emitted into the air in automotive exhaust gases, which contain H2 in the range of 1-5 % by volume. The nature of the oceanic source is not entirely clear but it is probably due to microbiological activity. However, the supersaturation of ocean waters unambiguously indicates hydrogen gas formation there. The emission from soils is caused by the fermentation of bacteria. 

Schmidt (1974) in the northern and southern Atlantic varied from 0.8 to 5.4 with an average of 2.5. From these data, extrapolated to other oceans, Schmidt (1974) and Seiler and Schmidt (1974b) estimated a global emission rate of 4Tg/yr. In contrast to CO, the aqueous concentrations of H2 do not undergo diurnal variations. This does not preclude a photochemical production of H2, as was shown for carbon monoxide, but other production mechanisms for hydrogen, such as by bacteria or in the digestive tracts of zooplankton, may be more effective. 

Streets, D. G. and S. T. Waldhoff (2000) Present and future emissions of air pollutants in China SO2, NO, and CO. Atmospheric Environment 34, 363-374 Stribling, R. and S. L. Miller (1987) Energy yields for hydrogen cyanide and formaldehyde synthesis the HCN and amino acid concentrations in the primitive ocean. Origins of Life... 

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