Sulfur compounds, atmosphere reduced

In the reducing atmosphere of the reactor, sulfur compounds form hydrogen sulfide and small amounts of carbonyl sulfide [463-58-1J, COS, in a molar ratio of approximately 24 1. 

In the manufacture of varnish, heat is necessary for formulation and purificahon. The same may be true of operations preparing paints, shellac, inks, and other protective or decorative coahngs. The compounds emitted to the atmosphere are gases, some with extremely low odor thresholds. Acrolein, with an odor threshold of about 4000 /xg/m, and reduced sulfur compounds, with odor thresholds of 2 are bofh possible emissions... 

Sulfur dioxide is an economically important gas that is used as a refrigerant, disinfectant, and reducing atmosphere for preserving food. Although it is also used in the manufacture of many other sulfur compounds, the most important use of S02 is as a precursor in producing sulfuric acid. It can be obtained by burning sulfur, but it is also produced in numerous other reactions. Sulfites react with acids by liberating so2. 

Mercury has several other characteristics that make it of particular environmental concern and make it likely to be found as many different species. It is a natural constituent of soil, although it occurs at low concentrations. It is widely used both in industry and in the laboratory, making it a common contaminant of reference soils. Metallic mercury has a relatively high vapor pressure, which means that it can occur in measurable amounts in the soil atmosphere. It has a high affinity for reduced sulfur compounds in soil solids and soluble organic matter that allows species to be present in the soil solution above mercury s solubility limit. 

If indeed S02 and S03 are effective in reducing the superequilibrium concentration of radicals in flames, sulfur compounds must play a role in NO formation from atmospheric nitrogen in flame systems. Since S02 and S03 form no matter what type of sulfur compound is added to combustion systems, these species should reduce the oxygen atom concentration and hence should inhibit NO formation. Wendt and Ekmann [46] have reported flame data that appear to substantiate this conclusion. 

The catalytic reduction of the radicals, particularly the O atom, by sulfur compounds will generally reduce the rates of reactions converting atmospheric nitrogen to NO by the thermal mechanism. However, experiments do not permit explicit conclusions [21], For example, Wendt and Ekmann [46] showed that high concentrations of S02 and H2S have an inhibiting effect on thermal NO in premixed methene-air flames, while deSoete [47] showed the opposite effect. To resolve this conflict, Wendt el al. [48] studied the influence of fuel-sulfur on fuel-NO in rich flames, whereupon they found both enhancement and inhibition. 

Biogenic processes, however, emit reduced forms of sulfur, including dimethyl sulfide and hydrogen sulfide, with lesser amounts of carbon disulfide (CS2), dimethyl disulfide (CH3SSCH3), carbonyl sulfide (COS), and methyl mercaptan (Cl I3SH). These reduced sulfur compounds are then oxidized in the atmosphere as described in detail in Chapter 8.E. 

The relative contribution that each reduced sulfur compound makes to the total sulfur flux is often of interest because the various compounds behave differently once they enter the atmosphere. Terrestrial biogenic sulfur emissions are dominated by COS (38%), DMS (35%) and H2S (21%). Emissions of CS2 and DMDS together represent about 6% of the total. DMS emissions dominate during the summer season with 41% of the total. 

The production of volatile reduced sulfur compounds in marine ecosystems and the subsequent efflux of these compounds to the marine atmospheric boundary layer is an important source of sulfur to the global atmosphere (1). Independent of its role in the atmospheric sulfur budget, Charlson et al. (2) have suggested that dimethylsulfide (DMS) also plays a major role in cloud formation over oceans. Oxidation products of DMS appear to serve as sites for cloud nucleation. 

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). 

Product analyses for the reaction of CH3S with NO2 were carried out in order to elucidate the mechanism for that reaction in air. S0180 was observed by means of FT-IR spectroscopy when N0180 was used as a reactant. This is a clear evidence for the formation of CH3SO and NO as products of the above reaction. Dependence of the yield of SO2 on the initial concentration of O2 ana NO2 was observed, which indicates that the secondary reactions of CH3SO with 02 or NOz are important in the atmospheric oxidation of reduced organic sulfur compounds. 

Reduced sulfur compounds are ubiquitous in aqueous and atmospheric systems (10,11). Natural sources of reduced sulfur species in aqueous environment result from biological reduction of sulfate, anaerobic microbial processes in sewage systems, putrefaction of sulfur-containing amino-acids (12), oxidative decomposition of pyrite (13), and activities of marine organisms in the upper layers of the ocean (14,15). The build-up of sulfides in areas such as the Black Sea is also giving cause for concern (8). 

Atmospheric Considerations. It is probably premature to assess the role of liquid phase oxidation of reduced sulfur compounds in atmospheric chemistry with the limited kinetic data available in the open literature. However, it is appropriate to discuss certain conclusions obvious from the information presented above. 

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