Sulfur compounds, reduced atmospheric reactions

Barnes, I., V. Bastian, K. H. Becker, E. H. Fink (1984). Reactions of OH radicals with reduced sulfur compounds under atmospheric conditions. In Physico-chemical Behaviour of Atmospheric Pollutants (B. Versino and G. Angeletti, eds.), Proc. Eur. Symp. 3rd, Varese, Italy, pp. 149-157. Dordrecht, Holland. 

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. 

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. 

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. 

On a global scale, natural emissions of reduced sulfur compounds account for about 50% of the total sulfur flux into the atmosphere (1-3). Hence, it is important to understand the natural sulfur cycle in order to establish a "base line" for assessing the significance of anthropogenic perturbations (primarily SO2 emissions). Dimethylsul-fide (DMS) is the predominant reduced sulfur compound entering the atmosphere from the oceans (4-9), and DMS oxidation represents a major global source of S(VI). The atmospheric oxidation of DMS can be initiated by reaction with either OH or NO3. In marine environments, however, NO3 levels are typically very low and DMS is destroyed primarily by OH ... 

Dimethyl sulfide is emitted mainly from the ocean where it is released from phytoplankton. Estimates of emission rates range from 30 to 68 Tg yr . Soils and vegetation contribute comparatively little to the global emission rate. The rate of DMS emissions evidently exceeds that of all other reduced sulfur compounds. Although this makes DMS the most important reduced sulfur compound globally, its impact is essentially confined to the marine atmosphere. The removal of DMS occurs primarily by reaction with OH radicals. 

As discussed in Section 2.2, one of the major removal processes for most reduced sulfur compounds is gas phase oxidation initiated by reaction with OH or NO3 radicals. Carbon disulfide is unreactive towards NO3 [7,8] reaction with OH is the only significant chemical sink for atmospheric CS2. Early direct studies exploring the kinetics of the CS2 + OH reaction indicated CS2 was unreactive towards OH [9-12]. These experiments were conducted on millisecond timescales in the absence of O2. However, when O2 was added to the reaction mixture, a rapid reaction was observed and bimolecular rate coefficients on the order of 2 x 10 cm molec" s" were measured [13-16]. Jones etal. [14], were the first to propose a reaction mechanism involving formation of a CS2-OH adduct which could react with O2 in competition with adduct decomposition to reform reactants, reactions (1,-1) and (2) ... 

Summary The flux of dimethyl sulfide through the atmosphere exceeds that of all other reduced sulfur compounds combined. Dimethyl sulfide is emitted from the ocean surfaces and is rapidly oxidized in the marine boundary layer via reactions with OH and NO3. These initial reactions in the oxidation of DMS have received considerable attention from the scientific community and are reasonably well understood. The reactions with OH and NO3 result in the production of the CH3SCH2OO radical. The reaction between DMS and OH also produces a radical adduct, CH3S(OH)CH3, which reacts with O2 to produce dimethyl sulfoxide (DMSO) with a yield of about 50%. The remaining pathway(s) for the adduct -I- O2 reaction are unidentified. 

Purification processing. The raw materials, in batches of proper composition, are blended and charged into the reaction vessel. The material is melted and heated to 700°C under an atmosphere of anhydrous HF to remove H2O with a minimum of hydrolysis. The HF is replaced with H2 for a period of 1 hr, during which the temperature is raised to 800°C, to reduce and U to (in the case of simulated fuel mixtures), and sulfur compounds to S, and extraneous oxidants (Fe +, for example) to... 

We cover each of these types of examples in separate chapters of this book, but there is a clear connection as well. In all of these examples, the main factor that maintains thermodynamic disequilibrium is the living biosphere. Without the biosphere, some abiotic photochemical reactions would proceed, as would reactions associated with volcanism. But without the continuous production of oxygen in photosynthesis, various oxidation processes (e.g., with reduced organic matter at the Earth s surface, reduced sulfur or iron compounds in rocks and sediments) would consume free O2 and move the atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of the planet is thus intimately tied to the biosphere. 

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