Reduced Sulfur Compounds Dimethyl Sulfide

Reduced sulfur-containing species RSR react with OH and NO3 radicals. For H2S, the dominant tropospheric removal process involves OH radical reaction  

The DMS-OH reaction proceeds via H-atom abstraction or OH addition to the sulfur atom in the DMS molecule (Yin et al. 1990a,b Barone et al. 1995, 1996 Tumipseed et al. 1996 Ravishankara et al. 1997)  

The abstraction path is favored at higher temperatures, the addition path at lower temperature. At 298 K, about 80% of the reaction proceeds by abstraction at 285 K the two channels are approximately equal. The CH3SCH2- radical behaves as an alkyl radical (Stickel et al. 1993 Wallington et al. 1993)  

CH3SCH2- + 02 + M — CH3SCH202- + M CH3SCH202- + NO — CH3SCH20- + N02 CH3SCH2Cb — CH3S- + HCHO 

The last reaction occurs rapidly, so that, once formed, the CH3SCH20- radical can be assumed to decompose immediately (Tyndall and Ravishankara 1991). In the marine boundary layer, where NO levels are relatively low, CH3SCH202- radicals react with H02 radicals as well as NO. There are two channels for this reaction  

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Other Techniques Continuous methods for monitoring sulfur dioxide include electrochemical cells and infrared techniques. Sulfur trioxide can be measured by FTIR techniques. The main components of the reduced-sulfur compounds emitted, for example, from the pulp and paper industry, are hydrogen sulfide, methyl mercaptane, dimethyl sulfide and dimethyl disulfide. These can be determined separately using FTIR and gas chromatographic techniques. 

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. 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

Dimethyl Sulfide. DMS is one of the major reduced sulfur compounds produced by biological activity, mainly in the oceans (2.3). For this reason its oxidation reactions have been intensively studied, especially the reaction... 

The third, fourth, and fifth sections investigate the distribution and biological and chemical transformations of reduced sulfur compounds in the oceans. The third section focuses primarily on dimethyl sulfide, which is the predominant form of volatile sulfur in the ocean. Research in the past has concentrated on documenting the distribution of DMS in various oceanic environments. The factors controlling this distribution are not well understood. These chapters examine laboratory and field investigations relating DMS production to productivity and spedation. 

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. 

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. 

The reducing properties of organic compounds of sulfur, such as methyl mercaptan, show up in partial reduction of trigeminal to geminal dihalides [243]. Dimethyl sulfide reduces hydroperoxides to alcohols and ozonides to aldehydes while being converted to dimethyl sulfoxide [244]. 

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