Oxidation by Chlorine Atoms

Atomic chlorine reacts rapidly with DMS, with an overall rate constant of (3.3 + 0.5) X 10 1(1 cm3 molecule 1 s 1 at 298 K and 700 Torr total pressure (Stickel et al., 1992). As is the case for the OH reaction, the chlorine atom reaction proceeds by two reaction channels, one an abstraction and the other addition to the sulfur atom  

A potential reaction path producing CH3S + CH3C1 can be ruled out based on the observation of very small yields of CH3C1, only 0.13% at 1 atm pressure (Langer et al., 1996), and Zhao et al. (1996) show a path producing CH3 is also not important. Stickel et al. (1992) suggest that at 298 K and 1 atm pressure, the 

The iodine atoms react with 03, forming the IO radical, which can potentially oxidize DMS to DMSO, regenerating iodine atoms in a chain process  

However, with the recent recognition of the potential importance of atomic chlorine and bromine under certain conditions in the Arctic at polar sunrise (e.g., see Barrie et al., 1988 and Niki and Becker, 1993), the potential for BrO and CIO chemistry has been reconsidered. As described in Chapter 6 J.4, at polar sunrise there is a rapid loss of ground-level 03 that appears to be associated with reaction with atomic bromine and at the same time, there is evidence that chlorine atoms are playing a major role in the organic removal (Jobson et al., 1994). This is consistent with reactions of sea salt particles generating atomic bromine and chlorine, although the exact nature of the reactions and halogen atom precursors remains unknown. 

FIGURE 8.25 Overview of oxidation of DMS by OH in the troposphere (note that many of the reactions after the first step are the same in DMS reactions with S03, Cl, etc.). 

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