Halogen Radical Oxidants

There has been longstanding interest in the possible role of halogen radicals as tropospheric oxidants (Singh and Kasting, 1988; Chatfield and Crutzen, 1990). The best evidence so far comes from measurements of alkanes and acetylene in Arctic surface air (Jobson et al., 1994), which indicate a sink in April (polar sunrise) consistent with oxidation by CI atoms present at a concentration of ~ l x 104 atoms/cm3. The data also suggest the presence of Br atoms to oxidize acetylene. The source of the halogen oxidants is not well established but likely involves chemical production from sea salt accumulated on the ice over the polar night (Impey et al, 1999).

Generation of halogen oxidants from sea salt would be of little interest for global tropospheric chemistry if it were confined to Arctic sunrise. However, measurements of hydrocarbons and nonradical CI species in the marine boundary layer (MBL) at midlatitudes and in the tropics suggest that CI atoms may be present at least occasionally at concentrations in the range 104 to 105 atoms/cm3 (Keene et al, 1990, 1996; Pszenny et al, 1993; Singh et al, 1996; Spicer et al, 1998). At such concentrations, oxidation by CI atoms would provide a major sink for dimethylsulfide and alkanes in the MBL. Even less is known about Br radical chemistry in the MBL, although Toumi (1994) has suggested that BrO could provide an important oxidant for dimethylsulfide. Field measurements of the halogen radicals and their reservoirs HOC1 and HOBr are needed.

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