In marine areas, wave action generates airborne droplets of seawater from which the water can evaporate, leaving a suspended particle of the dissolved solids. Because this is mainly NaCl, the possibility exists for the generation of atomic chlorine via the reactions of NaCl with gaseous species such as N205 or C10N02 (see reviews by Finlayson-Pitts, 1993; Graedel and Keene, 1995; Andreae and Crutzen, 1997; Finlayson-Pitts and Pitts, 1997; De Haan et al., 1999; and Hemminger, 1999), e.g.,

(Chlorine nitrate is formed from the reaction of CIO with NOz.) These reactions also occur when NaCl is in the aqueous phase, in competition with the hydrolysis of N205 and C10N02, i.e., above the deliquescence point of NaCl in sea salt (e.g., Behnke et al., 1997). Photolysis then generates chlorine atoms, e.g.,

It is likely that there are as yet ill-defined aqueous-phase reactions in the airborne seawater droplets that release photochemically labile chlorine gases. For example, Oum et al. (1998a) have shown that Cl2 is formed when sea salt aerosols above their deliques cence point are irradiated at 254 nm in the presence of 03, generating OH which initiates CI oxidation.

Such reactions may also be important in other situations in the troposphere. For example, Shaw (1991) has observed salt particles as far as 900 km inland in Alaska, and chloride salts are used on many roads in cold climates in the wintertime. In addition, in the plumes from oil well burning in Kuwait, salt particles were observed, due to the brine that was mixed with the oil in the wells (e.g., see Cahill et al., 1992).

Direct evidence for the potential importance of CI as an organic oxidant comes from recent measurements of inorganic chlorine-containing species other than HC1 in the marine troposphere in midlatitudes (Keene et al., 1993; Pszenny et al., 1993). In particular, Cl2 has been identified using atmospheric pressure ionization mass spectrometry (API-MS) in a coastal region (Spicer et al., 1998). Interestingly, the concentrations, up to 150 ppt, are much higher than can be explained by any known chemistry, again highlighting the contribution of some as yet unidentified chemistry in the marine boundary layer. During the day, any Cl2 formed will absorb strongly in the 300- to 400-nm region (Chapter 4), and dissociate, generating atomic chlorine.

Indirect evidence for the involvement of atomic chlorine in the chemistry of marine atmospheres comes from the measurement of simple organics, where their relative rates of decay frequently cannot be matched assuming attack only by OH (Wingenter et al., 1996). Estimates of the peak concentrations of atomic chlorine range from ~103 to 106 radicals cm"3 in the marine boundary layer (e.g., Pszenny et al., 1993; Singh et al., 1996a). However, on a global scale, the concentrations are likely much smaller. For example, Rudolph et al. (1996) and Singh et al. (1996b) have examined the budget for tetrachloroethene (TCE), which reacts relatively rapidly with CI compared to OH. The measured atmospheric concentrations of TCE are consistent with the known emissions and removal solely by reaction with OH, from which an upper limit for the global annual average CI atom concentrations was estimated to be < 103 atoms cm-3. However, most of it is in the marine boundary layer (MBL) so that these averaged values may not be inconsistent with peak MBL concentrations of 104-106 atoms cm-3.

In short, while there is evidence that atomic chlorine is generated from sea salt reactions and contributes to organic oxidations in the marine boundary layer, the nature and strength of the sources remain to be elucidated.

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