the actual mechanism of reaction (6) was not well understood until recently. A combination of theoretical (Morokuma and Muguruma, 1994; Hoffmann and Schleyer, 1994) and experimental work (e.g., Reiner and Arnold, 1993; Kolb et al., f 994; Phillips et al., 1995; Lovejoy et al., 1996; Jayne et al., 1991) has established that S03 forms a complex with H20, but this does not appear to form H2S04 directly. It appears likely that the H20-S03 complex reacts with a second water molecule, leading to H2S04 formation via a cyclic six-membered transition state such as that shown in Fig. 8.4. An alternate mechanism that cannot be ruled out is the reaction of S03 with a water dimer. This leads to the reaction being second order in H20 and first order in S03 (Lovejoy et al., 1996; Jayne et al., 1997).
While reaction of S03 with water vapor to form sulfuric acid is expected to be by far its major fate, there is also the possibility that some minor reaction paths could play a role under some circumstances. For example, S03 forms a complex with NH3 and ulti-
Possible transition state:
mately sulfamic acid (H2NS03H), which rapidly forms dimers (Shen et al., 1990; Lovejoy and Hanson, 1996). Jayne et al. (1997) measured a reaction probability of S03 of ~1 on surfaces that had adsorbed water, suggesting that such heterogeneous losses may become important at higher altitudes. In addition, as discussed in Chapter 4.K.2, the photolysis of S03 may compete with the reaction with water vapor in the upper stratosphere.
The production of H02 in reaction (5) is important to the overall chemistry of S02 oxidation in laboratory systems as well as in air. Thus in the presence of NO, the H02 reacts to regenerate OH:
As a result, the initial S02-0H reaction does not lead to the net loss of OH and a chain oxidation of S02 can result. Perhaps more important, the generation of H02 leads to increased H202 production. As discussed in Section C.3.e, this highly soluble gas is a major oxidant for S(tV) in the aqueous phase so that reaction (5) can affect not only gas-phase processes but also the oxidation in clouds and fogs (e.g., Stockwell, 1994).
b. Criegee Biradical
While OH is the major gas-phase oxidant for S02, Criegee biradicals may also contribute. This is so particularly at night when OH concentrations are small but significant concentrations of 03 and alkenes may exist, generating the Criegee intermediate (see Chapter 6.E.2).
The first indication of a reaction between the Criegee intermediate and S02 came from studies by Cox and Penkett (1971, 1972), who showed that although the oxidation of S02 by 03 alone was negligible, it was relatively fast in the presence of both ozone and alkenes. In addition, water vapor inhibited the S02 oxidation in this system. These observations can be understood in terms of the competition between the reactions of the Criegee intermediate with S02 and
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