H O

The reaction (10) of the adduct with S02 generates S03, which then reacts with water vapor to form H2S04. However, the adduct itself also reacts directly with water vapor to form H2S04 and HCHO (Martinez and Herron, 1981, 1983). Akimoto and co-workers have evaluated the relative rate constants for reaction of the HCHOO- -S02 adduct with H20 compared to S02 to be (0.6-2) X 10"4 (Hatakeyama and Akimoto, 1994). However, at 298 K and 50% relative humidity, the water vapor concentration corresponds to about 4 X 1017 molecules cm"3 compared to ~3 X 1012 molecules cm"3 for S02 in a polluted atmosphere. As a result, reaction with water vapor is favored by more than an order of magnitude.

One can estimate the relative contribution of the Criegee intermediate (CI) to S02 oxidation in the gas phase in the troposphere. The absolute value of the rate constant for the reaction of the CI with S02 is not known, with estimates ranging from 1.7 X 10"" to 3 X 10"15 cm3 molecule"1 s"1 (Hatakeyama and Akimoto, 1994). Using the highest value and a concentration of the CI of f X 105 molecules cm"3, one obtains ~10-6 s"1 for the first-order rate of removal of S02 by this reaction. This can be compared to the rate of removal of S02 by reaction with f X 106 OH radicals cm-, which is also

using the effective bimolecular rate constant cited earlier. Using the lower estimates for the CI-S02 rate constant, which is more reasonable, would lower its contribution proportionately.

In short, the Criegee intermediate from alkene-ozone reactions can contribute, in principle, to the gas-phase oxidation of S02. In practice, it is likely less important than reaction with OH. In addition, as we shall see, even the 0H-S02 gas-phase reaction is, under many conditions, swamped out by reactions occurring in the liquid phase found in clouds and fogs. As a result, the CI-S02 reaction may contribute in some circumstances but is unlikely to be a major contributor to S02 oxidation as a whole.

FIGURE 8.6 Loss of S02 and increase in HCHO as a function of S02 in the C2H4-03-S02 reactions. Triangle: AS02/AC2H4; solid circle: AAHCHO/AC2H4 (adapted from Hatakeyama and Akimoto, 1994).

FIGURE 8.6 Loss of S02 and increase in HCHO as a function of S02 in the C2H4-03-S02 reactions. Triangle: AS02/AC2H4; solid circle: AAHCHO/AC2H4 (adapted from Hatakeyama and Akimoto, 1994).

c. Other Gas-Phase Reactants

While one might expect that other tropospheric free radicals such as 0( P), H02, and R02 could react with S02 as well, such reactions are not significant.

Because the oxidants for S02 are generated in the VOC-NOx system discussed in Chapter 6, the overall gas-phase mechanism for the oxidation of S02 to H2S04 is quite complex. The reader is referred to the mechanism in the "RADM" model (.Regional Acid Deposition Model) for a treatment of the V0C-N0x-S02 chemistry (Stockwell et al, 1990; Gao et al., 1996). It should be noted that an earlier version of this mechanism is given in some of the examples included with the OZIPR model discussed in Chapter 16 and whose applications are included with this book.

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