Co Co

FIGURE 6.6 Calculated rates of HOt radical generation from various sources for a rural forested site in the southeastern United States (adapted from Paulson and Orlando, 1996).

oxides are known to decompose in solution to H202 and the carbonyl compounds. This decomposition, reaction (48c), which can occur during analysis in solution or on the walls of the reaction chamber, may be responsible for the highly varying yields of H202 and hydroxyhydroperoxides reported in various studies (e.g., see Gab et al., 1985, 1995; Hatakeyama and Akimoto, 1994; Horie et al., 1994; Hatakeyama et al., 1993; Hewitt and Kok, 1991; Simonaitis et al., 1991; Wolff et al., 1997; Neeb et al., 1997; and Sauer et al., 1999). At any rate, their formation is now accepted, although the yields as a function of water vapor concentration under typical atmospheric conditions are not well established for a variety of alkenes.

For example, in the isoprene reaction with 03, H202 has been observed as a product, with its yield increasing as the relative humidity increased from 1 to 9% (Sauer et al., 1999). The yields of methacrolein and methyl vinyl ketone also increased simultaneously, as expected if they were also formed in part from the decomposition of an intermediate hydroxyhydroperox-ide.

The reaction with S02 has been thought in the past to proceed via the direct formation of S03:

However, as discussed in detail by Hatakeyama and Akimoto (1994), there is increasing evidence for the formation of an addition complex such as that proposed by Martinez and Herron (1981) and observed earlier by mass spectrometry by Schulten and Schurath (1975):

In the presence of water vapor, the adduct then reacts with H20 to generate sulfuric acid.

Other reactions proposed for the Criegee intermediate include

R,CHOO- + NO -» R,CHO + N02, (51) R.CHOO- + N02 -» R,CHO + N03, (52) R,CHOO- + CO -> R,CHO + C02. (53)

However, the relative importance of these reactions is highly uncertain. Table 6.f2 gives one estimate of the range of possible rate constants for these reactions of the Criegee intermediate and the calculated lifetimes of the intermediate under some typical atmospheric conditions. Reaction with water vapor is expected to be the major fate of the Criegee intermediate, with per-

TABLE 6.12 Range of Reported Rate Constants for the Reactions of the Criegee Intermediate with Some Gases" and Associated Lifetimes of the Criegee Intermediate under Polluted Tropospheric Conditions

TABLE 6.12 Range of Reported Rate Constants for the Reactions of the Criegee Intermediate with Some Gases" and Associated Lifetimes of the Criegee Intermediate under Polluted Tropospheric Conditions

h2o

1 x 10" ,5-2 x 10" 19

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