Cc

CH, same time as N03. However, there may still be sufficient amounts for this to be a significant fate of the peroxy radical. Figure 6.9 traces the expected fates of this radical through to stable organic products. The dinitroxybutane is formed from the alkoxy radical-N02 reaction and acetone from the alkoxy radical decomposition. The observation of smaller amounts of other aldehydes and formaldehyde also suggests a small contribution from a f,4-H shift and subsequent reactions of the alkyl radical thus formed (Hjorth et al., 1990).

The results of laboratory studies of the mechanism of N03 radical reactions with alkenes tend to be slanted toward observing polyfunctional organics containing more than one nitro group. The reason for this is that the thermal decomposition of N205,

N2O5 <-> NO3 + NO2, is often used as the source of N03. As a result, relatively high concentrations of N02 may also be present, and depending on the concentration regimes employed, these can trap the alkylperoxy and alkoxy radicals more efficiently than would normally be the case in the atmosphere. On the other hand, sources such as the reaction of fluorine atoms with HN03,

FIGURE 6.8 Mechanism of the NO, reaction with 2,3-dimethyl-2-butene (adapted from Skov et al., f994).

are normally used at low pressures (~ Torr) and in the absence of air because such conditions are needed to generate the halogen atoms. Under these conditions, the products may be quite different from those at f atm in air. For example, the yields of epoxides are much greater, typically approaching 100%, under these low-pressure, low -02 conditions (Skov et al., 1994; Berndt and Boge, 1995).

However, it is interesting that vicinal dinitrates have been measured in air with higher concentrations at night, as would be expected if nitrate radical reactions were the source. For example, Schneider et al. (1998) measured the diurnal variation of a series of alkyl nitrates in rural air in Germany and found higher nighttime levels of vicinal dinitrates. They attributed these to the addition of N03 to double bonds, formation of the alkylperoxy radical, and then reaction with NO, with the minor channel in the R02 + NO reaction leading to a second nitrate group on the adjacent carbon.

As seen from the rate constants in Table 6.13, the reactions of biogenic hydrocarbons with N03 are quite fast, so this reaction is expected to be a major fate of these biogenics at night and to also contribute to the removal of NOx. The lifetime of isoprene with respect to reaction with N03 at 50 ppt, for example, is only about 20 min.. Addition of N03 occurs primarily at the 1-position and leads in the absence of NO to the ono2 oo« I I

CHo CHq no, ono2 oono2

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