Mpan

Like the other higher PANs, MPAN is expected to be formed in relatively small yields and indeed, this com pound has been identified and measured in rural areas where isoprene is present (e.g., see Bertman and Roberts, 1991; Williams et al., 1997; Nouaime et al., 1998; and Roberts et al., 1998). In one study in the rural southeastern United States, the MPAN concentration was found to track that of methacrolein but with a delay of several hours (Fig. 6.17); Nouaime et al. (1998) attributed this positive correlation to a more rapid formation of methacrolein compared to its rate of reaction to form MPAN.

Of all the possible fates for PANs in the atmosphere, thermal decomposition is usually the most important; e.g., for PAN

to a3

00:00

00:00

FIGURE 6.17 Diurnal variation of mean mixing ratios of MPAN and its precursor methacrolein measured near Nashville, Tennessee (adapted from Nouaime et al., f998).

to a3

00:00

00:00

FIGURE 6.17 Diurnal variation of mean mixing ratios of MPAN and its precursor methacrolein measured near Nashville, Tennessee (adapted from Nouaime et al., f998).

Table 6.21 gives the Arrhenius parameters for the decomposition of various PANs and the rate constants and corresponding lifetimes for PAN. The decomposition is strongly temperature dependent, with long lifetimes, of the order of a year or more at low temperatures of ~ 215 K, and very short lifetimes, < 1 h, at the higher temperatures around 298 K.

The effective rate of decomposition depends on the ratio of NO to N02. Thus the acetylperoxy radical formed in the decomposition must be removed by further reaction, e.g., with NO,

before it reacts back with N02 to regenerate PAN, reaction (75), in which case there has been no net reaction (see Problem 8). As a result, the ratio of rate constants k75/kls as well as the ratio of N02 to NO concentrations is important. This rate constant ratio

1997; Seefeld and Kerr, 1997; and Sehested et al., 1998b). The CH3C(0)0 radical rapidly decomposes to CH3 + C02. In regions where NO levels are low, CH3C(0)00 can also be removed by reactions with other RC(0)00, R02, H02, and N03, which then become important in determining the lifetime of PAN (Madronich and Calvert, 1990; Stockwell et al., 1995).

This strong temperature dependence has important implications for the role of PAN, and its higher homologs, in the troposphere. Thus, when it is formed at lower temperatures or is transported into colder regions, it is stabilized and acts as an NOx reservoir. When an air mass containing PAN is transported into warmer regions, however, the PAN decomposes, releasing both the N02 and the acetylperoxy free radical. If sufficient NO is present, the organic free radical reacts in the following manner:

has been measured to be k75/k7 and peroxypropionyl nitrate (e.£

TABLE 6.21 Rate Constants and Temperature Dependence" for the Thermal Decomposition of Some PANs

Compound

Temperature dependence

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