S 04

FIGURE 4.17 Energetics of NO 3 photodissociation (adapted from Davis et al, 1993).

1997). The absorption drops off strongly in the actinic region, with the result that they are not photodissoci-ated rapidly in the troposphere (Roberts and Fajer, 1989; Turberg et al., 1990; Clemitshaw et al., 1997).

There are several different possible primary processes, depending on the photolysis wavelength. For example, ethyl nitrate in principle has three paths:

C2H50N02 + hv^ C2H50 + N02, (22a) -> CH3CHO + HONO, (22b) -> C2H5ONO + O. (22c)

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants Up) for ethyl and «-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0.

2. Peroxyacetyl Nitrate

Peroxyacetyl nitrate, CH3C(0)00N02, commonly referred to as PAN, is important as a means of transporting NOx over large distances. It is quite stable at low temperatures but decomposes at higher temperatures to release N02. Hence whether photolysis can compete with the thermal decomposition is of great interest. Figure 4.23 shows the absorption spectrum of PAN, CH3C03N02, and Table 4.21 the absorption cross sections (Talukdar et al., 1995).

FIGURE 4.18 Quantum yields for NO, photolysis: dotted line, NO J -> N02 + O; solid line, NO, -» NO + 02; dashed line, fluorescence quantum yields (adapted from Johnston et ai, 1996).

Photolysis by several pathways is possible:

The wavelength thresholds for these at 298 K are f025, 990, and 445 nm, respectively. Mazely et al. (1995, 1997) have measured the quantum yields for production of N02 and N03 at 248 nm and find </>(N02) = 0.83 ± 0.09 and 4>(N03) = 0.3 ± O.f. The quantum yields at longer wavelengths have not been reported.

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