Nitrate Radical

The nitrate radical, NO-,, is an important intermediate in nightime chemistry. Its spectroscopy, photochemistry, and chemistry are reviewed in detail by Wayne et al. (1991) and by Atkinson (1991).

As shown in Figure 4.16, N03 is unusual in that it absorbs strongly in the red region (620-670 nm) of the visible spectrum, unlike most atmospherically important species whose absorptions typically fall in the UV. Its absorption in this region is banded, which allows its detection and measurement using spectroscopic techniques (see Sections A.Id and A.4a in Chapter 11).

Table 4.15 gives the absorption cross sections and quantum yields at 298 K. A number of studies report increased values at 662 nm at lower temperatures (e.g., Ravishankara and Mauldin, 1986; Sander, 1986; Yokel-son et al., 1994), while one (Cantrell et al., 1987) finds no change. This is important, since these cross sections are used to derive absolute concentrations of N03 in the atmosphere, where the temperature during measurement can vary considerably.

There are two possible decomposition pathways for N03:

Figure 4.17 shows the energetics of these pathways, including the possible formation of electronically excited singlet states of 02. The threshold for (f8a) is 585.5 nm (Johnston et al., f 996). While reaction (18b) is close to thermoneutral overall, there is a substantial energy barrier to the dissociation, 47.3 + 0.8 kcal mol-1; the threshold observed for this reaction is 594.5 nm (Johnston et al., 1996).

Figure 4.18 shows the results of a réévaluation of the quantum yields as a function of wavelength based on the observed energy thresholds for each channel and on consideration of the potential contributions of rotational and vibrational energy to the dissociation of N03 (Davis et ai, 1993; Johnston et al., 1996). These are in excellent agreement with the experiments of Orlando et al. (1993), except for the region from 595 to 635 nm, where the experimental quantum yields for (18a) are larger than those predicted based on the model of Johnston et al. (1996). The reasons for the discrepancy in this region are not clear, but Johnston and co-workers suggest several possibilities, including contributions from an as yet unrecognized low-lying electronic state.

cf>|Xa is ~ 1 from 570 to 585 nm and then decreases gradually to zero at 635 nm. On the other hand, as (18a) decreases at longer wavelengths, <^l8b first in-

FIGURE 4.16 Absorption spectrum of NO, at 298 K [adapted from DeMore et al., 1997 based on data from Ravishankara and Mauldin (1986), Sander (1986), and Canosa-Mas et al. (1987)].

FIGURE 4.16 Absorption spectrum of NO, at 298 K [adapted from DeMore et al., 1997 based on data from Ravishankara and Mauldin (1986), Sander (1986), and Canosa-Mas et al. (1987)].

creases to a peak value of ~ 0.36 at 595 nm and then drops off at longer wavelengths. Also shown is the increase in fluorescence quantum yield from zero at 595 nm. As expected (Chapter 3.A.2), as the quantum yields for photochemical channels decline, that for the photophysical process of fluorescence increases.

For a solar zenith angle of 0°, the photolysis rate constants are estimated to be in the range 0.17-0.19 s"1 for (18a) and 0.016-0.020 s"1 for (18b) at the earth's surface in the absence of clouds (Orlando et al., 1993; Johnston et al., 1996). The 0(3P) that is formed in the predominant path will add to 02 to generate 03, which can then react with N02 to regenerate N03.

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