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FIGURE 4.10 Relative yield of 02('Ag) irom the photolysis of O, (adapted from Ball et al, 1993).

Tables 4.7 and 4.8 show the recommended absorption cross sections (DeMore et al., 1997). Those in the 202-to 274-nm region are temperature independent. At the longer wavelengths there appears to be a small temperature dependence parameterized as shown in Table 4.8.

As discussed in Chapter 11, N02 is frequently measured in the atmosphere using UV-visible absorption spectroscopy. Because of the large number of possible absorbing species in the atmosphere, NOz is usually measured using the peak-to-valley absorbance of selected peaks, rather than peak to zero absorbance. While reducing the temperature from 298 K only slightly affects the peak absorption cross sections, it does decrease the minima in the spectrum, thus increasing the peak-to-valley absorbance differences used for measuring N02 in the atmosphere by differential optical absorption spectrometry (e.g., Davidson et al., 1988; Harwood and Jones, 1994; Vandaele et ai, 1996; Harder et al., 1997). This is expected since lowering the temperature will lower the population of the higher rotational energy levels in the ground state, leading to a smaller range of energies and wavelengths absorbed in a given transition. In addition to the temperature dependence, a pressure dependence of the absorption cross sections has been observed in high-resolution studies (Harder et al., 1997), which is important for application in the upper atmosphere.

One of the complications in measuring N02 absorption cross sections, particularly at lower temperatures and higher concentrations, is the presence of the dimer N204 in equilibrium with it:

TABLE 4.7 Averaged Absorption Cross Sections (Base e) from 202 to 274 nm for NOz at 298 Ku

\

102"<T*

\

io2V

(nm)

(cm2 molecule ~1 )

(nm)

(cm2 molecule ~1 )

0 0

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