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FIGURE 4.24 Absorption cross section of S02 (adapted from Manatt and Lane, 1993).

from this photolysis are much higher than for the larger molecules.

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies; as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.fd and A.4f in Chapter 11.).

TABLE 4.22 Absorption Cross Sections (Base e ) for S03u

Wavelength

102"<T

Wavelength

102"<T

(nm)

(cm2 molecule-

') (nm)

(cm2 molecule -1)

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