Photolysis Rates in the Atmosphere

The rate of photodissociation of a molecule, A, upon light absorption, 

In effect, kp takes into account the intensity of available light that the molecule can absorb, the intrinsic strength of light absorption in that region by A, i.e., the absorption cross section cr, and the quantum yield for photodissociation, . 

The light available to a molecule in air for absorption and photodissociation includes both direct and scattered and reflected radiation coming from all directions as described earlier and depicted in Fig. 3.16. The term actinic flux or spherically integrated actinic flux, denoted by F( A), is used to describe the total intensity of this light and is the quantity of interest in calculating kp. 

However, in practice, available light intensity is often measured using flat-plate devices such as the one 

FIGURE 3.17 Typical device (Eppley Laboratories Model 8-48) used to measure solar irradiance. The detector consists of a differential thermopile with the hot junction receivers blackened with flat black coating and the cold junction receivers whitened with BaS04 (photo supplied courtesy of G. L. Kirk, Eppley Laboratories). 

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Most commercial spectrometers report absorbance, as defined in Eq. (Q), versus wavelength. This is very important to recognize, since as we will see later, calculations of the rate of light absorption in the atmosphere require the use of absorption coefficients to the base e rather than to the base 10. While the recent atmospheric chemistry literature reports absorption cross sections to the base e, most measurements of absorption coefficients reported in the general chemical literature are to the base 10. If these are to be used in calculating photolysis rates in the atmosphere, the factor of 2.303 must be taken into account. 

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