Photolysis Rate as a Function of Altitude

Photolysis reactions are central to atmospheric chemistry, since the source of energy that drives the entire system of atmospheric reactions is the Sun. The general expression for the first-order rate coefficient j for photodissociation of a species A is given by (4.39). Because the rate of a photolysis reaction depends on the spectral actinic flux / and because 

The photolysis rate coefficient of a species A can be expressed as a function of altitude from (4.39) as 

Assume that the attenuation of radiation is due solely to absorption by species A. From the Beer-Lambert Law, I(z,X) can be expressed as 

To proceed further, consider the case in which the absorbing species A has a uniform mixing ratio, A (e.g., 02) then, its concentration can be expressed in terms of the molecular air density as a function of altitude, nan (z)  

The total number concentration of air as a function of altitude falls off approximately with the scale height H of atmospheric pressure  

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FIGURE 4.14 Photodissociation rate coefficient for 02, y o2, and 02 photolysis rate as a function of altitude at solar zenith angle 0q = 0°. (Courtesy Ross J. Salawitch, Jet Propulsion Laboratary.)... 

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