Example Photolysis of Acetaldehyde at the Earths Surface

To illustrate the application of Eqs. (OO) and (PP), let us calculate the rate of photolysis of acetaldehyde. Aldehydes such as CH3CHO play an important role in tropospheric chemistry because they photodissociate to produce free radicals. In the case of acetaldehyde, there are four possible sets of products  

For the purposes of illustration, the rate of photolysis will be calculated for conditions of a cloudless day at the earth s surface at 30°N latitude six hours after noon on July 1. 

From Eq. (PP), we need the primary quantum yields for each of the processes (9a)-(9d), the absorption cross sections, base e, and the actinic flux values, F(A). Table 3.18 gives IUPAC recommendations (Atkinson et a,l., 1997) for the absorption cross sections and primary quantum yields for CH3CHO. Primary quantum 

To use the actinic flux data in Table 3.7, we need to know the solar zenith angle at 30°N latitude six hours after noon on July 1. We can obtain this from Table 3.9. Since the values are symmetrical about noon, we use the data for 0600 on July 1 at 30°N latitude, which from Table 3.9 corresponds to a solar zenith angle of 78.7°. To obtain the actinic fluxes corresponding to this solar zenith angle, one returns to Table 3.7 and inter- 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products .,v(A) xlv(A)F(A) for each wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

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