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4. Example: Photolysis of Acetaldehyde at the Earth's 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 al., 1997) for the absorption cross sections and primary quantum yields for CH3CHO. Primary quantum

TABLE 3.19 Calculated Photolysis Rate Constants for CH3CHO Photolysis at 30°N Latitude

Six Hours after Noon on July 1

Actinic flux, Absorption cross Quantum yield for

Wavelength 'HI14

interval, photons (10"20cm2 <}..,/'\r,v /J/av <t>ilv''V„

A A (nm) cm"2«"1) molecule"1) 4>av9a(A.) <f>av9b(*-) (lO^s"1)" ilO s"1)"

Wavelength 'HI14

interval, photons (10"20cm2 <}..,/'\r,v /J/av <t>ilv''V„

A A (nm) cm"2«"1) molecule"1) 4>av9a(A.) <f>av9b(*-) (lO^s"1)" ilO s"1)"

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