Photochemistry of Minor Atmospheric Constituents

The atmosphere contains many trace gases that are photochemically active. Few, however, attain a significance similar to oxygen or ozone in driving atmospheric chemistry. Table 2-6 gives an overview on the photochemical behavior of atmospheric constituents. The last column indicates whether the photodecomposition of the substance is important to atmospheric chemistry or not. The molecules listed may be subdivided into three groups. (1) For a number of gases such as methane or ammonia that enter 

In this section we discuss absorption spectra, quantum yields, and photodissociation coefficients for several atmospheric trace species. DeMore et al. (1985) and Baulch et al. (1980, 1982, 1984) have critically reviewed data relevant to atmospheric processes. Table 2-7 summarizes important photochemically active molecules and associated photodissociation coefficients. These were computed with the full, unattenuated solar spectrum so that they represent maximum values. But an attempt has been made to separate critical wavelength regions as a discriminator for the significance of the process in the stratosphere and troposphere, respectively. 

Constituent long-wavelength limit (nm) limit (nm) process yield the atmosphere 

185-215 nm Important in the stratosphere in the atmospheric absorption window, 185-215 nm Important in the stratosphere in the atmospheric absorption window, 185-215 nm Stratosphere Not established 

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The photochemistry of the Mars and Venus atmospheres may be Ixih, understood if the minor constituents, such as H202, 03, and H2, can I.. measured and if the rate constants involving H02 and ClOO radicals >.m be measured more accurately in the laboratory. McElroy et al. (678) bclm, that CO and H20 are converted to C02 and H2 in the hot region neai tin surface... 

Some molecules in this group (HONO, NC j 0, HONC ) have been extensively studied because the photofragments OH and NO can be probed by tunable lasers. These molecules are important minor constituents in the earth atmosphere and their photochemistry plays a major role in air pollution. Atmospheric pollutants N0X (NO, NO2, NO3) are formed from combustion of fuel and subsequent chemical reactions in the atmosphere. Photolysis of alkyl oxides produces NO and NO2 that can be probed by LIF the internal energy distribution provides an important clue to the mechanism of photodissociation. 

Some minor constituents are produced or destroyed in the atmosphere by photochemical reactions. The altitude at which these processes occur depends on the penetration depth of the solar radiation which initiates the photochemistry. 

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