Photochemical Reactions

In the presence of air, SO2 is slowly oxidized to SO3 when exposed to solar radiation. If water is present, the SO2 rapidly converts to sulfuric acid. Since no radiation wavelengths shorter than 2900 A reach the earth's surface and the dissociation of SO2 to SO and O is possible only for wavelengths below 2180 A, the primary photochemical processes in the lower atmosphere following absorption by SO2 involve activated SO2 molecules and not direct dissociation. Thus, the conversion of SO2 to SO3 in clear air is a result of a several-step reaction sequence involving excited SO2 molecules, oxygen, and oxides of sulfur other than SO2. In the presence of reactive hydrocarbons and nitrogen oxides, the conversion rate of SO2 to SO3 increases markedly. In addition, oxidation of SO2 in systems of this type is frequently accompanied by aerosol formation.

A survey of possible reactions by Bufalini (1971) and Sidebottom et al. (1972) concludes that the most important oxidation step for the triplet state 3SO2 from among those involving radiation only is:

Other primary substances absorbing UV radiation include sulfur and nitrogen oxides and aldehydes. UV radiation excites the molecules of these substances, which then react with atmospheric molecular oxygen to yield atomic oxygen. Analogous to SO2 oxidation, aldehydes react as follows:

Atomic oxygen can also be formed by the following reactions:

SO2 and aldehydes react irreversibly, whereby the amount of atomic oxygen formed by these processes is relatively small and corresponds to the amount of SO2 and aldehy des in the atmosphere. In the reaction of nitrogen dioxide, however, the absorption of UV radiation leads to the destruction of one bond between the nitrogen and oxygen atoms and to the formation of atomic oxygen and nitrogen oxide. Further reactions lead to the formation of atomic oxygen and nitrogen oxide as follows:

The regenerated nitrogen dioxide can reenter the reaction, and this process can repeat until the nitrogen dioxide converts into nitric acid or reacts with organic substances to form nitrocompounds. Therefore, a low concentration of nitrogen dioxide in the atmosphere can lead to the formation of a considerable amount of atomic oxygen and ozone. This nitrogen dioxide is significant in the formation of oxidation smog.

Olefins with a large number of double bonds also react photochemically to form free radicals. Inorganic substances in atomic form in the atmosphere also contribute to the formation of free radicals. On reacting with oxygen, some free radicals form peroxy compounds from which new peroxides or free radicals are produced that can cause polymerization of olefins or be a source of ozone. The photochemistry is described by the thirty-six reactions for the twenty-seven species in Table 5.4.2 which includes four reactive hydrocarbon groups: olefins, paraffins, aldehydes, and aromatics.

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