Ozone and Related Compounds Photochemical Smog

Ozone is formed in the stratosphere by the short-wavelength ( 240-nm) homolysis of molecular oxygen (O2) to two oxygen atoms (O), followed by the subsequent collision of an oxygen atom with an oxygen molecule to produce O3. The absorption spectrum of ozone (X ax = 255 nm) is such that virtually all the potentially damaging ultraviolet wavelengths between 200 and 300 nm are screened out before they reach 

The initiation step for the smog-producing reaction sequence is the photodissocia- 

In air, the O atoms produced photochemically combine with O2 in the presence of a third body to yield O3 by the reaction already discussed (Section 1.A.4). Ozone itself, however, is readily photodissociated, and also reacts with NO to reform O2 and NO2. The rates of production and destruction of O3 in an air-nitrogen oxide system are such that the concentrations of NO and O3 should be roughly equal. In practice, however, the combination of NO and hydrocarbons plus solar ultraviolet leads to a net production of O3 and other oxidants. 

The products of the photochemical reactions of aliphatic hydrocarbons in the presence of nitrogen oxides are reasonably well understood. For example, -butane is converted to CH2O, CH3CHO, and methyl ethyl ketone, together with the nitrogen-containing species methyl, ethyl and isopropyl nitrate (14-16) and perox-yacetyl nitrate ( PAN, 17 Altshuller et al., 1969). Radical (RO-, RC = 0 and ROO-) recombination reactions with NO2 probably account for the formation of 

14-17 17 is of special interest because of its phytotoxic, oxidizing, and irritant properties. PAN is also a nearly universal constituent of the troposphere, occurring even in extremely clean mid-ocean samples (Temple and Taylor, 1983 Singh and Salas, 1983). 

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