SMOG-FORMING REACTIONS OF ORGANIC COMPOUNDS IN THE ATMOSPHERE

16.7 SMOG-FORMING REACTIONS OF ORGANIC COMPOUNDS IN THE ATMOSPHERE

Hydrocarbons are eliminated from the atmosphere by a number of chemical and photochemical reactions. These reactions are responsible for the formation of many noxious secondary pollutant products and intermediates from relatively innocuous hydrocarbon precursors. These pollutant products and intermediates make up photo- chemical smog.

Hydrocarbons and most other organic compounds in the atmosphere are thermodynamically unstable toward oxidation and tend to be oxidized through a series of steps. The oxidation process terminates with formation of CO 2 , solid organic particulate matter that settles from the atmosphere, or water-soluble products (for example, acids, aldehydes), which are removed by rain. Inorganic species such as ozone or nitric acid are byproducts of these reactions.

This section addresses the conditions that are characteristic of a smoggy atmos- phere and the overall processes involved in smog formation. In atmospheres that receive hydrocarbon and NO pollution accompanied by intense sunlight and stagnant air masses, oxidants tend to form. In air-pollution parlance, gross photochemical oxidant is a substance in the atmosphere capable of oxidizing iodide ion to elemental iodine. Sometimes other reducing agents are used to measure oxidants.

© 2001 CRC Press LLC

The primary oxidant in the atmosphere is ozone, O 3 . Other atmospheric oxidants include H 2 O 2 , organic peroxides (ROOR'), organic hydroperoxides (ROOH), and peroxyacyl nitrates such as peroxyacetyl nitrate (PAN).

HCC O O NO 2 Peroxyacetyl nitrate (PAN)

H Nitrogen dioxide, NO 2 , is not regarded as a gross photochemical oxidant.

However, it is about 15% as efficient as O 3 in oxidizing iodide to iodine(0), and a correction is made in measurements for the positive interference of NO 2 . Sulfur dioxide is oxidized by O 3 and produces a negative interference for which a measurement correction must also be made. Peroxyacetyl nitrate and related compounds containing the -C(O)OONO 2 moi- ety, such as peroxybenzoyl nitrate (PBN),

C O O NO 2

a powerful eye irritant and lachrymator, are produced photochemically in atmos- pheres containing alkenes and NO x , and the presence of compounds of this type are important indicators of the presence of photochemical smog. PAN, especially, is a notorious organic oxidant. In addition to PAN and PBN, some other specific organic oxidants that may be important in polluted atmospheres are peroxypropionyl nitrate

(PPN); peracetic acid, CH 3 (CO)OOH; acetylperoxide, CH 3 (CO)OO(CO)CH 3 ; butyl hydroperoxide, CH 3 CH 2 CH 2 CH 2 OOH; and tert-butylhydroperoxide, (CH 3 ) 3 COOH. As shown in Figure 16.11 , smoggy atmospheres show characteristic variations with time of day in levels of NO, NO 2 , hydrocarbons, aldehydes, and oxidants. Examination of the figure shows that, shortly after sunrise, the level of NO in the atmosphere decreases markedly, a decrease that is accompanied by a peak in the

concentration of NO 2 . During midday (significantly, after the concentration of NO has fallen to a very low level), the levels of aldehydes and oxidants become relatively high. The concentration of total hydrocarbons in the atmosphere peaks sharply in the morning, then decreases during the remaining daylight hours.

An overview of the processes responsible for the behavior just discussed is summarized in Figure 16.12 . The chemical bases for the processes illustrated in this figure are explained in the following section.