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Tropospheric air pollution has impacts on scales ranging from local to global. Reactive intermediates in the oxidation of mixtures of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) play central roles: the hydroxyl radical (OH), during the day; the nitrate radical (NO3), at night; and ozone (O3), which contributes during the day and night. Halogen atoms can also play a role during the day. Here the implications of the complex VOC-NOx chemistry for O3 control are discussed. In addition, OH, NO3, and O3 are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles.

The yields of alkyl nitrates formed in the NOx-air photooxidations of the homologous series of n-alkanes from ethane through n-octane have been determined at 299 ± 2 K and 735 torr total pressure for two different chemical systems. Alkyl peroxy radicals were generated by reaction of the n-alkanes with OH radicals (generated from the photolysis of methyl nitrite in air) or Cl atoms (from photolysis of Cl2 in air). The alkyl nitrate yields obtained from the two systems, corrected for secondary reactions, were in agreement within the experimental errors and increased monotonically with the carbon number of the n-alkane, from <1% for ethane to ~33% for n-octane, with the yields apparently approaching a limit of ~35% for large n-alkanes. The relative yields of the various secondary alkyl nitrate isomers in the n-pentane through n-octane systems were in good agreement with those expected from OH radical or Cl atom reaction with the corresponding secondary C-H bonds. However, the relative yields of the primary alkyl nitrates in the propane and butane systems were a factor of ~2 lower than expected. The data are consistent with the alkyl nitrates being formed almost entirely from the reaction of peroxy radicals with NO, and the ratios of the corrected alkyl nitrate yields thus reflect the fraction of R02 radicals which react with NO to form alkyl nitrates. These nitrate yields from the reaction of R02 radicals with NO are important inputs into chemical computer models of the atmospheric NO*-air photooxidations of the large n-alkanes.

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Directly active mutagens are formed on exposure of the promutagen benzo[a]pyrene to gaseous pollutants in smog. In simulated atmospheres containing 1 part per million nitrogen dioxide and traces of nitric acid, directly mutagenic nitro derivatives are readily formed from both benzo[a]pyrene and perylene, a non-mutagen in the Ames reversion assay. Possible formation of direct mutagens by such reactions on sample collection filters, in exhaust effluents, and in the atmosphere should be recognized.