Anthropogenic ozone, acids and mutagens: Half a century of Pandora’S No_x

It has been four decades since the phenomenon of photochemical air pollution was first characterized and, in the same year, a tragic London smog episode caused 4,000 excess deaths. Since then, there has been a substantial increase in our understanding of the chemistry involved in both types of air pollution, and a recognition that there is a very close chemical interrelationship between them. In this overview, we provide a brief historical perspective on the atmospheric chemistry of photochemical smog and illustrate how fundamental studies on the gas-phase chemistry of uv-irradiated mixtures of volatile organic compounds (VOC) and oxides of nitrogen (NO(x)) in polluted laboratory and ambient air masses have contributed to our understanding of three environmental problems: the atmospheric formation of ozone, nitric acid and airborne mutagens. In particular, we demonstrate the central role played by nitrogen dioxide and the hydroxyl radical in each case. We also show how certain reactive toxic and acidic species, e.g., formaldehyde and nitrous and formic acids, have been characterized in smog chambers and ambient smog by long pathlength spectroscopic techniques. It is shown that by using the same methods they now have been identified unequivocally, along with NO2, in certain common types of polluted indoor atmospheres ... and at much higher concentrations than outdoors. This has significant health implications for indoor HCHO and quite possibly the acids. We then trace the history of the direct mutagenicity of respirable particles in polluted ambient air and show how, through use of the Ames test in biologically-directed assays of products coupled with fundamental studies of gas-phase reactions of polycyclic aromatic hydrocarbons (PAH) and NO(x) in irradiated air, much of this activity can be accounted for in terms of the formation of nitro-PAH and oxygenated derivatives. Finally, we discuss the application of basic kinetic, mechanistic and analytical, experimental techniques and theoretical concepts to the development of a new set of "reactivity-based" regulatory controls on motor vehicle emissions of VOC's. This novel regulatory approach applied by California's Air Resources Board, which takes effect in 1994, illustrates the continuing need for fundamental research in the area of atmospheric chemistry and how it may be applied to "real world" environmental problems.