Photochemical smog refers to a number of species that are chemically produced in highly polluted environments in processes driven by sunlight. The most prominent of these species is ozone (03), which reaches levels that violate government health standards in urban areas throughout the world. Other components of photochemical smog include peroxyacetyl nitrate (PAN, CH3C03N02) and nitric and sulfuric acid (HN03, H2S04). The latter is associated with the formation of acid aerosols, which have serious impacts on both human health and visibility. Photochemical smog typically forms during conditions characterized by high sunlight (though often with haze and reduced visibility), light winds, and warm temperatures. This chapter focuses on ozone and its precursors.
Episodes with high ozone were first observed in Los Angeles in the 1950s (Haagen-Smit and Fox, 1954) and have generally been found in cities with high automobile traffic. This type of photochemical smog should be distinguished from the type of smog driven by primary emissions (primarily of coal-based S02, N02, CO, and soot), which characterized the city of London during the early 1900s (Brimblecombe, 1987) and Beijing today. In primary smog, high concentrations are associated with patterns of atmospheric circulation that "trap" the emitted pollutants in an atmospheric layer close to emission sources. The most severe events tend to occur in fall or winter, when atmospheric vertical mixing at ground level is minimal, and may coincide with fog (hence the origin of the word smog for the combination of smoke and fog). By contrast, photochemical smog can only occur in meteorological conditions that favor photochemical activity (i.e., high sunlight, warm temperatures)
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and not necessarily during conditions with restricted meteorological dispersion. The most severe events have occurred in large urban areas with warm, dry climates (Los Angeles, Mexico City, Athens). However severe photochemical smog has been observed in virtually all major cities of North America and Europe and more recently in developing nations. Although the most severe episodes have occurred in locations with high automobile traffic, elevated 03 has also been associated with coal-fired power plants (Miller et al„ 1978; White et al., 1983; Gillani and Pleim, 1996; Ryerson et al., 2001). In the eastern United States and in Europe elevated 03 occurs in regionwide events and is characterized by transport over distances of 500 km or more. Elevated 03 has also been found in association with biomass burning in the tropics (see Chapter 14).
The relation between ozone formation and precursor emissions has been the subject of much uncertainty and controversy. Ozone is formed from two general classes of precursors: hydrocarbons (including oxygenated organic species)* and nitrogen oxides (NO +N02, or NO, ). The chemistry of ozone formation typically falls into one of two recognizable patterns: a NO,-limited regime in which the rate of formation increases with NOx and is largely independent of hydrocarbon concentrations and a hydrocarbon-limited (or light-limited) regime in which the rate of formation increases with hydrocarbons and decreases with increasing NOx. An analogous split into NO,-limited and light-limited regimes also occurs in the remote troposphere. The split into NO,-limited and HC-limited regimes has generated a debate on policy, especially in the United States, concerning the best way to reduce urban ozone. Because this represents a major uncertainty associated with ozone formation, much of this chapter will address the complex relation between ozone, NOx, and HC.
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