It has been recognized since the early work of Haagen-Smit and co-workers (e.g., see Haagen-Smit et al., 1953; Haagen-Smit and Fox, 1956) that different types of organic compounds react at different rates to form 03 and other secondary pollutants in irradiated VOC-NOx mixtures. Indeed, it was this recognition of differing reactivities that has led to the development of traditional control strategies for non-methane hydrocarbons, NMHC. Thus, methane reacts sufficiently slowly over the time scale of hours to a few days that it does not contribute significantly to photochemical smog formation. It reacts only with the OH radical at a significant rate (also with chlorine atoms in coastal regions), with a calculated lifetime of about 5 years at an OH concentration of 1 x 106 cm~3. Thus although methane is important in the global troposphere and stratosphere (see Chapters 12 and 14), it is essentially unreactive in terms of photochemical oxidant production on urban and regional scales. On the other extreme are most alkenes and aldehydes, which are highly reactive (vide infra).
Because of this recognition of different reactivities of organics, various regulations have been promulgated over the years that attempt to take into account the fact that 2-butene, for example, will produce more 03 than ethane. An early example is what was known as "Rule 66," implemented by the Los Angeles Air Pollution Control District in 1966 to limit solvent emissions based on their reactivity. For a discussion of some of the reactivity scales used prior to 1975, see Darnall et al. (1976), Bufalini et al. (1976), Bufalini and Dodge (1983), Dodge (1984), Hough and Derwent (1987), Der-went and Jenkin (1991), and the papers on applications to motor vehicle exhaust discussed below.
More recently, there has been a major focus on applying the same principle to the emissions from motor vehicles. We discuss here the scientific basis for this approach, with some recent examples of their application.
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