If one were to increase the total mass of VOC emissions in a city, such as Los Angeles, by 20% through additional emissions of ethane, ozone levels would increase slightly. However, if the same amount of propene were added instead, there would be a large increase in ozone. Why the big difference between the two, given that both are rather simple hydrocarbons? The primary cause of the difference is the differing rates at which these two species react in the atmosphere. Ethane has an atmospheric lifetime of weeks. Little of the ethane emitted in an urban area reacts within that area before it is transported away. Its contribution to ozone formation within the urban area is therefore very small. Propene, on the other hand, has a lifetime of hours. Most of it will typically react near its source and thus be able to contribute to the photochemical production of ozone in the area in which it is emitted (or immediately downwind). A secondary, but smaller, cause for the differing impacts of the two species is the different number of ozone molecules formed in that environment for each molecule of ethane and propene that reacts. Differences in ozone productivity arising from the first effect are often expressed in terms of the kinetic reactivity (KR), and differences from the second are expressed in terms of the mechanistic reactivity (MR).

There is, in fact, a significant historical precedent for accounting for VOC reactivity in U.S. regulatory policy, albeit to a limited extent (see Dimitriades 1996, for a history of VOC regulation in the United States). During the early years of ozone mitigation, it was recognized that there were some organics, for example ethane, that did not contribute significantly to smog formation on urban scales, whereas others, such as propene, did. Thus, two categories of organic gases were defined for regulatory purposes: unreactive and reactive.² (see Table 3-1). However, the term