of solvents, together classified as reactive organic gases (ROGs). The EPA established a limit on these emission sources, and when McRae began modeling the atmosphere over Los Angeles in the early 1980s, one of the inevitable elements of his work was an evaluation of the effectiveness of lower emissions of ROGs. That evaluation, as it developed in the larger context of his model and in the work of other atmospheric scientists, surprised everyone.
"EPA's approach was basically right if you're looking very close to the source. But the role of hydrocarbons is essentially that of controlling the speed of the chemistry" that is leading to atmospheric pollution, said McRae. Other elements, primarily NOx, turn out to be more instrumental in whether the pollution is actually generated. "If you put only organic controls on a city, it slows the reactions so that ozone forms downwind, outside the central city," McRae remarked. This generally means that regions and counties away from the center of a city—such as the Riverside and San Bernadino areas east of Los Angeles—are thus inundated with the smog. Because the formation of the pollutant was slower, ''our national policy was leading to a situation where we may have reduced ozone in cities," but at the expense of suburban and rural areas downwind, so that "we were creating a problem on the regional scale." The sins of the generating region—as with the acid rain experience—were being visited upon nearby or faraway neighbors, depending upon the meteorology transporting them.
Running his model suggested to McRae an even more startling possibility. Regulating ROGs was not reducing ozone formation at all, because the role of NOx had been misperceived and as a consequence, underestimated. Although, as Lloyd pointed out, "ozone levels in Los Angeles have been cut by about half over the last 15 years," the model indicated that in the rest of the country where NOx was not being regulated, uncontrolled emissions of NOx could confound the intended results of ROG reduction. "Despite massive control efforts," concluded McRae, "the average ozone levels in the eastern United States and in western cities such as Houston, Phoenix, and Denver have not been lowered. Part of the reason is that the chemistry of oxidant production is highly nonlinear. There is no simple relationship between the emissions of the major precursors and the resulting ozone."
Lloyd noted that the EPA is now turning to control of NOx as well as ROGs, an approach taken early on by regulators in California, and he emphasized as well the amount of work still to be done by scientists in addressing problems of air pollution. Especially important, he maintained, is the need to obtain adequate amounts of high-