growing and coherent real-world test case permits him to evaluate what has now become a generic model, which he continues to develop, perfect, and apply from his present post at Carnegie Mellon University. The model's historical database provides useful comparative and baseline information for future simulations, of Los Angeles and of other urban applications. Because of the severity of pollution in Southern California, McRae and his model have been drawn deeply into the political decision-making process. Alan Lloyd, another session participant and chief scientist for the South Coast Air Quality Management District in California, emphasized that "it is critical to have available a wide group of scientists and institutions that regulators can turn to for unbiased and impartial information."

McRae credited the session's third participant, Arthur Winer, an atmospheric chemist at the University of California, Los Angeles, with "unraveling some of the very interesting things—the chemistry—going on at nighttime in the atmosphere." Winer brought to the symposium discussion his perspective as a longtime scientific advisor to California's regulators, and he also singled out as a challenge for atmospheric science the need "to arrest the declining numbers of really gifted young scientists" entering the field. He found it ironic that the surge in interest and academic programs in his field in the 1970s withered during the 1980s, just when dramatic environmental problems called for galvanizing society's resources.

HELPING REGULATORS DEFINE THE PROBLEM

Atmospheric scientists use the phrase photochemical oxidant air pollution to refer to a mixture of chemical compounds commonly known as smog. "The oxidants are not emitted directly, but rather are formed as products of chemical reactions in the atmosphere. It is this latter property that makes their control so difficult," explained McRae, because time of day and meteorological conditions directly affect the formation of pollution, as do the characteristics of the chemical compounds themselves. The precursors of smog are primarily nitrogen oxides, referred to by the symbol NOx to indicate a sum of nitric oxide (NO) and nitrogen dioxide (NO2). products of high-temperature fuel combustion, and reactive hydrocarbons resulting from solvent use, vegetative emissions, motor vehicles, and a broad spectrum of stationary sources.

Once the polluting sources have been identified, the next logical and traditional step taken by regulators has been to mediate the problem by reducing or redirecting the emitting source. But McRae pointed out one of the basic truths of this field that applies to most ecosys-



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