To summarize the chemistry: certain photolytically driven reactions (such as the interaction of hydroxyl radicals with organics) tend to oxidize NO to NO2 and/or to produce more hydroxyl radicals. Such chemical reactions can contribute to a cycle or chain reaction that intensifies the concentration of ozone, and hence increases smog. Knowing how to identify and apply the reactions is the important framework, but obtaining accurate measurements of these substances—which can exist in a transient condition, swirling around the atmosphere, in concentrations as small as a few parts per trillion—presents a formidable challenge to atmospheric chemists.

GETTING THE NUMBERS RIGHT

Winer necessarily brings a historical perspective to any consideration of atmospheric science. For 20 years he has been a prominent participant in a number of important developments in the field, watching the science of measurement, the use of computers, and the role of the scientist in the public forum all evolve. In his early work, Winer was concerned "that too much attention was being given to six so-called criteria pollutants under the Clean Air Act" in the mid- to late-1970s and that as a consequence not much was known about a whole spectrum of other species such as formaldehyde (HCHO), nitrous acid (HONO), nitric acid (HNO3), the nitrate radical (NO3), dinitrogen pentoxide (N2O5), and formic acid (HCOOH). "These species either had never been measured in the atmosphere, or had, at best, been detected by wet chemical methods of notorious unreliability," said Winer, who added that, "not surprisingly, almost nothing was known about the possible health impacts of these compounds." Then at the Statewide Air Pollution Research Center (SAPRC), a research institute of the University of California based on the Riverside campus, Winer and a SAPRC team led by James N. Pitts, Jr., began a series of experiments to "try to understand what's really in a smog cloud," using Fourier transform infrared (FT-IR) spectroscopy for the first time over long optical paths of a kilometer or more. The parts-per-billion detection sensitivities afforded by such pathlengths led to the first spectroscopic detection of HCHO and HNO3 in polluted air.

This success spurted Winer and his colleagues to collaborate with German physicists Uli Platt and Dieter Perner, who had devised a novel spectroscopic instrument operating in the UV-visible region. By exploiting long optical paths—up to 17 kilometers—and a rapid scanning computer-based capability in which a signal-averaging algorithm was applied to thousands of superimposable spectral records, Winer, Platt, and their collaborators were able to achieve parts-per-



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