DMS concentrations did not swing higher after dark. The nitrate radical turns out to be extremely efficient at titrating DMS, said Winer, and thus suppresses its buildup over the oceans at night (Winer et al., 1984).

The spectroscopic measurements made by Winer and his colleagues explained one final piece of the nighttime puzzle, which is suggestive of a larger pattern of diurnal pollution cycling that may be fairly independent of the meteorology. The ubiquitous hydroxyl radical, he said, "is the single most important reactive intermediate in the atmosphere. It catalyzes and drives smog formation." Winer and his colleagues confirmed the accumulation of substantial amounts of nitrous acid (HONO) at night in the Los Angeles basin. Since HONO is a photolytic species that readily breaks apart into the hydroxyl radical and nitric oxide during the day, Winer continued, "the accumulated nitrous acid that photolyzes at sunrise leads to a pulse of OH radicals that represents a kicking off of the next day's smog." In the fall and winter seasons in California when ozone and formaldehyde (which also photolyze to produce OH radicals) are reduced, this OH radical kickoff by nitrous acid can become a dominant source of OH radicals.

McRae clarified for the symposium three important implications from discoveries by Winer, Pitts, and others that have helped to redefine the urban pollution problem. First, nitric acid can be created in a way that requires neither sunlight nor OH radicals; second, he said, "previous model simulations that did not include this reaction would underpredict the importance of controlling NOx emissions; and finally, peroxyacetyl nitrate (PAN) can lead to long-distance transport of reactive nitrogen," a phenomenon that had been known about previously but was further clarified here.

And they also demonstrate the utility of computer models, which, in this case, revealed a crucial part of the picture that scientists had overlooked. Compared to the immediate troposphere where pollutants often accumulate in high and easily detectible concentrations, the upper atmosphere where these reactions occur was harder to observe. Thus the trace concentrations of the nitrogen-containing NO3 radical, to most scientists, had seemed less significant to their models than had the more apparent phenomena. Computer models can be constructed to exclude such judgmental criteria, however. Therefore serendipities can emerge such as this detection of a chemically rapid path by which NO3 can produce nitric acid and PAN without the sunlight's driving energy, especially in the upper portion of the polluted layer. There is something particularly compelling about such discoveries, which seem bred of the inherent complexities of a dy-



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