trillion detection limits and hence the first observation of the nitrate radical and nitrous acid in the polluted troposphere (Platt et al., 1980a, b). Thus the SAPRC-German teams established the methodological basis for later, more sophisticated models predicated on accurate measurements of atmospheric components. They worked on what Winer called "an historical antecedent" to McRae's powerful model, based on the premise that the more definitive the measurable data used as input, the more likely the model will be to mimic real-world processes.

NIGHTTIME CHEMISTRY

The long optical-path measurements by the SAPRC-German collaborators opened inquiry into an area that had been previously overlooked, nighttime chemistry. Sunlight drives most of the chemical reactions that atmospheric scientists model, and thus conventional wisdom held that nighttime—with its absence of hydroxyl radicals—might offer a respite from the creation of secondary pollutants. General pollution measurements taken after dark seemed to confirm that nighttime chemistry "was of minor importance," said McRae, who added that it "was even disregarded in some models." NO3, for example, is a trace nitrogen-containing radical with such small predicted concentrations (in the parts-per-trillion range) in the troposphere that scientists largely ignored its possible significance. But Platt, Winer, and Pitts and their co-workers focused their special spectrograph (Figure 8.1) on just such overlooked yet potentially significant species. They were the "first to discover the nitrate radical in the polluted troposphere," and then went on to detect it in the cleaner troposphere as well, said McRae. "We have now shown systematically in cleaner atmospheres that after sunset the nitrate radical concentration grows into more or less steady-state levels," said Winer, ''which persist throughout the night and disappear at sunrise due to photolysis'' (Platt et al., 1984).

This discovery, notwithstanding the small amount of the nitrate radical they found, could hardly have been more confounding and significant to the atmospheric sciences community. Suddenly, the formation of nitric acid in the troposphere at night could be explained. Previously, it was believed that nitric acid could be produced only by the reaction of the hydroxyl radical with NO2 during daylight hours. With the discovery of the nitrate radical concentration arising after dark, the leftover pollutants O3 and NO2 were provided a pathway leading to the nighttime production of nitric acid.

Winer, as a Californian, immediately recognized the implications of this discovery, since "nitric acid is, in fact, two-thirds of our acid



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