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Trace gas measurements pertinent to understanding the transport and photochemical formation of 03 were made at a surface site in rural Georgia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between 03 and the oxidation products of NOx: O3(ppb) = 27 + 2 11.4 (NOy(ppb) -NOx(ppb)), r = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background 03 level of 27 ppb; values higher than 27 ppb are due to photochemical production in the recent past, which varied from near zero to -•50 ppb. The origin of the 03 above background was investigated by using a free radical budget equation to calculate an in situ 03 production rate in terms of measured concentrations of NO and free radical precursors (03, HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in 0 3 indicates significant 03 production in the afternoon at a time when 03 concentration is either steady or decreasing. The afternoon near-surface layer is thereby a source region for 03 which can be exported. In situ production accounts for approximately one half of the morning increase in 03 concentration on days with high 03; the remainder is due to entrainment of dirty air aloft by the growing convective boundary layer. Additional evidence for the role of vertical transport in controlling the hour-to-hour changes in 03 is found in the diurnal cycles of SO2 and HNO3 which also have rapid increases in the morning. The day-to-day variability of 03 was investigated using a back trajectory model. NOy concentration at the measurement site could be reasonably accounted for by considering NOx emission sources located within 1-day transport distance. In as much as there is a strong correlation between 03 and NOy, the coincidence between trajectory location and NOx emission sources appears to t•e an important factor influencing midday 03 concentration. Hydrocarbon measurements are consistent with NOx being the limiting factor for formation of 03. 20-30 ppb. During pollution episodes, 03 levels in excess ofthe 120 ppb National Ambient Air Quality Standard have been measured at rural sites [Meagher et al.southeastern United States differs from more industrialized and populated regions in that NOx emissions are lower and natural HC emissions are higher. In addition to precursor emission rates the formation of 03 depends on meteorological conditions. An active photochemistry is favored by high solar intensity, temperature, and absolute humidity which are common summertime conditions in the southeastern United States. Stagnation episodes, which are also common, allow emitted pollutants and their photochemically produced reaction products to accumulate over a several day period. Considerable progress has been made in using photochemical models to simulate the production of 03 and the effects of emission changes [e.g., Seinfeld, 1988; McKeen et al., 1991a, b; NRC, 1991; Roselle et al., 1991]. However, the coupled em...

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