Ozone formation at a rural site in the southeastern United States

Trace gas measurements pertinent to understanding the transport and photochemical formation of O 3 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 O 3 and the oxidation products of NO x : O 3 (ppb) = 27 + 11.4 (NO y (ppb) − NO x (ppb)), r 2 = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O 3 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 O 3 above background was investigated by using a free radical budget equation to calculate an in situ O 3 production rate in terms of measured concentrations of NO and free radical precursors (O 3 , HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in O 3 indicates significant O 3 production in the afternoon at a time when O 3 concentration is either steady or decreasing. The afternoon near‐surface layer is thereby a source region for O 3 which can be exported. In situ production accounts for approximately one half of the morning increase in O 3 concentration on days with high O 3 ; 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 O 3 is found in the diurnal cycles of SO 2 and HNO 3 which also have rapid increases in the morning. The day‐to‐day variability of O 3 was investigated using a back trajectory model. NO y concentration at the measurement site could be reasonably accounted for by considering NO x emission sources located within 1‐day transport distance. In as much as there is a strong correlation between O 3 and NO y , the coincidence between trajectory location and NO x emission sources appears to be an important factor influencing midday O 3 concentration. Hydrocarbon measurements are consistent with NO x being the limiting factor for formation of O 3 .