OZONE FORMATION AT A RURAL SITE IN THE SOUTHEASTERN UNITED-STATES

Trace gas measurements pertinent to understanding the transport and ph otochemical formation Of O3 were made at a surface site in rural Georg ia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between O3 and the oxid ation products of NO(x): O3(ppb) = 27 + 11.4 (NO(y)(ppb) - NO(x)(ppb)) , r2 = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O3 level of 27 ppb; values higher than 27 ppb are due to photochemical producti on in the recent past, which varied from near zero to almost-equal-to 50 ppb. The origin of the O3 above background was investigated by usin g a free radical budget equation to calculate an in SitU O3 production rate in terms of measured concentrations of NO and free radical precu rsors (O3, HCHO, peroxides, and other carbonyls). A comparison of obse rved and predicted diurnal trends in O3 indicates significant O3 produ ction in the afternoon at a time when O3 concentration is either stead y or decreasing. The afternoon near-surface layer is thereby a source region for O3 which can be exported. In situ production accounts for a pproximately one half of the morning increase in O3 concentration on d ays with high O3; the remainder is due to entrainment of dirty air alo ft by the growing convective boundary layer. Additional evidence for t he role of vertical transport in controlling the hour-to-hour changes in O3 is found in the diurnal cycles Of SO2 and HNO3 which also have r apid increases in the morning. The day-to-day variability Of O3 was in vestigated using a back trajectory model. NO(y) concentration at the m easurement 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 O3 and NO(y) the coincidence between trajectory location and NO(x) emission sources appears to be a n important factor influencing midday O3 concentration. Hydrocarbon me asurements are consistent with NO(x) being the limiting factor for for mation Of O3.