Abstract. An expression for the production rate of 03, P(O 3), is derived based on a radical budget equation applicable to low and high NOx conditions. Differentiation of this equation with respect to NO or hydrocarbons (HC) gives an approximate analytic formula in which the relative sensitivity of P(O3) to changes in NO or HC depends only on the fraction of radicals which are removed by reactions with NOx. This formula is tested by comparison with results from a photochemical calculation driven by trace gas observations from the 1995 Southern Oxidants Study (SOS) campaign in Nashville, Tennessee.

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...

Steady state photochemical calculations were performed using observed or estimated trace gas concentrations as constraints. According to these calculations the local rate of 03 production P(O3) in all four plumes is VOC sensitive, sometimes strongly so. The local sensitivity calculations show that a specified fractional decrease in VOC concentration yields a similar magnitude fractional decrease in P(O3). Imposing a decrease in NO•, however, causes P(O3) to increase. The question of primary interest from a regulatory point of view is the sensitivity of 03 concentration to changes in emissions of NO• and VOCs. A qualitative argument is given that suggests that the total 03 formed in the plume, which depends on the entire time evolution of the plume, is also VOC sensitive. Indicator ratios O3/NO z and H202/NO z mainly support the conclusion that plume 03 is VOC sensitive.

As part of the Southern Oxidants Study, Brookhaven National Laboratory operated an intensive measurement site near Metter, Georgia, during parts of the summers of 1991 and 1992. Measurements were made of photochemically active trace gases and meteorological parameters relevant to determining causes for elevated ambient ozone concentration. The 1992 data set was used to calculate peroxy radical concentration and ozone formation rate based on determining the departure from the photostationary state (PSS) and based on a radical budget equation, such as applied previously to the 1991 data set. Averaged over the 28-day experimental period, we find maximum radical production occurring near noon at 2.5 ppb h -1 , maximum peroxy radical concentration also occurring near noon at 80 ppt, and maximum ozone production of 8 ppb h -1 occurring near 1000 EST. Ozone photolysis accounts for 55% of radical production, HCHO and other carbonyl compounds about 40%. The radical budget and PSS methods depend in different ways on atmospheric photochemistry and a comparison between them affords a test of our understanding of the photochemical production of 03. We find that these methods agree to the extent expected based on uncertainty estimates. For the data set as a whole, the median estimate for fractional error in hourly average peroxy radical concentration determined from the radical budget method is approximately 30% and from the PSS method, 50%. Error estimates for the PSS method are highly variable, becoming infinite as peroxy radical concentration approaches zero. This behavior can be traced back to the difference form of the PSS equations. To conduct a meaningful comparison between the methods, the data set was segregated into subsets based on PSS uncertainty estimates. For the lowuncertainty subset, consisting of a third of the whole data set, we find that the ratio of peroxy radical concentration predicted from the PSS method to that predicted from the radical budget method to be 1.22 + 32%. 1.Paper number 95JD00215. 0148-0227/95/95 JD-00215 $05.00 a radical budget calculation and based on the photostationary state (PSS) relations. We compare these two approaches using data from the 1992 campaign. In a previous study using the 1991 data set, the radical budget approach was used to determine production rates for 03 which were then compared with other trace gas measurements [Kleinman et al., 1994a]. We will refer to the 1991 study as "SOS91." Estimates of peroxy radical concentration and 03 formation rate have been obtained for a diverse range of locations via the use of the PSS equations [Parrish et al., 1986; Chameides et al., 1990; Ridley et al., 1992; Davis et al., 1993; Cantrell et al., 1993; Bawkin et al., 1994; Poulida et al., 1994]. Diurnal cycles of peroxy radical concentration and their relation to other atmospheric variables such as NOx(NO + NO 2) concentration and solar UV have been discussed. Several studies including those at Mauna Loa [Ridley et al., 1992] and at the Rural Oxidants in the Southern Environment (RO...

A nonenzymatic method Is developed and compared with the well-known enzymatic (p-hydroxyphenyl)acetlc acid (pOH-PAA) method for the determination of hydrogen peroxide In aqueous atmospheric samples. The new method Is based on the Fe( 11)-catalyzed oxidation of benzoic acid by H202 to form hydroxylated products (OHBA), which are analyzed by fluorescence detection. The limit of detection and linear response range of the new method are comparable to those of the pOHPAA technique. In addition, the new Fenton-OHBA method has the advantage of using Inexpensive, stable, easily available chemical reagents that do not require refrigeration. The new method Is insensitive to moderate transition-metal concentrations often found in atmospheric samples.