During the summer of 1988, measurements of photochemical trace species were made at a coordinated network of seven rural sites in the eastern United States and Canada. At six of these sites concurrent measurements of ozone and the sum of the reactive nitrogen species, NOy, were made, and at four of the sites a measure for the reaction products of the NO x oxidation was obtained. Common to all sites, ozone, in photochemically aged air during the summer, shows an increase with increasing NOy levels, from a background value of 30-40 parts per billion by volume (ppbv)at NOy mixing ratios below 1 ppbv to values between 70 to 100 ppbv at NOy levels of 10 ppbv. Ozone correlates even more closely with the products of the NOx oxidation. The correlations from the different sites agree closely at mixing ratios of the oxidation products below 5 ppbv, but systematic differences appear at higher levels. Variations in the biogenic hydrocarbon emissions may explain these differences. IntroductionElevated and potentially harmful levels of ozone are being found in many rural areas of North America during summer. Daily maximum 03 levels measured in rural areas are often comparable to those found in urban areas and daily average levels can exceed urban levels. There is substantial evidence from field measurements and model calculations that most of this ozone is being produced photochemically from ozone precursors emitted within the region [Research Triangle Institute, 1975; Vukovich et al., 1977Vukovich et al., , 1985Cleveland et al., 1977;Spicer et al., 1979;Wolff and Lioy, 1980;Fehsenfeld et al., 1983;Kelly et al., 1984;Liu et al., 1987]. A similar situation appears to exist for western Europe [Cox et al., 1975;Guicherit and Van Dop, 1977;Hov, 1984]. The photochemical processes responsible for these high levels are thought to be quite similar to the processes that operate in urban photochemical smog but with important differences. In

[1] Accurate emission inventories and their temporal trends must be incorporated into pollutant inventories to allow for reliable modeling of the country's past, present, and future air quality. Measured carbon monoxide (CO) and nitrogen oxide (NO x ) concentrations from two urban areas show that the CO/NO x vehicular emission ratio has decreased at an average rate of 7-9% per year from 1987 to 1999. This amounts to a factor of nearly 3 over the 12 years. The current U.S. Environmental Protection Agency tabulations of estimated pollutant emission trends indicate a rate of decrease smaller by a factor of 2-3. The trend in maximum ambient CO levels in U.S. cities suggests a 5.2 ± 0.8% per year average annual decrease in CO vehicular emissions, which implies a 2-3% annual increase in NO x emissions from vehicles. Thus over the decade of the 1990s, annual U.S. CO emissions from vehicles have decreased from $65 to $38 Tg, representing approximate decreases of 6 and 3% in the annual global fuel-use CO emissions and in total global anthropogenic CO emissions, respectively. It is expected that the volatile organic compound (VOC)/NO x vehicular exhaust emission ratio has decreased similarly, implying that the character of atmospheric photochemistry in U.S. urban areas has changed significantly over the decade.

Abstract. An informal intercomparison of NOy measurement techniques was conducted from

Measurements of O3, NO, NO2, peroxyacetyl nitrate (PAN), HNO3, and NOy were made during a 6‐week period in the summer of 1991 in Giles County, Tennessee. These data were analyzed to determine the factors controlling the relationship between O3 and NOy at this rural site. A strong association was observed between the O3 and NOx oxidation product (NOz = NOy ‐ NOx) levels. The higher O3 levels were associated with air masses impacted by higher NOx emissions that had been photochemically processed. An analysis of the data indicates that the ultimate O3 production is about 10 molecules of O3 produced for each molecule of NOx emitted. The analysis results also suggest that O3 net production continues until about 70% of the NOx has been converted into NOz. The PAN/HNO3 ratios observed suggest that the air masses in Giles County are composed of higher volatile organic carbon/NOx ratios than the air masses observed at other rural sites in eastern North America. A comparison of the data analysis results to model simulations and smog chamber experiments suggests that most of the time, Giles County is in an NOx‐limited regime for O3 production.

During the late summer and early fall of 1988, measurements of many trace species of tropospheric photochemical interest, including NO, NO2, PAN, HNO3, NO3-, NOy, and ozone were made at seven surface stations in the eastern United States and Canada. The NOy (as well as ozone) levels and its partitioning were strongly influenced by the diurnal evolution of the boundary layer at the sites that are beneath the nocturnal inversion. At the higher elevation sites the median levels of all species were much more nearly constant. During the daytime the median NOy levels were 2 to 5 ppbv at all sites, which may be representative of rural areas in the populated regions of eastern North America. Each site showed variations in the NOy levels of an order of magnitude or more. Measurements from all of the sites are consistent with the major contributors to NOy being NOx (the sum of NO and NO2), PAN, and nitric acid with a minor contribution from aerosol nitrate. At the lower elevation sites the median [NOx] to [NOy] ratios were 70% or more during the night and declined to minima of 25 to 40% during the day. During the daytime the ranges of the median contributions of PAN and HNO3 to NOy were 12 to 25% and approximately 20 to 30%, respectively. The distributions of the contributions about these medians are discussed. Results from all of the sites are consistent with the individually measured species accounting for about 90% of the simultaneously measured NOy. IntroductionThe family of tropospheric reactive oxidized nitrogen species, generically referred to here as NO¾, is composed of principally NO, NO2, peroxyacetyl nitrate (PAN), HNO3, and NO3-aerosol [Fahey et al., 1986]. Other inorganic and organic species may make additional minor contributions to the total family concentration. These species play several significant roles in tropospheric photochemistry. The primary pollutant, 1Aeronomy Laboratory, NOAA, Boulder, Colorado. 2Also at NO, is ultimately oxidized to HNO3 whose removal from the atmosphere by wet and dry deposition constitutes the nitrogen contribution to acid deposition, which in eastern North America is significant, second to sulfate deposition. Organic peroxy and hydroperoxy radicals are responsible for much of the oxidation of NO to NO2; hydroxyl radicals oxidize NO2 to HNO3; and peroxyacetyl radicals combine with NO2 to form PAN. These reactions exert a controlling influence on the radical balance in the troposphere. To the extent that the products are removed from the atmosphere before dissociating, these reactions provide sinks for the radicals and thus also affect the total radical concentration in the troposphere. Since the radicals are responsible for forming the major oxidants of the troposphere (ozone, hydrogen peroxide, and organic hydroperoxides), the levels of these oxidants are strongly coupled to the levels of the NOy family. Thus the characterization of the levels of these species is essential to the understanding of tropospheric photochemistry. Up to the time of the measurement campaign repo...