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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

The profound changes in global SO2 emissions over the last decades have affected atmospheric composition on a regional and global scale with large impact on air quality, atmospheric deposition and the radiative forcing of sulfate aerosols. Reproduction of historical atmospheric pollution levels based on global aerosol models and emission changes is crucial to prove that such models are able to predict future scenarios. Here, we analyze consistency of trends in observations of sulfur components in air and precipitation from major regional networks and estimates from six different global aerosol models from 1990 until 2015. There are large interregional differences in the sulfur trends consistently captured by the models and observations, especially for North America and Europe. Europe had the largest reductions in sulfur emissions in the first part of the period while the highest reduction came later in North America and East Asia. The uncertainties in both the emissions and the representativity of the observations are larger in Asia. However, emissions from East Asia clearly increased from 2000 to 2005 followed by a decrease, while in India a steady increase over the whole period has been observed and modelled. The agreement between a bottom-up approach, which uses emissions and process-based chemical transport models, with independent observations gives an improved confidence in the understanding of the atmospheric sulfur budget.

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...

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Data from the National Atmospheric Deposition Program/National Trends Network (NADP/ NTN) indicate significant changes have occurred in precipitation chemistry and the chemical climate in the United States (US). A Seasonal Kendall Trend (SKT) analysis shows statistically significant increases in precipitation ammonium concentrations at 64% of 159 continental US NADP/NTN sites evaluated from Winter 1985 to Fall 2004 (Dec. 1984 -Nov. 2004). Sulfate decreases were widespread, with an SKT analysis indicating statistically significant decreases at 89% of sites evaluated. Ratios of chemical equivalent concentrations of ammonium to sulfate in precipitation have risen to the extent that ammonium now exceeds sulfate over more than half of the continental U.S. on a precipitation-weightedmean annual basis. These trends in the concentrations of ammonium, sulfate, and other species have been accompanied by significant decreases in the frequency of acidic precipitation (pH<5.0) in the last decade.

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