The total reactive oxidized nitrogen levels and the partitioning between the individual species at six rural sites in eastern North America

Abstract

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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