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] Hydroxyl (OH), hydroperoxy (HO 2 ) radicals, collectively known as HO x , and OH reactivity, were measured during the PMTACS-NY (PM2.5 Technology Assessment and Characteristics Study-New York) summer 2002 intensive at Whiteface Mountain, Wilmington, New York. The measurement results of OH and HO 2 for 4 weeks are presented. Diurnal cycles show that the average noontime maximum mixing ratios were about 0.11 pptv (2.6 Â 10 6 cm À3 ) for OH and 20 pptv for HO 2 . Measured HO 2 to OH ratios were typically between 40 and 400, which are greater than those obtained in polluted and semipolluted rural environments. Low but significant mixing ratios of OH and HO 2 persisted into early evening and were frequently observed during nighttime, consistent with previous studies in different environments. Steady state OH and HO 2 were calculated with a zero-dimensional chemical model using a complete Regional Atmospheric Chemical Mechanism (RACM) and a parameterized RACM which was constrained to the measured OH reactivity. Good agreement was obtained between the complete RACM and the parameterized RACM models. On average, the complete RACM model reproduced the observed OH with a median measured-to-modeled OH ratio of 0.82 and daytime HO 2 with a median measured-to-modeled HO 2 ratio of 1.21. The reasonably good agreement in this study is inconsistent with the significant underestimation of OH in the Program for Research on Oxidants: Photochemistry, Emissions, and Transport in 1998 (PROPHET98) study at a similar forested site. HO x budget analysis indicates that OH was primarily from the photolysis of HONO and O 3 during the day and from O 3 + alkenes reactions at night. The main HO x loss was the self reaction of HO 2 . The good agreement between the measured and calculated OH reactivity in this environment contrasts with findings in the PROPHET2000 study, in which significant OH reactivity was missing and the missing OH reactivity was temperature-dependent.
[1] HONO, HCHO, and O 3 concentrations were measured at the summit of Whiteface Mountain, New York, during the summer of 1999. Concentrations were in the range of 5-400 pptv with a median of 27 pptv and a mean of 46 pptv for HONO, in the range of 30-6170 pptv with a median of 1260 pptv and a mean of 1340 pptv for HCHO, and in the range of 20-105 ppbv with a median of 51 ppbv and a mean of 49 ppbv for O 3 . The daily HO x productions from the photolysis of O 3 , HONO, and HCHO were 6.1, 2.8, and 1.9 ppbv d À1, respectively, contributing 57, 26, and 17% to the overall daily radical budget from these precursors. Significant diurnal variation of average HONO concentrations was observed, with a late morning maximum and a late afternoon/early evening minimum. HNO 3 photolysis on the mountain slope surfaces is proposed as the major daytime source for HONO, sustaining the majority of the observed daytime HONO concentrations against its photolytic loss. The daytime HONO/NO x ratio was also found to be high, $0.33 between 0800 and 1600 LT, suggesting that the transport of HONO from the ground surface to the observation height should be short, perhaps limited to the top portion of the mountain slope. The late afternoon HONO concentrations ($27 pptv) may be considered as typical daytime HONO concentration in the aloft air mass not influenced by surface processes. While particulate nitrate photolysis may contribute significantly as a HONO source, more HONO formation/transport mechanisms are still required to account for the majority of the observed HONO concentration. HCHO concentrations exhibited only a small diurnal variation, with daytime concentrations slightly higher than nighttime concentration. HCHO was mainly transported to the site rather than produced locally from the biogenic photooxidation. O 3 concentrations exhibited a diurnal variation that was similar to but more pronounced than HCHO, suggesting that the daytime in situ O 3 photochemical production was small compared to its loss through photolysis and dry deposition.
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...
An intercomparison was made near Niwot Ridge, Colorado, of three different instruments for measuring NO2 at low concentrations in ambient air: (1) the photolysis/chemiluminescence (PC) instrument, (2) the tunable diode laser absorption spectrometer (TDLAS), and (3) the Luminox instrument. Calibrated mixtures of NO2 in air and NO2 with possible interferants (HNO3, peroxyacetyl nitrate (PAN), H2O2, n‐propyl nitrate, and O3) were provided in simultaneous tests. In addition, ambient air measurements were made using the three instruments. Blind procedures were followed in preparing all results. Several conclusions were reached concerning the performance of these instruments during this intercomparison: (1) For NO2 levels above 2 parts per billion by volume (ppbv), similar results were obtained for all instruments; (2) Below 2 ppbv, the expected interferences from ozone and PAN influenced the NO2 measurements made using the Luminox instruments. Those interferences were sufficiently consistent that they could be corrected for by using the measured values Of O3 and PAN down to about 0.3 ppbv NO2; (3) The ozone interference on the Luminox instruments was removed by an ozone scrubber placed in the sampled air stream of the Luminox instrument. However, this did not remove PAN. In addition, the scrubber appeared to remove about 50% of the NO2 as well; (4) Although no interferences were identified for the TDLAS technique, care must be taken in the data analysis near (or below) the detection limit for the instrument. At these levels the data reduction program provided with the TDLAS will tend to find background noise that is correlated with the reference NO2 spectrum and calculate levels of NO2 that are too high; (5) No interferences or artifacts were found for the final results reported by the PC technique. However, these results for ambient measurements were corrected by subtracting an artifact that averaged 5 parts per trillion by volume (pptv) and by calculating a correction for the effect of ambient ozone. This latter correction averaged 1.0% in magnitude.
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