Total Reactive Nitrogen Oxides Research Articles

A numerical study of lightning-induced NOx and formation of NOy observed at the summit of Mt. Fuji using an explicit bulk lightning and photochemistry model

This study coupled a meteorological model with explicit bulk lightning and chemical transport models to investigate the impacts of lightning-induced nitrogen oxides (LNOx) on nitrogen monoxide (NO), nitrogen dioxide (NO2), and total reactive nitrogen oxide (NOy) measured on August 22, 2017, at the top of Mt. Fuji, Japan. Our simulation results indicated that the LNOx emitted around Wakasa Bay in the windward area of Mt. Fuji largely contributed to the NOy content measured at the top of Mt. Fuji. Furthermore, sensitivity experiments regarding the height of LNOx emissions indicated that the NOy content measured atop Mt. Fuji originated from LNOx emitted below 6 km. Our simulation assumed that a two-mode vertical distribution of LNOx emissions was more consistent with measured NOy at Mt. Fuji than a single-mode structure assumption in this case. A comparison of simulated NOx (= NO + NO2) and measured NOx at Mt. Fuji indicated that the reaction rates of the NO and NO2 cycles were well reproduced in our model; however, the ratio of NOz (NOy species other than NOx) to NOy estimated by the model were lower than the observed value, implying that the model either underestimated the reaction rate of LNOx or overestimated the wet removal of lightning-induced NOz. Finally, our results also suggest that the simultaneous observation of NOy and NOx is important for understanding LNOx emissions, subsequent atmospheric chemical reactions, and removal processes, as well as validating chemical transport models.

基于腔衰荡光谱技术(CRDS)对大气总活性氮氧化物(NOy)的实时测量

基于腔衰荡光谱技术(CRDS)对大气总活性氮氧化物(NOy)的实时测量

Sensitivity to grid resolution in the ability of a chemical transport model to simulate observed oxidant chemistry under high-isoprene conditions

Abstract. Formation of ozone and organic aerosol in continental atmospheres depends on whether isoprene emitted by vegetation is oxidized by the high-NOx pathway (where peroxy radicals react with NO) or by low-NOx pathways (where peroxy radicals react by alternate channels, mostly with HO2). We used mixed layer observations from the SEAC4RS aircraft campaign over the Southeast US to test the ability of the GEOS-Chem chemical transport model at different grid resolutions (0.25° × 0.3125°, 2° × 2.5°, 4° × 5°) to simulate this chemistry under high-isoprene, variable-NOx conditions. Observations of isoprene and NOx over the Southeast US show a negative correlation, reflecting the spatial segregation of emissions; this negative correlation is captured in the model at 0.25° × 0.3125° resolution but not at coarser resolutions. As a result, less isoprene oxidation takes place by the high-NOx pathway in the model at 0.25° × 0.3125° resolution (54 %) than at coarser resolution (59 %). The cumulative probability distribution functions (CDFs) of NOx, isoprene, and ozone concentrations show little difference across model resolutions and good agreement with observations, while formaldehyde is overestimated at coarse resolution because excessive isoprene oxidation takes place by the high-NOx pathway with high formaldehyde yield. The good agreement of simulated and observed concentration variances implies that smaller-scale non-linearities (urban and power plant plumes) are not important on the regional scale. Correlations of simulated vs. observed concentrations do not improve with grid resolution because finer modes of variability are intrinsically more difficult to capture. Higher model resolution leads to decreased conversion of NOx to organic nitrates and increased conversion to nitric acid, with total reactive nitrogen oxides (NOy) changing little across model resolutions. Model concentrations in the lower free troposphere are also insensitive to grid resolution. The overall low sensitivity of modeled concentrations to grid resolution implies that coarse resolution is adequate when modeling continental boundary layer chemistry for global applications.

Trapping, chemistry, and export of trace gases in the South Asian summer monsoon observed during CARIBIC flights in 2008

Abstract. The CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container) passenger aircraft observatory performed in situ measurements at 10–12 km altitude in the South Asian summer monsoon anticyclone between June and September 2008. These measurements enable us to investigate this atmospheric region (which so far has mostly been observed from satellites) using the broad suite of trace gases and aerosol particles measured by CARIBIC. Elevated levels of a variety of atmospheric pollutants (e.g. carbon monoxide, total reactive nitrogen oxides, aerosol particles, and several volatile organic compounds) were recorded. The measurements provide detailed information about the chemical composition of air in different parts of the monsoon anticyclone, particularly of ozone precursors. While covering a range of 3500 km inside the monsoon anticyclone, CARIBIC observations show remarkable consistency, i.e. with distinct latitudinal patterns of trace gases during the entire monsoon period. Using the CARIBIC trace gas and aerosol particle measurements in combination with the Lagrangian particle dispersion model FLEXPART, we investigated the characteristics of monsoon outflow and the chemical evolution of air masses during transport. The trajectory calculations indicate that these air masses originated mainly from South Asia and mainland Southeast Asia. Estimated photochemical ages of the air were found to agree well with transport times from a source region east of 90–95° E. The photochemical ages of the air in the southern part of the monsoon anticyclone were systematically younger (less than 7 days) and the air masses were mostly in an ozone-forming chemical mode. In its northern part the air masses were older (up to 13 days) and had unclear ozone formation or destruction potential. Based on analysis of forward trajectories, several receptor regions were identified. In addition to predominantly westward transport, we found evidence for efficient transport (within 10 days) to the Pacific and North America, particularly during June and September, and also of cross-tropopause exchange, which was strongest during June and July. Westward transport to Africa and further to the Mediterranean was the main pathway during July.

Observations of reactive nitrogen oxide fluxes by eddy covariance above two midlatitude North American mixed hardwood forests

Abstract. Significant knowledge gaps persist in the understanding of forest–atmosphere exchange of reactive nitrogen oxides, partly due to a lack of direct observations. Chemical transport models require representations of dry deposition over a variety of land surface types, and the role of canopy exchange of NOx (= NO + NO2) is highly uncertain. Biosphere–atmosphere exchange of NOx and NOy (= NOx + HNO3 + PANs + RONO2 + pNO3− + ...) was measured by eddy covariance above a mixed hardwood forest in central Ontario (Haliburton Forest and Wildlife Reserve, or HFWR), and a mixed hardwood forest in northern lower Michigan (Program for Research on Oxidants: Photochemistry, Emissions and Transport, or PROPHET) during the summers of 2011 and 2012 respectively. NOx and NOy mixing ratios were measured by a custom-built two-channel analyser based on chemiluminescence, with selective NO2 conversion via LED photolysis and NOy conversion via a hot molybdenum converter. Consideration of interferences from water vapour and O3, and random uncertainty of the calculated fluxes are discussed. NOy flux observations were predominantly of deposition at both locations. In general, the magnitude of deposition scaled with NOy mixing ratios. Average midday (12:00–16:00) deposition velocities at HFWR and PROPHET were 0.20 ± 0.25 and 0.67 ± 1.24 cm s−1 respectively. Average nighttime (00:00–04:00) deposition velocities were 0.09 ± 0.25 cm s−1 and 0.08 ± 0.16 cm s−1 respectively. At HFWR, a period of highly polluted conditions (NOy concentrations up to 18 ppb) showed distinctly different flux characteristics than the rest of the campaign. Integrated daily average NOy flux was −0.14 mg (N) m−2 day−1 and −0.34 mg (N) m−2 day−1 (net deposition) at HFWR and PROPHET respectively. Concurrent wet deposition measurements were used to estimate the contributions of dry deposition to total reactive nitrogen oxide inputs, found to be 22 and 40% at HFWR and PROPHET respectively.

Measurement of NOy during Campaign of Air Quality Research in Beijing 2006 (CAREBeijing‐2006): Implications for the ozone production efficiency of NOx

Total reactive nitrogen oxides (NOy) are among the key components in the chemistry of ozone production. In order to improve the understanding of the formation mechanisms of high‐ozone events in Beijing, China, an intensive experiment on the reactive nitrogen oxides was carried out at the observatory of Peking University during the Campaign of Air Quality Research in Beijing and surrounding areas in 2006 (CAREBeijing‐2006) campaign. In this study, analysis focusing on the data of high O3 episodes was performed to examine the relationship between the concentration of total oxidant and the composition of NOy. High levels of NO and NOy were observed in the morning rush hours, indicating the influences of fresh emissions from local traffic. However, the ratio of [NO] to [NOy] was only 11–60% in the morning, implying that there was a substantial amount of aged air pollutants remained overnight in the stagnant air mass. Significant increases in the NOz mixing ratio (= [NOy]‐[NOx]) were observed during the period from the morning toward early afternoon, consistent with the increasing oxidant level. Ozone production efficiency of NOx (OPEx), which was derived from the NOz‐Ox regression, was found to range from 3.9 to 9.7 mol/mol in Beijing. Furthermore, it was revealed that the daily [NOz] maximum was proportional to the NOx peak level in the morning, and that OPEx decreased with the increases of [NOz] in a hyperbolic form. According to the results, abatement in NOx emission would not be effective toward reducing ozone concentrations in Beijing.

Seasonal variation of nitrogen oxides in the central North Atlantic lower free troposphere

Measurements of NO, NO2, and NOy (total reactive nitrogen oxides) made at the Pico Mountain station, 38.47°N, 28.40°W, 2.2 km above sea level, from July 2002 to August 2005 are used to characterize the seasonal and diurnal variations of nitrogen oxides in the background lower free troposphere (FT) over the central North Atlantic Ocean. These observations reveal a well‐defined seasonal cycle of nitrogen oxides (NOx = NO + NO2 and NOy), with higher mixing ratios during the summertime. Observed NOx and NOy levels are consistent with long‐range transport of emissions, with significant removal en route to the measurement site. Larger summertime nitrogen oxides levels are attributed to boreal wildfire emissions and more efficient export and transport of NOy from eastern North America during that season. In addition, measurements of NOx and NOy obtained during in‐cloud and cloud‐free conditions are used to estimate PAN and HNO3 mixing ratios and examine the partitioning of the reactive nitrogen species. These estimates indicate that reactive nitrogen over the central North Atlantic lower FT largely exists in the form of PAN and HNO3 (∼80–90% of NOy) year‐round. The composition of NOy shifts from dominance of PAN in winter‐spring to dominance of HNO3 in summer‐fall, as a result of changes in temperature and photochemistry over the region. A further comparison of the nitrogen oxides measurements with results from the global chemical transport model GEOS‐Chem finds that simulated nitrogen oxides are significantly larger than the observations.

Large‐scale impacts of anthropogenic pollution and boreal wildfires on the nitrogen oxides over the central North Atlantic region

Transport of North American anthropogenic and boreal wildfire emissions is a large source of nitrogen oxides over the North Atlantic region. To characterize the influence of transport of these emissions on nitrogen oxides levels over the central North Atlantic lower free troposphere (FT) and their further implications for hemispheric O3, we analyze measurements of NOx (NO + NO2), total reactive nitrogen oxides (NOy), CO, and O3 made at the Pico Mountain station (38.47°N 28.40°W, 2.2 km above sea level) in the Azores archipelago from July 2002 to August 2005. Transport of pollution from North America causes significant enhancements of nitrogen oxides year‐round. An analysis of the export of United States NOx emissions to the FT based on observed ΔNOy/ΔCO in the anthropogenic plumes indicates that more than 94% of the NOx emitted over the United States is lost within the continent and/or during export out of the United States boundary layer and to the Azores, consistent with previous studies. However, our observations indicate that about 30% of the NOy initially exported out of the United States boundary layer reach the Azores lower FT. NOx was also significantly enhanced in these plumes. Since the lifetime of NOx is shorter than the transport timescale of most events, PAN decomposition and potentially photolysis of HNO3 provide a supply of NOx over the central North Atlantic lower FT. Observed ΔO3/ΔNOy ratios and significant NOy levels remaining in the North American plumes suggest a potential for O3 formation well downwind from North America. Summertime boreal wildfires in North America are responsible for important shifts in the nitrogen oxides distributions toward higher levels, with medians of NOy (117–175 pptv) and NOx (9–30 pptv) greater in boreal wildfire plumes. These findings demonstrate the potential hemispheric scale impact that boreal wildfire and North American anthropogenic events have on background NOx and NOy levels and on the tropospheric O3 budget.

Photochemical evolution of submicron aerosol chemical composition in the Tokyo megacity region in summer

We investigate the chemical transformation of submicron aerosol in the Tokyo megacity region in summer. An Aerodyne quadrupole aerosol mass spectrometer (AMS) was deployed both at an urban site in Tokyo (35°39′N, 139°40′E) and a suburban site (downwind site) in Saitama (36°05′N, 139°33′E) in the summer of 2004. The temporal evolution of size‐resolved chemical compositions of submicron (PM1) aerosols during photochemical smog episodes are investigated using the photochemical age derived from the combination of alkyl nitrate‐to‐hydrocarbon ratio and NOz/NOy ratio (where NOz is defined as the total reactive nitrogen oxides (NOy) excluding nitrogen oxides (NOx)). The photochemical age observed at the downwind site was about 12 h in most aged air. Organic aerosols (OA) and sulfate (SO42−) were major constituents of PM1 aerosols (40–50% and 20–30%, respectively) at both sites during the observation period and their fractions showed no large variation with the NOz/NOy ratio. Mass ratios of OA and SO42− to black carbon (BC) largely increased with the NOz/NOy ratio (by factors of ∼3 and ∼2, respectively), indicating the significance of secondary formation of these compounds in controlling PM1 mass concentrations. We also investigate the photochemical evolution of OA mass spectra observed by the AMS. The mass‐to‐charge ratio (m/z) peaks of organic compounds relative to BC mass generally showed an increase with the NOz/NOy ratio. These increasing trends vary significantly for different m/z peaks, suggesting the complexity of the temporal evolution of organic functional groups. The m/z 44 and 45 peaks, which are good markers of carboxylic groups in organic particles, showed larger increases than any other m/z peaks, suggesting an efficient formation of carboxylic functional groups on a timescale of hours during the measurement period.

Significant enhancements of nitrogen oxides, black carbon, and ozone in the North Atlantic lower free troposphere resulting from North American boreal wildfires

Extensive wildfires burned in northern North America during summer 2004, releasing large amounts of trace gases and aerosols into the atmosphere. Emissions from these wildfires frequently impacted the PICO‐NARE station, a mountaintop site situated 6–15 days downwind from the fires in the Azores Islands. To assess the impacts of the boreal wildfire emissions on the levels of aerosol black carbon (BC), nitrogen oxides and O3 downwind from North America, we analyzed measurements of CO, BC, total reactive nitrogen oxides (NOy), NOx (NO + NO2) and O3 made from June to September 2004 in combination with MOZART chemical transport model simulations. Long‐range transport of boreal wildfire emissions resulted in large enhancements of CO, BC, NOy and NOx, with levels up to 250 ppbv, 665 ng m−3, 1100 pptv and 135 pptv, respectively. Enhancement ratios relative to CO were variable in the plumes sampled, most likely because of variations in wildfire emissions and removal processes during transport. Analyses of ΔBC/ΔCO, ΔNOy/ΔCO and ΔNOx/ΔCO ratios indicate that NOy and BC were on average efficiently exported in these plumes and suggest that decomposition of PAN to NOx was a significant source of NOx. High levels of NOx suggest continuing formation of O3 in these well‐aged plumes. O3 levels were also significantly enhanced in the plumes, reaching up to 75 ppbv. Analysis of ΔO3/ΔCO ratios showed distinct behaviors of O3 in the plumes, which varied from significant to lower O3 production. We identify several potential reasons for the complex effects of boreal wildfire emissions on O3 and conclude that this behavior needs to be explored further in the future. These observations demonstrate that boreal wildfire emissions significantly contributed to the NOx and O3 budgets in the central North Atlantic lower free troposphere during summer 2004 and imply large‐scale impacts on direct radiative forcing of the atmosphere and on tropospheric NOx and O3.

Ship‐based nitric acid measurements in the Gulf of Maine during New England Air Quality Study 2002

Gas phase nitric acid (HNO3) was measured at 5‐min resolution on board the National Oceanographic and Atmospheric Administration (NOAA) research vessel Ronald H. Brown during the second leg (29 July to 10 August) of the New England Air Quality Study (NEAQS) 2002 cruise. A primary objective of the cruise was to improve understanding of the oxidation of NOx in, and removal of the oxidation products from, the polluted marine boundary layer east of northeastern North America. For the first 9 days of this leg the ship remained north of Cape Cod, and the cruise track did not extend much farther north than the New Hampshire‐Maine border. During this period, HNO3 averaged 1.1 ppb and accounted for 19% of total reactive nitrogen oxides (measured NOy). On all days, peak HNO3 mixing ratios were observed in the early afternoon (average 2.3 ppb), at levels twofold to fourfold higher than the minima around sunrise and sunset. In these daytime peaks, HNO3/NOy averaged 28%. There were secondary nighttime peaks of HNO3 (0.9 ppb average), when HNO3 accounted for 16% of total reactive nitrogen oxides. This pronounced diurnal pattern confirms that production, and subsequent deposition, of HNO3 in the polluted marine boundary layer downwind of New England removes a significant fraction of the NOx exported to the atmosphere over the Gulf of Maine. Nitric acid was correlated with O3, particularly during the early afternoon interval when both molecules reached maximum mixing ratios (R2 = 0.66). The ozone production efficiency (OPE) inferred from the slope (10 ppb O3/ppb HNO3) was similar to the OPE of 9 estimated at the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) Thompson Farm station in coastal New Hampshire during the study period.

Seasonality of reactive nitrogen oxides (NOy) at Neumayer Station, Antarctica

NO, NOy (total reactive nitrogen oxides), gaseous HNO3, and particulate nitrate (p‐NO3−) were measured at Neumayer Station from February 1999 to January 2000. In addition, during February 1999, the NOy component species peroxyacetyl nitrate (PAN) and methyl, ethyl, i‐propyl, and n‐propyl nitrates were determined. We found a mean NOy mixing ratio of 46 ± 29 pptv, with significantly higher values between February and end of May (58 ± 35 pptv). Between February and November, the (HNO3 + p‐NO3−)/NOy ratio was extremely low (around 0.22) and in contrast to NOy the seasonality of p‐NO3− and HNO3 showed a distinct maximum in November and December, leading to a (HNO3 + p‐NO3−)/NOy ratio of 0.66. Trajectory analyses and radioisotope measurements (7Be, 10Be, 210Pb, and 222Rn) indicated that the upper troposphere or stratosphere was the main source region of the observed NOy with a negligible contribution of ground‐level sources at northward continents. Frequent maxima of NOy mixing ratios up to 100 pptv are generally associated with air mass transport from the free troposphere of continental Antarctica, while air masses with the lowest NOy mixing ratios were typically advected from the marine boundary layer.

NOx and NOy over the northwestern North Atlantic: Measurements and measurement accuracy

Measurements of NOx (NO+NO2) and NOy (total reactive nitrogen oxides) during February‐April 1996 at the northern tip of Newfoundland are used to determine levels in the local marine boundary layer (MBL) and assess the adequacy of current understanding of the processes controlling NOx levels over the northern North Atlantic, as expressed through previously reported simulations using the Geophysical Fluid Dynamics Laboratory (GFDL) Global Chemical Transport Model (GCTM). Median mixing ratios of NOx and NOy in the local MBL were 24 parts per trillion by volume (pptv) and 200 pptv, respectively. These levels are 35–64% above background levels measured in the remote MBL in summer and fall during the Chemical Instrumentation Test and Evaluation (CITE 2), North Atlantic Regional Experiment (NARE‐93), and Pacific Exploratory Mission‐West (PEM‐West) A measurement campaigns and are similar to or somewhat higher than anthropogenically influenced levels observed in winter‐spring during the PEM‐West B campaign. The magnitude of median NOx and NOy, levels in the local MBL is not due to events with high reactive nitrogen oxides levels. Instead, these relatively high median levels are likely the result of dispersion of anthropogenic emissions over a large region. A detailed comparison with results from the GFDL GCTM indicates that measured March and April average NOx levels are significantly lower than simulated levels over the north central North Atlantic. The frequency and magnitude of modeled and observed elevated‐NOx events were similar, indicating that the conditions responsible for relatively direct long‐range transport events were similar. This indicates that interannual variability probably did not cause the discrepancy in monthly average NOx values. However, simulated elevated NOx events are much longer than are observed. This difference appears to be at least partially responsible for the higher average NOx values simulated by the model. These results indicate that model‐based estimates of this region's contributions to the global ozone budget may be too high. Accuracy of the NOx measurements is estimated to be 6%, while conservative analysis of conversion efficiencies indicates a negative bias of ≲18% in the determination of gas‐phase NOy compounds.

Air‐snow exchange of HNO3 and NOy at Summit, Greenland

Ice core records of NO3− deposition to polar glaciers could provide unrivaled information on past photochemical status and N cycling dynamics of the troposphere, if the ice core records could be inverted to yield concentrations of reactive N oxides in the atmosphere at past times. Limited previous investigations at Summit, Greenland, have suggested that this inversion may be difficult, since the levels of HNO3 and aerosol‐associated NO3− over the snow are very low in comparison with those of NO3− in the snow. In addition, it appears that some fraction of the NO3− in snow may be reemitted to the atmosphere after deposition. Here we report on extensive measurements of HNO3, including vertical gradients between 1.5 and 7 m above the snow, made during the summers of 1994 and 1995 at Summit. These HNO3 data are compared with NO3− concentrations in surface snow and the first measurements of the concentrations and fluxes of total reactive nitrogen oxides (Ny) on a polar glacier. Our results confirm that HNO3 concentrations are quite low (mean 0.5 nmol m−3) during the summer, while NO3− is the dominant ion in snow. Daytime peaks in HNO3− appear to be due at least partly to emissions from the snow, an assertion supported by gradients indicating a surface source for HNO3− on many days. Observed short‐term increases in NO3− inventory in the snow can be too large to be readily attributed to deposition of HNO3− suggesting that deposition of one or more other N oxides must be considered. We found that the apparent fluxes of HNO3 and NOy were in opposite directions during about half the intervals when both were measured, with more cases of HNO3 leaving the snow, against an NOy flux into the snow, than the reverse. The concentrations of NOy are generally about 2 orders of magnitude greater than those of HNO3; hence deposition of only a small, non‐HNO3, fraction of this pool could dominate NO3− in snow, if the depositing species converted to NO3−, either in the snowpack or upon melting for analysis.

Seasonal transition from NOx‐ to hydrocarbon‐limited conditions for ozone production over the eastern United States in September

Concentrations of O3, CO, NO, total reactive nitrogen oxides (NOy), H2O2, and HCHO were measured from September 4 to October 1, 1990, at a mountain ridge site in Shenandoah National Park, Virginia. The data show evidence for a transition from NOx‐limited to hydrocarbon‐limited conditions for O3 production over the course of September. The transition is diagnosed by large decreases of the H2O2/(NOy‐NOx) and HCHO/NOy concentration ratios, weakening of the correlation between O3 and NOy‐ NOx concentrations, and decrease of the slope ΔO3/Δ(NOy‐NOx). A high‐O3 episode occurring in late September was associated with only 0.34 ppbv H2O2, indicative of hydrocarbon‐limited conditions. A seasonal transition in photochemical regime over the eastern United States in September would be expected from theory; the production rate of odd hydrogen radicals decreases by a factor of 2 over the course of the month, due to decreasing UV radiation and humidity, allowing HNO3 production to become the dominant sink for odd hydrogen in the boundary layer and resulting in hydrocarbon‐limited conditions for O3 production. Seasonal decline of isoprene emission can greatly accentuate the transition.

Measurements of nitrogen oxides at Barrow, Alaska during spring: Evidence for regional and northern hemispheric sources of pollution

Total reactive nitrogen oxide (NOy) and NO were measured at the National Oceanic and Atmospheric Administration Geophysical Monitoring for Climatic Change (NOAA GMCC) site at Barrow, Alaska, for 5 weeks during the spring of 1989. The data demonstrate the existence of a complex array of pollution sources which influence arctic air chemistry. During periods when 10‐day back‐trajectories indicated southerly flow, median NOy concentrations ranged from 284 to 467 ppt. When the 10‐day back‐trajectories originated in the Arctic, median NOy, concentrations ranged from 617 to 734 ppt, providing evidence for the long‐range transport of pollutants to the Arctic known as “arctic haze.” The NOy concentrations observed during the spring background periods are significantly higher than previous measurements of NOy at Barrow taken during summer, regardless of where the air mass originated. These observations provide evidence for a greatly increased NOy lifetime during the colder months over a large region of the background troposphere as well as the presence of much higher NOy concentrations in the Arctic. During four 12–60 hour events, substantially elevated NOy were observed in air not impacted by emissions from the town of Barrow. During these four events, peak NOy concentrations of 3 to 16 ppb were observed. Other data, including NO, SO2, SO4, and meteorology, were used to help identify the source region for the elevated NOy levels. These data present a consistent picture which indicates that during certain periods, elevated NOy levels are associated with air coming from the Prudhoe Bay industrial area approximately 300 km to the ESE and demonstrate that this facility is a major source of nitrogen oxides, and possibly other air pollutants, to the Alaskan Arctic.