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Dinosaur National Monument (DINO) is located near the northeastern edge of the Uinta Basin and often experiences elevated levels of wintertime ground-level ozone. Previous studies have shown that high ozone mixing ratios in the Uinta Basin are driven by elevated levels of volatile organic compounds (VOCs) and nitrogen oxides (NOx) from regional oil and gas development coupled with temperature inversions and enhanced photochemistry from persistent snow cover. Here, we show that persistent snow cover and temperature inversions, along with abundant ammonia, also lead to wintertime haze in this region. A study was conducted at DINO from November 2018 through May 2020 where ozone, speciated fine and coarse aerosols, inorganic gases, and VOCs were measured. Three National Ambient Air Quality Standards (NAAQS) ozone exceedances were observed in the first winter, and no exceedances were observed in the second winter. In contrast, elevated levels of particulate matter were observed both winters, with 24-h averaged particle light extinction exceeding 100 Mm−1. These haze events were dominated by ammonium nitrate, and particulate organics were highly correlated with ammonium nitrate. Ammonium nitrate formation was limited by nitric acid in winter. As such, reductions in regional NOx emissions should reduce haze levels and improve visibility at DINO in winter. Long-term measurements of particulate matter from nearby Vernal, Utah, suggest that visibility impairment is a persistent issue in the Uinta Basin in winter. From April through October 2019, relatively clean conditions occurred, with average particle extinction of ~10 Mm−1. During this period, ammonium nitrate concentrations were lower by more than an order of magnitude, and contributions from coarse mass and soil to haze levels increased. VOC markers indicated that the high levels of observed pollutants in winter were likely from local sources related to oil and gas extraction activities. Implications: Elevated ground-level ozone and haze levels were observed at Dinosaur National Monument in winter. Haze episodes were dominated by ammonium nitrate, with 24-h averaged particle light extinction exceeding 100 Mm−1, reducing visual range near the surface to ~35 km. Despite elevated ammonium nitrate concentrations, additional gas-phase ammonia was available, such that any increase in NOx emissions in the region is likely to lead to even greater haze levels. [ABSTRACT FROM AUTHOR]

For the first time in the 40-year history of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN), a unique urban-to-rural transect of wet deposition monitoring stations was operated as part of the NTN in 2017 to quantify reactive inorganic nitrogen wet deposition for adjacent urban and rural, montane regions. The transect of NADP stations (sites) was used to collect continuous precipitation depth and weekly wet-deposition samples in the Denver – Boulder, Colorado, urban corridor. Gradients in reactive inorganic nitrogen (Nr) concentrations and wet deposition were identified along the transect, which included Rocky Mountain National Park. Back trajectory modeling and stable isotopes suggested contribution of agricultural ammonia (NH 3) to urban Nr wet deposition in Denver, but apportionment of wet-deposited Nr to agricultural versus urban mobile sources was not possible for this study. The results demonstrate the importance of multiple monitoring sites across an urban area in defining fine-scale geographic patterns in atmospheric deposition and its sources. Data from new sites located within 50 km of the urban area demonstrate that the urban influence does not extend as far as the inverse distance weighting would have suggested without such empirical monitoring data. It is important to determine the radius of influence of urban emissions and associated deposition on the interpolated deposition raster, which is constrained by a paucity of monitoring sites east of Denver. Unlabelled Image • First assessment of reactive inorganic nitrogen wet deposition in the Denver metropolitan area. • Nitrogen wet deposition varies with land surface elevation and population density. • Differences in nitrogen sources, transport and deposition evident over urban – montane transect. • Chemical data and air mass trajectories indicate nitrogen deposition seasonality. • Urban data improved spatially interpolated N atmospheric deposition maps. [ABSTRACT FROM AUTHOR]

Photochemical reactions of nitrate in snow release reactive nitrogen species via two channels, which produce (1) nitrogen dioxide (NO2) and hydroxyl radical (•OH) and (2) nitrite (NO2-) and oxygen atom (O(³P)). Quantum yields (Φ) for these channels are generally well characterized, except for channel 2 in ice. In this study, we quantify Φ(NO2-) in water ices and examine the impacts of pH and organic scavengers of •OH. Compared to solution results, we find that nitrite quantum yields in ice are more sensitive to pH and that •OH scavengers are less effective, although 2-propanol appears to work well. The temperature dependence (-30 to 25 °C) of Φ(NO2-) in samples containing 2-propanol is well described by a single regression line, ln(Φ(NO2-)) = -(1330 ± 100)(1/T(K)) + (0.09 ± 0.39). At -10 °C, the resulting quantum yield is 4.6 times larger than the previously reported (and recommended) value without an •OH scavenger. Although some reports suggest nitrite is a minor product from nitrate photolysis, based on our current and past results, rates of photoproduction of NO2- and NO2 are similar at room temperature, while NO2- production dominates at lower temperatures in solution and ice. [ABSTRACT FROM AUTHOR]

Photolysis of nitrate (NO3-) produces reactive nitrogen and oxygen species via three different channels, forming: (1) nitrogen dioxide (NO2) and hydroxyl radical (*OH), (2) nitrite (NO2-) and oxygen atom (O(³P)), and (3) peroxynitrite (ONOO-). These photoproducts are important oxidants and reactants in surface waters, atmospheric drops, and snowpacks. While the efficiency of the first channel, to form NO2, is well documented, a large range of values have been reported for the second channel, nitrite, above 300 nm. In part, this disagreement reflects secondary chemistry that can produce or destroy nitrite. In this study, we examine factors that influence nitrite production and find that pH, nitrate concentration, and the presence of an *OH scavenger can be important. We measure an average nitrite quantum yield (F(NO2-)) of (1.1 ± 0.2)% (313 nm, 50 µM nitrate, pH = 5), which is at the upper end of past measurements and an order of magnitude above the smallest--and most commonly cited--value reported for this channel. Nitrite production is often considered a very minor channel in nitrate photolysis, but our results indicate it is as important as the NO2 channel. In contrast, at 313 nm we observe no formation of peroxynitrite, corresponding to F(ONOO-) < 0.26%. [ABSTRACT FROM AUTHOR]

During the summer of 2012 the Hewlett Gulch and High Park wildfires burned an area of 400 km 2 northwest of Fort Collins, Colorado. These fires both came within 20 km of the Department of Atmospheric Science at Colorado State University, allowing for extensive measurements of smoke-impacted air masses over the course of several weeks. In total, smoke plumes were observed at the measurement site for approximately 125 h. During this time, measurements were made of multiple reactive nitrogen compounds, including gas phase species NH 3 , NO x , and HNO 3 , and particle phase species NO 3 − and NH 4 + , plus an additional, unspeciated reactive nitrogen component that is measured by high temperature conversion over a catalyst to NO. Concurrent measurements of CO, levoglucosan and PM 2.5 served to confirm the presence of smoke at the monitoring site. Significant enhancements were observed for all of the reactive nitrogen species measured in the plumes, except for NH 4 + which did not show enhancements, likely due to the fresh nature of the plume, the presence of sufficient regional ammonia to have already neutralized upwind sulfate, and the warm conditions of the summer measurement period which tend to limit ammonium nitrate formation. Excess mixing ratios for NH 3 and NO x relative to excess mixing ratios of CO in the smoke plumes, ΔNH 3 /ΔCO (ppb/ppb) and ΔNO x /ΔCO (ppb/ppb), were determined to be 0.027 ± 0.002 and 0.0057 ± 0.0007, respectively. These ratios suggest that smoldering combustion was the dominant source of smoke during our plume interceptions. Observations from prior relevant laboratory and field measurements of reactive nitrogen species are also briefly summarized to help create a more comprehensive picture of reactive nitrogen and fire. [ABSTRACT FROM AUTHOR]

Abstract: Increasing rates of nitrogen deposition are a concern in many protected ecosystems. Understanding the sources influencing these regions can be a challenge as there are often few observations available to understand the transport of key species. Several field campaigns were conducted in and around Rocky Mountain National Park (RMNP) from 2006 to 2009 to assess the impacts of various reactive nitrogen sources and regional meteorology on reactive nitrogen deposition. Measurements of ammonia, ammonium, nitric acid, and nitrate at ground-level sites across northern Colorado were used to examine spatial gradients in atmospheric reactive nitrogen concentrations, the influence of wind direction on reactive nitrogen transport in the regional atmosphere, and anthropogenic contributions to reactive nitrogen concentrations in RMNP. The highest concentrations of reduced nitrogen occurred on the eastern plains of Colorado while oxidized nitrogen concentrations were highest along the Front Range urban corridor. Both regions lie east of RMNP. Upslope (easterly) winds associated with mountain–valley wind patterns and larger, synoptic scale forcing, transport emissions from these sources westward up the eastern slope of the Rockies and into RMNP; the highest ammonium and nitrate concentrations in RMNP were clearly associated with this upslope transport pattern. Concentrations of key reactive nitrogen species east of the Continental Divide in RMNP were, on average, more than 50% higher than those observed at a background site located west of the park, further indicating large contributions from sources east of the park. These observations highlight the need to focus on controlling reactive nitrogen emissions east of the park as part of ongoing efforts to reduce reactive nitrogen deposition in RMNP. [Copyright &y& Elsevier]

Abstract: Relatively few measurements exist of cloud water composition over the open ocean, despite the importance of clouds as processors of atmospheric aerosols and trace gases in such regions. In order to improve understanding of cloud water composition in the southeast Pacific, and to determine the capacity of regional clouds for aqueous sulfate production, cloud water was collected aboard the NCAR C-130 during VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment) in October and November, 2008. A total of 72 cloud water samples were collected on 14 flights using the NCAR/CSU airborne cloud water collector. Cloud water samples were collected over a region extending from approximately 71–85° west and from approximately 18–30° south. Cloud water pH ranged from 2.9 to 7.2 with an average of 4.3. Samples were also analyzed for major inorganic ions, hydrogen peroxide, sulfur(IV), carboxylic and dicarboxylic acids, formaldehyde, total Fe and Mn, and total organic carbon. Sodium and chloride were the most abundant solutes, while organic matter was found to contribute only 12%, on average, of the total measured solute load. Cloud water and trace gas measurements aboard the aircraft were used to examine the importance of various oxidation pathways for in-cloud sulfate production. S(IV) oxidation by H2O2 was found to be the most important pathway and excess cloud water H2O2 concentrations revealed considerable potential for additional, rapid in-cloud sulfate production by this mechanism. S(IV) oxidation by ozone was important when cloud water pH climbed above 5.5 and was the fastest oxidation pathway 24% of the time. [ABSTRACT FROM AUTHOR]

The National Park Service is tasked with protecting the lands it oversees, including from impacts from air pollutants. While ozone is regularly monitored in many parks across the United States, precursors to ozone formation are not routinely measured. In this work we characterize volatile organic compounds (VOCs) at four national parks in the southwestern United States: Carlsbad Caverns (CAVE), Great Basin (GRBA), Grand Canyon (GRCA), and Joshua Tree (JOTR). Whole air samples were collected for VOC analysis for five months (mid-April through mid-September) in 2017. Samples were collected from 3 p.m. to 5 p.m. local time, corresponding approximately to the time of expected peak ozone concentrations, and were analyzed using gas chromatography for a variety of compounds including alkanes, alkenes, aromatics, biogenics, and alkyl nitrates. Among the four parks, the total measured VOC mixing ratio was greatest at CAVE, mostly due to an abundance of light alkanes (on average 94% of all VOCs measured) from oil and gas sources. VOC concentrations at the other three parks were similar to each other and approximately 7–10 times lower than at CAVE. While VOC sources varied across sites, VOC-OH reactivity was dominated by biogenic compounds at all sites except CAVE, which had similar contributions from biogenics and from light alkanes. JOTR had the highest levels of ozone, and the highest concentrations of biogenic VOCs among the parks studied. Back trajectory analysis suggests that the high levels of ozone occurred when air flow came from the west, in the direction of the greater Los Angeles area. The two parks most removed from oil and gas and urban sources, GRBA and GRCA, had the lowest concentrations of total VOCs, and were likely influenced by long range transport of pollutants. To better characterize source influences at the park with the highest VOC concentrations, intensive measurements were conducted in and around CAVE for one week in September 2017. These measurements showed an oil and gas influence throughout the region and indicated that the whole air samples collected over the five-month study did not capture the full range of VOC mixing ratios present at other times of the day. • VOC were measured at Carlsbad Caverns, Great Basin, Grand Canyon, and Joshua Tree National Parks. • CAVE had the highest levels of light alkanes, by approximately an order of magnitude. • VOC concentrations at the other three parks were similar to each other. • Measurements in and around CAVE showed an oil and gas influence. [ABSTRACT FROM AUTHOR]

Ambient reactive nitrogen is a mix of nitrogen-containing organic and inorganic compounds. These various compounds are found in both aerosol- and gas-phases with oxidized and reduced forms of nitrogen. Aerosol-phase reduced nitrogen is predominately thought to include ammonium and amines. In ambient samples, the ammonium concentration is routinely determined, but the contribution of amines is not. We developed a method to discretely measure amines from ambient aerosol samples. It employs ion chromatography using a Thermo Scientific IonPac Dionex CS-19 column with conductivity detection and a three-step separation using a methanesulfonic acid eluent. This method allows for the quantification of 18 different amines, including the series of methylamines and the different isomers of butylamine. Almost all amines quantifiable by this technique were measured regularly when applying this method to ambient filter samples collected in Rocky Mountain National Park (RMNP) and Greeley, CO. The sum of the amines was ~0.02 µg m−3 at both sites. This increased to 0.04 and 0.09 µg m−3 at RMNP and Greeley, respectively, at the same time they were impacted by smoke. Analysis of separate, fresh biomass burning source samples, however, suggests that smoke is likely a minor emission source of amines in most environments. [ABSTRACT FROM AUTHOR]

Carlsbad Caverns National Park (CAVE), located in southeastern New Mexico, experiences elevated ground-level ozone (O3) exceeding the National Ambient Air Quality Standard (NAAQS) of 70 ppbv. It is situated adjacent to the Permian Basin, one of the largest oil and gas (O&G) producing regions in the US. In 2019, the Carlsbad Caverns Air Quality Study (CarCavAQS) was conducted to examine impacts of different sources on ozone precursors, including nitrogen oxides (NOx) and volatile organic compounds (VOCs). Here, we use positive matrix factorization (PMF) analysis of speciated VOCs to characterize VOC sources at CAVE during the study. Seven factors were identified. Three factors composed largely of alkanes and aromatics with different lifetimes were attributed to O&G development and production activities. VOCs in these factors were typical of those emitted by O&G operations. Associated residence time analyses (RTA) indicated their contributions increased in the park during periods of transport from the Permian Basin. These O&G factors were the largest contributor to VOC reactivity with hydroxyl radicals (62%). Two PMF factors were rich in photochemically generated secondary VOCs; one factor contained species with shorter atmospheric lifetimes and one with species with longer lifetimes. RTA of the secondary factors suggested impacts of O&G emissions from regions farther upwind, such as Eagle Ford Shale and Barnett Shale formations. The last two factors were attributed to alkenes likely emitted from vehicles or other combustion sources in the Permian Basin and regional background VOCs, respectively. Implications: Carlsbad Caverns National Park experiences ground-level ozone exceeding the National Ambient Air Quality Standard. Volatile organic compounds are critical precursors to ozone formation. Measurements in the Park identify oil and gas production and development activities as the major contributors to volatile organic compounds. Emissions from the adjacent Permian Basin contributed to increases in primary species that enhanced local ozone formation. Observations of photochemically generated compounds indicate that ozone was also transported from shale formations and basins farther upwind. Therefore, emission reductions of volatile organic compounds from oil and gas activities are important for mitigating elevated O3 in the region. [ABSTRACT FROM AUTHOR]

Abstract Very few comparative studies on atmospheric dry and wet nitrogen (N) deposition have been conducted in agro-pastoral catchment areas. In an effort to fill this knowledge gap, three interconnected land use types (cropland, mountain pasture, and plain pasture) with six sampling sites (Chinese cropland (CC), Chinese mountain pasture (CM), Chinese plain pasture (CP) Mongolia cropland (MC), Mongolia mountain pasture (MM) and Mongolia plain pasture (MP)) were selected in the transboundary regions of northwest China and western Mongolia. During 16 months from May 2014 to September 2015 atmospheric and precipitation samples were collected to assess levels of nitrogen deposition. Cropland had the highest NO 3 −N (1.0 mg N L−1 in China and 1.2 mg N L−1 in Mongolia) and NH 4 +-N concentrations (1.6 mg N L−1 in China and 2.0 mg N L−1 in Mongolia) in our six sampling sites. The CC experienced the highest wet deposition (5.2 kg N ha−1 yr−1), followed by MM pasture (3.2 kg N ha−1 yr−1) while wet deposition in other sites ranged from 2.1 to 2.8 kg N ha−1 yr−1. The ambient concentrations of NH 3 , NO 2 and dry N deposition were significant higher in cropland than being other land use with 3.0, 3.8 and 1.2 μg N m−3 respectively in China and 2.2, 2.8 and 0.6 μg N m−3 respectively in Mongolia. The highest total N deposition was at CC (14.5 kg N ha−1 yr−1) and MC (9.4 kg N ha−1 yr−1), followed by the mountain pasture (5.8–6.8 kg N ha−1 yr−1), and the plains pasture (5.6–5.7 kg N ha−1 yr−1). Overall, agro-pastoral catchments areas had relatively low total N deposition compared to other studies, and cropland had the highest wet and dry N deposition than mountain and plain pastures, and dry N deposition absolutely dominated the percentage (64–70% in cropland and 53–64% in pasture areas) of total atmospheric N deposition. Comparing the total N deposition in China and Mongolia, we found the CC had 54% higher value than in MC while the reverse was true for mountain pastures Mongolia received 17% more N. Graphical abstract Image 1 Highlights • Passive samplers, PM 10 samplers and rain gauges equipments were used to quantify the nitrogen deposition in six typical land use types. • The cropland sites had significant higher NH 3 and NO 2 concentration and total N deposition than pasture areas in our research area. • Dry deposition absolutely dominated the percentage of total atmospheric N deposition in agro-pastoral catchment areas of China and Mongolia. • Nitrogen deposition in CC had 54% higher value than in MC while the reverse was true for mountain pastures Mongolia received 17% more N. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is estimated to have the second largest anthropogenic radiative forcing in earth‐systems models (ESMs), but there is significant uncertainty in its impact due to complex mixing with organics. Laboratory‐generated particles show that co‐mixed non‐absorbing material enhances absorption by BC by a factor of 2–3.5 as predicted by optical models. However, weak or no enhancements are often reported for field studies. The cause of lower‐than‐expected absorption is not well understood and implies a lower radiative impact of BC compared to how many ESMs currently treat aerosols. By analyzing BC aerosol particle‐by‐particle we reconcile observed and expected absorption for ambient smoke plumes varying in geographic origin, fuel types, burn conditions, atmospheric age and transport. Although particle‐by‐particle tracking is computationally prohibitive for sophisticated ESMs we show that realistic BC absorption is reliably estimated by bulk properties of the plume providing a suitable parameterization to constrain black carbon radiative forcing. Plain Language Summary: Particles that contain a mix of black carbon and organic aerosol will absorb more light due to the presence of the organics. The increase in absorption due to mixing is uncertain in ambient smoke plumes. In this work, we show that models overestimate black carbon absorption due to particle‐to‐particle differences in black carbon size and amount of organic coating. When smoke plumes remain in the lower troposphere and don't experience temperatures below freezing, black carbon absorption can be predicted by an empirical relationship related to the ratio of coating material to black carbon volume in the plume. This simple parameterization is suitable for incorporation into complex earth systems models to for determining the climate impact of black carbon. Key Points: Ambient smoke plumes exhibited absorption enhancement of black carbon (BC) aerosol, which has not always been reportedPlume BC absorption enhancement was accurately predicted with a particle‐by‐particle Mie model suggesting biases in other methodsParameterization of plume BC absorption enhancement is developed based on the ratio of coating volume to BC volume concentration [ABSTRACT FROM AUTHOR]

Changes in ecosystem function at Rocky Mountain National Park (RMNP) are occurring because of nitrogen deposition associated with emissions of nitrogen from sources in Colorado as well as other areas of the North American continent and beyond. Nitrogen species are in both reduced and oxidized forms. A year-long monitoring program was initiated to better understand their origins as well as the complex chemistry occurring during transport from source to receptor. Specifically, the goals of the study were to characterize the atmospheric concentrations of nitrogen species in gaseous, particulate, and aqueous phases in RMNP and to identify the emission sources of these various species. The apportionment strategy was designed to focus on differentiating between sources within and outside the state of Colorado and then further differentiate between sources along the Front Range of Colorado and the rest of Colorado. It was also desirous to identify the relative contributions to atmospheric nitrogen species from mobile sources, agricultural activities, and large and small point sources within the state of Colorado. The Particle Source Apportionment Technology (PSAT) module available in the chemical transport model, the Comprehensive Air quality Model with extensions (CAMx), is used to develop first-principle estimates of the contributions from different areas of North America. The CAMx_PSAT results are combined with measured species concentrations in a receptor modeling approach to develop final estimates of source apportionment of the various species' concentrations and deposition. [ABSTRACT FROM AUTHOR]

The Rocky Mountain Atmospheric Nitrogen and Sulfur (RoMANS II) study with field operations during November 2008 through November 2009 was designed to evaluate the composition and sources of reactive nitrogen in Rocky Mountain National Park, Colorado, USA. As part of RoMANS II, a mesoscale meteorological model was utilized to provide input for back trajectory and chemical transport models. Evaluation of the model's ability to capture important transport patterns in this region of complex terrain is discussed. Previous source-receptor studies of nitrogen in this region are also reviewed. Finally, results of several back trajectory analyses for RoMANS II are presented. The trajectory mass balance (TrMB) model, a receptor-based linear regression technique, was used to estimate mean source attributions of airborne ammonia concentrations during RoMANS II. Though ammonia concentrations are usually higher when there is transport from the east, the TrMB model estimates that, on average, areas to the west contribute a larger mean fraction of the ammonia. Possible reasons for this are discussed and include the greater frequency of westerly versus easterly winds, the possibility that ammonia is transported long distances as ammonium nitrate, and the difficulty of correctly modeling the transport winds in this area. [ABSTRACT FROM AUTHOR]

Changes in ecosystem function at Rocky Mountain National Park (RMNP) are occurring because of emissions of nitrogen and sulfate species along the Front Range of the Colorado Rocky Mountains, as well as sources farther east and west. The nitrogen compounds include both oxidized and reduced nitrogen. A year-long monitoring program of various oxidized and reduced nitrogen species was initiated to better understand their origins as well as the complex chemistry occurring during transport from source to receptor. Specifically, the goals of the study were to characterize the atmospheric concentrations of nitrogen species in gaseous, particulate, and aqueous phases (precipitation and clouds) along the east and west sides of the Continental Divide; identify the relative contributions to atmospheric nitrogen species in RMNP from within and outside of the state of Colorado; identify the relative contributions to atmospheric nitrogen species in RMNP from emission sources along the Colorado Front Range versus other areas within Colorado; and identify the relative contributions to atmospheric nitrogen species from mobile sources, agricultural activities, and large and small point sources within the state of Colorado. Measured ammonia concentrations are combined with modeled releases of conservative tracers from ammonia source regions around the United States to apportion ammonia to its respective sources, using receptor modeling tools. [ABSTRACT FROM AUTHOR]

This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange in natural ecosystems of North America are then summarized, focusing on the U.S. and Canada. Drawing on this synthesis, research needed to address the incompleteness of dry deposition budgets, more fully characterize temporal and geographical variability of fluxes, and better understand air-surface exchange processes is identified. Our assessment points to several data and knowledge gaps that must be addressed to advance dry deposition budgets and air-surface exchange modeling for North American ecosystems. For example, recent studies of particulate (NO 3 −) and gaseous (NO x , HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental framework of unidirectional flux from the atmosphere to the surface employed in most deposition models. Measurements in forest ecosystems document the importance of in-canopy chemical processes in regulating the net flux between the atmosphere and biosphere, which can result in net loss from the canopy. These results emphasize the need for studies to quantify within- and near-canopy sources and sinks of the full suite of components of the Nr chemical system under study (e.g., NO y or HNO 3 -NH 3 -NH 4 NO 3). With respect to specific ecosystems and geographical locations, additional flux measurements are needed particularly in agricultural regions (NH 3), coastal zones (NO 3 − and organic N), and arid ecosystems and along urban to rural gradients (NO 2). Measurements that investigate non-stomatal exchange processes (e.g., deposition to wet surfaces) and the biogeochemical drivers of bidirectional exchange (e.g., NH 3) are considered high priority. Establishment of long-term sites for process level measurements of reactive chemical fluxes should be viewed as a high priority long-term endeavor of the atmospheric chemistry and ecological communities. Unlabelled Image • Dry and total reactive nitrogen (Nr) deposition budgets for the continental U.S. are presented. • Analytical techniques and micrometeorological flux measurement methods for Nr are reviewed. • Measurements of Nr air-surface exchange in North American ecosystems are synthesized. • Long-term measurement sites for reactive chemical fluxes are needed across North America. • Better understanding of the influence of in-canopy chemistry on canopy-scale fluxes is needed. [ABSTRACT FROM AUTHOR]