Modeling the Oxygen Isotope Anomaly (Δ17O) of Reactive Nitrogen in the Community Multiscale Air Quality Model: Insights into Nitrogen Oxide Chemistry in the Northeastern United States.

Atmospheric nitrate, including nitric acid (HNO3), particulate nitrate (pNO3), and organic nitrate (RONO2), is a key atmosphere component with implications for air quality, nutrient deposition, and climate. However, accurately representing atmospheric nitrate concentrations within atmospheric chemistry models is a persistent challenge. A contributing factor to this challenge is the intricate chemical transformations involving HNO3 formation, which can be difficult for models to replicate. Here, we present a novel model framework that utilizes the oxygen stable isotope anomaly (Δ17O) to quantitatively depict ozone (O3) involvement in precursor nitrogen oxides N O x = N O + N O 2 photochemical cycling and HNO3 formation. This framework has been integrated into the US EPA Community Multiscale Air Quality (CMAQ) modeling system to facilitate a comprehensive assessment of NO x oxidation and HNO3 formation. In application across the northeastern US, the model Δ17O compares well with recently conducted diurnal Δ17O(NO2) and spatiotemporal Δ17O(HNO3) observations, with a root mean square error between model and observations of 2.6 ‰ for Δ17O(HNO3). The model indicates the major formation pathways of annual HNO3 production within the northeastern US are NO+OH (46 %), N2O5 hydrolysis (34 %), and organic nitrate hydrolysis (12 %). This model can evaluate NO x chemistry in CMAQ in future air quality and deposition studies involving reactive nitrogen.

Solid Phase Extraction Methodology for Robust Isotope Analysis of Atmospheric Ammonium.

The stable nitrogen isotope composition (δ15N) of atmospheric ammonia (NH3) and ammonium (NH4+) has emerged as a potent tool for improving our understanding of the atmospheric burden of reduced nitrogen. However, current chemical oxidation methodologies commonly utilized for characterizing δ15N values of NH4+ samples have been found to lead to low precision for low concentration (i.e., < 5 µmol L-1) samples and often suffer from matrix interferences. Here, we present an analytical methodology to extract and concentrate NH4+ from samples through use of a sample pretreatment step using a solid phase extraction technique involving cation exchange resins. Laboratory control tests indicated that 0.4 g of cation exchange resin (Biorad AG-50W) and 10 mL of 4 M sodium chloride extraction solution enabled the complete capture and removal of NH4+. Using this sample pretreatment methodology, we obtained accurate and precise δ15N values for NH4+ reference materials and an in-house quality control sample at concentrations as low as 1.0 µM. Additionally, the sample pretreatment methodology was evaluated using atmospheric aerosol samples previously measured for δ15N-NH4+ (from Changdao Island, China), which indicated an excellent δ15N-NH4+ match between sample pretreatment and no treatment (y = (0.98 ± 0.05)x + (0.11 ± 0.6), R2 = 0.99). Further, this methodology successfully extracted NH4+ from aerosol samples and separated it from present matrix effects (samples collected from Oahu, Hawaii; pooled standard deviation δ15N-NH4+ = ± 0.5‰,n = 16 paired samples) that without pretreatment originally failed to quantitatively oxidize to nitrite for subsequent δ15N isotope analysis. Thus, we recommend applying this sample pretreatment step for all environmental NH4+ samples to ensure accurate and precise δ15N measurement.

Nitrogen isotopic characteristics of aerosol ammonium in a Chinese megacity indicate the reduction from vehicle emissions during the lockdown period.

The role of agricultural versus vehicle emissions in urban atmospheric ammonia (NH3) remains unclear. The lockdown due to the outbreak of COVID-19 provided an opportunity to assess the role of source emissions on urban NH3. Concentrations and δ15N of aerosol ammonium (NH4+) were measured before (autumn in 2017) and during the lockdown (summer, autumn, and winter in 2020), and source contributions were quantified using SIAR. Despite the insignificant decrease in NH4+ concentrations, significantly lower δ15N-NH4+ was found in 2020 (0.6 ± 1.0‰ in PM2.5 and 1.4 ± 2.1‰ in PM10) than in 2017 (15.2 ± 6.7‰ in PM2.5), which indicates the NH3 from vehicle emissions has decreased by∼50% during the lockdown while other source emissions are less affected. Moreover, a reversed seasonal pattern of δ15N-NH4+ during the lockdown in Changsha has been revealed compared to previous urban studies, which can be explained by the dominant effect of non-fossil fuel emissions due to the reductions of vehicle emissions during the lockdown period. Our results highlight the effects of lockdown on aerosol δ15N-NH4+ and the importance of vehicle emissions to urban atmospheric NH3, providing conclusive evidence that reducing vehicle NH3 emissions could be an effective strategy to reduce PM2.5 in Chinese megacities.

Triple oxygen stable isotope analysis of nitrite measured using continuous flow isotope ratio mass spectrometry.

Oxygen stable isotopes (i.e., 16O, 17O, 18O) of nitrite (NO2-) are useful for investigating chemical processes and sources contributing to this important environmental contaminant and nutrient. However, it remains difficult to quantify the oxygen isotope compositions of NO2- due to the lack of internationally recognized NO2- reference materials with a well-known Δ(17O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ(17O) of NO2- by reducing nitrate (NO3-) materials with internationally recognized Δ(17O) values to NO2- using activated cadmium catalyzed by chloride in a basic solution while conserving Δ(17O). The NO3- reference materials reduced to NO2- and sample NO2- unknowns are converted to N2O using sodium azide/acetic acid reagent and decomposed to O2 by passing through a heated gold tube and introduced into a continuous flow isotope ratio mass spectrometer for analysis at m/z 32, 33, and 34 for Δ(17O) quantification. The adapted method involves the following main points:•NO3- reference materials with internationally recognized oxygen isotope composition are reduced to NO2- under high pH conditions that conserve Δ(17O) values.•The NO3- reference materials reduced to NO2- and sample NO2- with unknown Δ(17O) values are reduced to N2O using chemical methods involving sodium azide/acetic acid.•The product N2O is extracted, purified, decomposed to O2, and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ(17O) quantification. The Δ(17O) of NO2- samples are calibrated with respect to the NO3- reference materials with known Δ(17O) values.

Long-term trends in inorganic aerosol chemical composition and chemistry at an urban and rural site in the northeastern US.

Atmospheric nitrate and sulfate are major inorganic particulate matter components that impact human and ecosystem health and air quality. Over the last several decades, emissions of the precursor gases, nitrogen oxides (NOx = NO + NO2) and sulfur dioxide (SO2), have dramatically decreased in the US in response to federal regulations. However, the response in concentrations of particulate nitrate (pNO3) and sulfate (pSO4) have not followed predictions due to complex non-linear chemistry feedbacks that may differ amongst environments (i.e., urban vs. rural). In this study, we explored the long-term response of particle chemistry for urban and rural environments in southern New England, a region historically impacted by NOx and SO2 emissions. Particulate matter (PM10) samples collected via the same method from 2005 to 2015 at urban and rural locations in Rhode Island were analyzed for their major inorganic components, and air mass trajectories and statistical analysis were used to identify source regions over time. Our results indicated a significant urban-rural aerosol chemical composition gradient for sampling locations within 40 km. Over time, as anthropogenic influences have decreased, the relative contribution of marine and crustal sources has increased greatly, impacting fine and coarse particle chemistry in recent years. Total mass concentrations of chemical species, particularly anthropogenic pSO4 and particulate ammonium (pNH4), have shown dramatic decreases over the ten years at both the urban and rural sites; however, pNO3 concentration increased by 95 % and 57 % in the urban and rural sites, respectively, despite significant NOx emission reductions. Our results demonstrate that changes in chemical mechanisms due to the decrease in SO2 emissions contributed to decreases in pNH4, along with enhanced pNO3 concentration. Furthermore, the change in SO2 emissions has significantly impacted the atmospheric lifetime and transport distance of pNH4, favoring more localized contributions in recent years.

Collection of Nitrogen Dioxide for Nitrogen and Oxygen Isotope Determination─Laboratory and Environmental Chamber Evaluation.

The family of atmospheric oxides of nitrogen, NOy (e.g., nitrogen oxides (NOx) + nitric acid (HNO3) + nitrous acid (HONO) + peroxyacetyl nitrate (PAN) + particulate nitrate (pNO3-) + other), have an influential role in atmospheric chemistry, climate, and the environment. The nitrogen (δ15N) and oxygen (δ18O and Δ17O) stable isotopes of NOy are novel tools for potentially tracking emission sources and quantifying oxidation chemistry. However, there is a lack of well-established methods, particularly for speciated gas-phase components of NOy, to accurately quantify δ15N, δ18O, and Δ17O. This work presents controlled laboratory experiments and complex chamber α-pinene/NOx oxidation experiments of a sampling apparatus constructed for the simultaneous capture of multiple NOy species for isotope analysis using a series of coated denuders, with a focus on nitrogen dioxide (NO2•). The laboratory tests indicate complete NO2• capture for the targeted concentration of 15 ppbv for at least 24 h collections at 10 liters per minute, with δ15N and δ18O precisions of ±1.3‰ and 1.0‰, respectively, and minimal (2.2% ± 0.1%) NO2• collection on upstream denuders utilized for the capture of HNO3 and other acidic gases. The multispecies NOy collection system showed excellent concentration correlations with online instrumentation for both HNO3 and NO2• and isotope reproducibility of ±1.7‰, ±1.8‰, and ±0.7‰ for δ15N, δ18O, and Δ17O, respectively, for replicate experiments and highly time-resolved collections. This work demonstrates a new method that can enable the simultaneous collection of HNO3 and NO2• for accurate quantification of concentration and isotopic composition.

Atmospheric Particles Are Major Sources of Aged Anthropogenic Organic Carbon in Marginal Seas.

Deposition of atmospheric particulates is a major pathway for transporting materials from land to the ocean, with important implications for climate and nutrient cycling in the ocean. Here, we report the results of year-round measurements of particulate organic carbon (POC) and black carbon (BC) in atmospheric aerosols collected on Tuoji Island in the coastal Bohai-Yellow Sea of China (2019-2020) and during a cruise in the western North Pacific. Aerosol POC contents ranged from 1.9 to 11.9%; isotope values ranged from -18.8 to -29.0‰ for δ13C and -150 to -892‰ for Δ14C, corresponding to 14C ages of 1,235 to 17,780 years before present (BP). Mass balance calculations indicated that fossil carbon contributed 19-66% of the POC, with highest values in winter. BC produced from fossil fuel combustion accounted for 18-54% of the POC. "Old" BC (mean 6,238 ± 740 yr BP) was the major contributor to POC, and the old ages of aerosol POC were consistent with the 14C ages of total OC preserved in surface sediments of the Bohai-Yellow Sea and East China Sea. We conclude that atmospheric deposition is an important source of aged OC sequestered in marginal sea sediments and thus represents an important sink for carbon dioxide from the atmosphere.

Atmospheric nitrate formation pathways in urban and rural atmosphere of Northeast China: Implications for complicated anthropogenic effects.

Effects of human activities on atmospheric nitrate (NO3-) formation remain unclear, though the knowledge is critical for improving atmospheric chemistry models and nitrogen deposition reduction strategies. A potentially useful way to explore this is to compare NO3- oxidation processes in urban and rural atmospheres based upon the oxygen stable isotope composition of NO3- (Δ17O-NO3-). Here we compared the Δ17O-NO3- from three-years of daily-based bulk deposition in urban (Shenyang) and forested rural sites (Qingyuan) in northeast China and quantified the relative contributions of different formation pathways based on the SIAR model. Our results showed that the Δ17O in Qiangyuan (26.2 ± 3.3‰) is significantly higher (p < 0.001) than in Shenyang (24.0 ± 4.0‰), and significantly higher in winter (Shenyang: 26.1 ± 6.7‰, Qingyuan: 29.6 ± 2.5‰) than in summer (Shenyang: 22.7 ± 2.9‰, Qingyuan: 23.8 ± 2.4‰) in both sites. The lower values in the urban site are linked with conditions that favored a higher relative contribution of nitrogen dioxide reaction with OH pathway (0.76-0.91) than in rural site (0.47-0.62), which should be induced by different levels of human activities in the two sites. The seasonal variations of Δ17O-NO3- in both sites are explained by a higher relative contribution of ozone-mediated oxidation chemistry and unfavorable conditions for the OH pathway during winter relative to summer, which is affected by human activities and seasonal meteorological condition change. Based on Δ17O, wintertime conditions led to a contribution of O3 related pathways (NO3 + DMS/HC and N2O5 hydrolysis) of 0.63 in Qingyuan and 0.42 in Shenyang, while summertime conditions led to 0.15 in Qingyuan and 0.05 in Shenyang. Our comparative study on Δ17O-NO3- between urban and rural sites reveals different anthropogenic effects on nitrate formation processes on spatial and temporal scales, illustrating different responses of reactive nitrogen chemistry to changes in human activities.

Persistent Nitrate in Alpine Waters with Changing Atmospheric Deposition and Warming Trends.

Nitrate concentrations in high-elevation lakes of the Colorado Front Range remain elevated despite declining trends in atmospherically deposited nitrate since 2000. The current source of this elevated nitrate in surface waters remains elusive, given shifts in additional nitrogen sources via glacial inputs and atmospheric ammonium deposition. We present the complete isotopic composition of nitrate (δ15N, δ18O, and Δ17O) from a suite of nitrate-bearing source waters collected during the summers of 2017-2018 from two alpine ecosystems to constrain the provenance of elevated nitrate in surface waters during the summer open-water season. The results indicate a consistent contribution of uncycled atmospheric nitrate throughout the summer (13-23%) to alpine lakes, despite seasonal changes in source water inputs. The balance of nitrate (as high as 87% in late summer) is likely from nitrate production within the catchment via nitrification of reduced nitrogen sources (e.g., thawed soil organic matter and ammonium deposition) and released with rock glacier meltwater. The role of microbially produced nitrate has become increasingly important over time based on historical surface water samples from the mid-90s to present, a trend coincident with increasing ammonium deposition to alpine systems.

Speciated Collection of Nitric Acid and Fine Particulate Nitrate for Nitrogen and Oxygen Stable Isotope Determination.

Stable isotopic composition of atmospheric nitrate (nitric acid (HNO3) + particulate nitrate (pNO3-)) provides a higher-order dimensional analysis of critical atmospheric components, enabling a process-level understanding of precursor emissions, oxidation chemistry, aerosol acidity, and depositional patterns. Current methods have not been evaluated for their ability to accurately speciate and determine nitrogen (δ15N) and oxygen (δ18O and Δ17O) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO3(g) and pNO3- for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or Na2CO3 solutions were used to collect HNO3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO3(g), with the Na2CO3 solution demonstrating a higher operative capacity (>1470 µg of HNO3; n = 25) compared to the NaCl solution (∼750 µg of HNO3; n = 25). The precision values for laboratory-tested HNO3(g) collections are ±0.6‰ and ±1.2‰ for δ15N and δ18O for the NaCl solution and ± 0.8‰ and ±1.2‰ for the Na2CO3 solution. Replicate (urban) samples indicate that the Na2CO3 solution is significantly less selective for HNO3(g) collection than the NaCl solution. Nylon filters were found to collect efficiently and retain laboratory-generated NaNO3 and NH4NO3 particles, with maximum standard deviations for δ15N and δ18O of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ15N and δ18O of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO3(g) and pNO3- for isotopic measurements would best utilize the NaCl solution and Nylon filters.

Isotopic evidence that recent agriculture overprints climate variability in nitrogen deposition to the Tibetan Plateau.

The stable isotopes of nitrogen in nitrate archived in polar ice have been interpreted as reflecting a shift in reactive nitrogen sources or changes in atmospheric chemical reactivity. Here, we present a novel concentration and isotopic record of nitrate (δ15N-NO3-) from a central Tibetan Plateau ice core over the last ~200 years. We find that nitrate concentration increased from 6.0 ± 2.3 µeq/L (mean ± 1σ) in the preindustrial period (prior to 1900s) to 7.3 ± 2.7 µeq/L in post-1950. Over the same time period, the δ15N-NO3- decreased from 8.7 ± 3.7‰ to 4.2 ± 3.1‰, with much larger interannual variation in δ15N-NO3- during the preindustrial period. We present a useful framework for quantifying the sensitivity of the isotopic composition of atmospheric nitrate to changes in both sources and chemistry (gas and aerosol phase). After 1950, nitrogen deposition is primarily driven by fertilizer use, leading to significant increases in concentration and decreases in δ15N-NO3-. The large interannual variability of ice core δ15N-NO3- in the preindustrial reflects natural processes, namely the El Niño Southern Oscillation (ENSO) and dust events. Our results highlight a new connection between the nitrogen cycle and ENSO, and the overprinting of natural climate signals by recent anthropogenic increases in reactive nitrogen release.

Soil Carbon and Nitrogen Dynamics in Two Agricultural Soils Amended with Manure-Derived Biochar.

Biochar has been promoted as a means to sequester C and improve soil quality. Biochar produced from agricultural waste streams and recycled as a soil amendment also provides a strategy for improved nutrient management in agricultural systems. The effects of biochar amendment on soil C and N cycling remain poorly constrained. This study aimed to examine the effects of biochar on soil C and N storage, N mineralization, and soil physiochemical properties. Soils were collected from a field experiment in which biochar derived from poultry manure was applied for 2 yr in two croplands differing in soil texture (sandy and silt-loam). Samples from biochar-amended and control soils were physically fractionated to separate water-stable soil aggregates and analyzed for C and N. Biochar amendments increased total soil C by 16 (sandy soil) and 30% (silt-loam soil). These increases were observed in aggregate size classes associated with short-term C and N storage in silt-loam soils and intermediate-term C and N storage in sandy soils. Net N mineralization rates observed in a short-term incubation were small or negative (1.79 and -24.7 µg N g soil for sandy and silt-loam soils, respectively), indicating little or no new N mineralization from biochar over short timescales. Biochar amendment had a positive impact on cation exchange capacity at both sites, increasing it by 7 and 11% in the silt-loam soil and sandy soil, respectively. These results suggest that biochar amendments to cropping systems can improve the capacity of soil to retain nutrients and store C and N.

Selective Collection of Particulate Ammonium for Nitrogen Isotopic Characterization Using a Denuder-Filter Pack Sampling Device.

Nitrogen stable isotope analysis (δ15N) of particulate ammonium (NH4+) may provide additional constraints on this critical component of fine particulate matter; however, no previous collection method has been verified for its ability to accurately and precisely characterize δ15N(NH4+). This is a critical point due to the difficulty of quantitative NH4+ collection and possible sampling artifacts. Here, we report on δ15N(NH4+) precision using an established denuder-filter pack combination with two filter configurations including (1) a nylon filter plus an acid-impregnated cellulose filter and (2) an acid-impregnated glass fiber filter for NH4+ collection in both laboratory-controlled environments and ambient air samples. Laboratory NH4+ were generated from the nebulization of ammonium salt solutions and collected using a filter pack sampling train for off-line concentration and isotopic measurement. Quantitative collection of NH4+ was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δ15N(NH4+) precision of ±0.9‰ (1σ; n = 24) and ±0.6‰ ( n = 9) for the nylon plus citric acid impregnated cellulose filter and for the citric acid impregnated glass fiber filter, respectively. Field sample reproducibility was assessed from 24 h collected side-by-side samples and indicated δ15N(NH4+) to be reproducible within 1.1‰, consistent with the laboratory findings. This work represents the first established method for speciated NH4+ collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.

Development of a Fuel-Based Oil and Gas Inventory of Nitrogen Oxides Emissions.

In this study, we develop an alternative Fuel-based Oil and Gas inventory (FOG) of nitrogen oxides (NO x) from oil and gas production using publicly available fuel use records and emission factors reported in the literature. FOG is compared with the Environmental Protection Agency's 2014 National Emissions Inventory (NEI) and with new top-down estimates of NO x emissions derived from aircraft and ground-based field measurement campaigns. Compared to our top-down estimates derived in four oil and gas basins (Uinta, UT, Haynesville, TX/LA, Marcellus, PA, and Fayetteville, AR), the NEI overestimates NO x by over a factor of 2 in three out of four basins, while FOG is generally consistent with atmospheric observations. Challenges in estimating oil and gas engine activity, rather than uncertainties in NO x emission factors, may explain gaps between the NEI and top-down emission estimates. Lastly, we find a consistent relationship between reactive odd nitrogen species (NO y) and ambient methane (CH4) across basins with different geological characteristics and in different stages of production. Future work could leverage this relationship as an additional constraint on CH4 emissions from oil and gas basins.

Collection of Ammonia for High Time-Resolved Nitrogen Isotopic Characterization Utilizing an Acid-Coated Honeycomb Denuder.

Nitrogen stable isotope analysis (δ15N) of ammonia (NH3) has shown potential to be a useful tool for characterizing emission sources and sink processes. However, to properly evaluate NH3 emission sources and sink processes under ambient conditions, it is necessary to collect and characterize the chemical speciation between NH3 and particulate ammonium (p-NH4+), together referred to as NH x. Current NH3 collection methods have not been verified for their ability to accurately characterize δ15N-NH3 and/or provide necessary chemical speciation (i.e., δ15N-NH3 and δ15N-NH4+). Here, we report on the suitability of an established collection device that can provide NH x speciation, an acid-coated (2% citric acid (w/v) + 1% glycerol (w/v) in 80:20 methanol to water solution) honeycomb denuder (HCD) with a downstream filter pack housed in the ChemComb Speciation Cartridge (CCSC), for characterizing δ15N-NH3 under a variety of laboratory-controlled conditions and field collections. The collection method was tested under varying NH3 concentration, relative humidity, temperature, and collection time at a flow rate of 10 L per minute (LPM). The acid-coated HCD collection device and subsequent chemical processing for δ15N-NH3 analysis is found to have excellent accuracy and precision of ±1.6‰ (2σ), with an operative capacity of ∼400 µg of collected NH3 for concentrations ≤207 ppbv. This work presents the first laboratory verified method for δ15N-NH3 analysis and will be useful in future air quality studies.

Nitrogen cycling during secondary succession in Atlantic Forest of Bahia, Brazil.

Carbon accumulation in tropical secondary forests may be limited in part by nitrogen (N) availability, but changes in N during tropical forest succession have rarely been quantified. We explored N cycle dynamics across a chronosequence of secondary tropical forests in the Mata Atlântica of Bahia, Brazil in order to understand how quickly the N cycle recuperates. We hypothesized that N fixation would decline over the course of succession as N availability and N gaseous losses increased. We measured N fixation, KCl-extractable N, net mineralization and nitrification, resin-strip sorbed N, gaseous N emissions and the soil δ15N in stands that were 20, 35, 50, and > 50 years old. Contrary to our initial hypothesis, we found no significant differences between stand ages in any measured variable. Our findings suggest that secondary forests in this region of the Atlantic forest reached pre-disturbance N cycling dynamics after just 20 years of succession. This result contrasts with previous study in the Amazon, where the N cycle recovered slowly after abandonment from pasture reaching pre-disturbance N cycling levels after ~50 years of succession. Our results suggest the pace of the N cycle, and perhaps tropical secondary forest, recovery, may vary regionally.

Collection Method for Isotopic Analysis of Gaseous Nitrous Acid.

The sources and chemistry of gaseous nitrous acid (HONO) in the environment are of great interest. HONO is a major source of atmospheric hydroxyl radical (OH), which impacts air quality and climate. HONO is also a major indoor pollutant that threatens human health. However, the large uncertainty of HONO sources and chemistry hinders an accurate prediction of the OH budget. Isotopic analysis of HONO may provide a tool for tracking the sources and chemistry of HONO. In this study, a modified annular denuder system (ADS) was developed to quantitatively capture HONO for offline nitrogen and oxygen isotopic analysis (δ15N and δ18O) using the denitrifier method. The ADS method was tested using laboratory generated HONO (400 ppbv to 1 ppmv) and validated by parallel HONO collection with a standard, basic impinger (BI) method. The ADS system shows complete capture of HONO without isotopic fractionation. The uncertainty (1σ) based on repeated measurements across the entire analytical procedure is 0.6‰ for δ15N and 0.5‰ for δ18O. The ADS method was also tested in roadside collections of ambient HONO (0.4-1.3 ppbv) for isotopic analysis and was found to be robust for low concentration collections over 3 and 12 h collection times. In order to ensure ability to use this method in the laboratory and in the field, storage conditions for the collected HONO samples were tested and samples can be stored with consistent δ15N and δ18O for 60 days. This method enables future work to utilize the isotopic composition of HONO for studying HONO chemical formation pathways, as well as atmospheric sources and chemistry.