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Air quality management commonly aims to mitigate nitrogen oxide (NO x ) emissions from combustion, reducing ozone (O 3 ) and particulate matter (PM) pollution. Despite such ongoing efforts, regulations have recently proven ineffective in rural areas like the Salton Sea Air Basin of Southern California, which routinely violates O 3 and PM air quality standards. With over $2 billion in annual agricultural sales and low population density, air quality in the region is likely influenced by the year-round farming activity. We conducted a source apportionment of NO x (an important precursor to both O 3 and PM) using nitrogen stable isotopes of ambient NO 2 , which revealed a significant contribution from soil-emitted NO x to the regional budget. The soil source strength was estimated based on the mean δ 15 N-NO x from each emission category in the California Air Resources Board's NO x inventory. Our annual average soil emission estimate for the air basin was 11.4 ± 4 tons/d, representing ~ 30% of the extant NO x inventory, 10× larger than the state's inventory for soil emissions. Unconstrained environmental factors such as nutrient availability, soil moisture, and temperature have a first-order impact on soil NO x production in this agriculturally intensive region, with fertilization and irrigation practices likely driving most of the emissions variability. Without spatially and temporally accurate data on fertilizer application rates and irrigation schedules, it is difficult to determine the direct impacts that these variations have on our observations. Nevertheless, comparative analysis with previous studies indicates that soil NO x emissions in the Imperial Valley are likely underrepresented in current inventories, highlighting the need for more detailed and localized observational data to constrain the sizeable and variable emissions from these arid, agricultural soils., Competing Interests: Declarations. Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

The introduction of three-way catalytic converter (TWC) to meet stringent vehicular NO x emission standards worldwide has led to an unintended consequence of vehicle-derived ammonia (NH 3 ) emission, which might degrade air quality and affect human health, especially in urban areas. The nitrogen stable isotope composition of NH 3 (δ 15 N-NH 3 ) may be a useful tool to trace NH 3 sources, but the isotopic signature of vehicle-emitted NH 3 is lacking. Here we report the δ 15 N-NH 3 measured from tailpipe exhausts collected directly from 19 different vehicles equipped with TWC using "grab" sample technique optimized to avoid isotopic fractionation. We found a large of range of NH 3 concentration (from 1 to 37 ppm, x̄ ± 1σ = 10 ± 8 ppm) and δ 15 N value (from -38.1 ‰ to 49.0 ‰, x̄ ± 1σ = 1.2 ± 20.9 ‰) for our samples (n = 57). Our results indicate a correlation between δ 15 N-NH 3 and NH 3 concentrations (i.e. δ 15 N-NH 3 (‰) = 16.9(± 2.2)In(NH 3 concentration; ppm) - 31.7(± 4.7) (R 2  = 0.53, P < 0.01) that is associated with the catalyst temperature, which is generally agree with the results of theoretical calculation. After consideration of the vehicle emission evolution, dominant driving condition and tunnel δ 15 N-NH 3 variation, a representative δ 15 N-NH 3 value (mean min max ) of vehicle source is identified as 0.8000 -2.2000 5.2000 , which is distinct from (larger than) other NH 3 volatilization-related emission sources. Using our newly measured δ 15 N values and Bayesian mixing model, we approximate the overall contribution of vehicles to ambient NH 3 before, during, and after an international event in Beijing was 14.1 %, 7.9 %, and 21.5 %, respectively, which agree well with the course of traffic restriction. Collectively, our findings demonstrate the importance of using vehicle-emitted δ 15 N-NH 3 to quantify vehicular NH 3 contribution in urban atmospheres., (Copyright © 2024. Published by Elsevier B.V.)

Atmospheric nitrate, including nitric acid (HNO 3 ), particulate nitrate (pNO 3 ), and organic nitrate (RONO 2 ), 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 HNO 3 formation, which can be difficult for models to replicate. Here, we present a novel model framework that utilizes the oxygen stable isotope anomaly (Δ 17 O) to quantitatively depict ozone (O 3 ) involvement in precursor nitrogen oxides N O x = N O + N O 2 photochemical cycling and HNO 3 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 HNO 3 formation. In application across the northeastern US, the model Δ 17 O compares well with recently conducted diurnal Δ 17 O(NO 2 ) and spatiotemporal Δ 17 O(HNO 3 ) observations, with a root mean square error between model and observations of 2.6 ‰ for Δ 17 O(HNO 3 ). The model indicates the major formation pathways of annual HNO 3 production within the northeastern US are NO+OH (46 %), N 2 O 5 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.

The role of agricultural versus vehicle emissions in urban atmospheric ammonia (NH 3 ) remains unclear. The lockdown due to the outbreak of COVID-19 provided an opportunity to assess the role of source emissions on urban NH 3 . Concentrations and δ 15 N of aerosol ammonium (NH 4 + ) 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 NH 4 + concentrations, significantly lower δ 15 N-NH 4 + was found in 2020 (0.6 ± 1.0‰ in PM 2.5 and 1.4 ± 2.1‰ in PM 10 ) than in 2017 (15.2 ± 6.7‰ in PM 2.5 ), which indicates the NH 3 from vehicle emissions has decreased by∼50% during the lockdown while other source emissions are less affected. Moreover, a reversed seasonal pattern of δ 15 N-NH 4 + 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 δ 15 N-NH 4 + and the importance of vehicle emissions to urban atmospheric NH 3 , providing conclusive evidence that reducing vehicle NH 3 emissions could be an effective strategy to reduce PM 2.5 in Chinese megacities., Competing Interests: Declaration of competing interest The authors declare no competing interests., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The stable nitrogen isotope composition (δ 15 N) of atmospheric ammonia (NH 3 ) and ammonium (NH 4 + ) 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 δ 15 N values of NH 4 + 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 NH 4 + 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 NH 4 + . Using this sample pretreatment methodology, we obtained accurate and precise δ 15 N values for NH 4 + 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 δ 15 N-NH 4 + (from Changdao Island, China), which indicated an excellent δ 15 N-NH 4 + match between sample pretreatment and no treatment ( y = (0.98 ± 0.05) x + (0.11 ± 0.6), R 2 = 0.99). Further, this methodology successfully extracted NH 4 + from aerosol samples and separated it from present matrix effects (samples collected from Oahu, Hawaii; pooled standard deviation δ 15 N-NH 4 + = ± 0.5‰, n = 16 paired samples) that without pretreatment originally failed to quantitatively oxidize to nitrite for subsequent δ 15 N isotope analysis. Thus, we recommend applying this sample pretreatment step for all environmental NH 4 + samples to ensure accurate and precise δ 15 N measurement., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Ammonia (NH 3 ) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NH x ). Accurately quantifying its contribution to regional atmospheric NH x deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ 15 N) is a promising indicator to trace atmospheric NH x sources, presupposing a reliable nitrogen isotopic signature of NH 3 emission sources. To obtain more specific seasonal δ 15 N values of soil NH 3 volatilization for reliable regional seasonal NH 3 source partitioning, we utilized an active dynamic sampling technique to measure the δ 15 N-NH 3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ 15 N-NH 3 variations were related to the main NH 3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ 15 N variation in soil-volatilized NH 3 could result in an overestimate by up to absolute 38% for agricultural volatile NH 3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ 15 N profile of agricultural volatile NH 3 in regional source apportionment.

The concentration of atmospheric ammonia (NH 3 ) in urban Beijing substantially decreased during the COVID-19 lockdown (24 January to 3 March 2020), likely due to the reduced human activities. However, quantifying the impact of anthropogenic interventions on NH 3 dynamics is challenging, as both meteorology and chemistry mask the real changes in observed NH 3 concentrations. Here, we applied machine learning techniques based on random forest models to decouple the impacts of meteorology and emission changes on the gaseous NH 3 and ammonium aerosol (NH 4 + ) concentrations in Beijing during the lockdown. Our results showed that the meteorological conditions were unfavorable during the lockdown and tended to cause an increase of 8.4 % in the NH 3 concentration. In addition, significant reductions in NO x and SO 2 emissions could also elevate NH 3 concentrations by favoring NH 3 gas-phase partitioning. However, the observed NH 3 concentration significantly decreased by 35.9 % during the lockdown, indicating a significant reduction in emissions or enhanced chemical sinks. Rapid gas-to-particle conversion was indeed found during the lockdown. Thus, the observed reduced NH 3 concentrations could be partially explained by the enhanced transformation into NH 4 + . Therefore, the sum of NH 3 and NH 4 + (collectively, NH x ) is a more reliable tracer than NH 3 or NH 4 + alone to estimate the changes in NH 3 emissions. Compared to that under the scenario without lockdowns, the NH x concentration decreased by 26.4 %. We considered that this decrease represents the real decrease in NH 3 emissions in Beijing due to the lockdown measures, which was less of a decrease than that based on NH 3 only (35.9 %). This study highlights the importance of considering chemical sinks in the atmosphere when applying machine learning techniques to link the concentrations of reactive species with their emissions., Competing Interests: Declaration of competing interest The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

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 (NO x  = NO + NO 2 ) and sulfur dioxide (SO 2 ), have dramatically decreased in the US in response to federal regulations. However, the response in concentrations of particulate nitrate (pNO 3 ) and sulfate (pSO 4 ) 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 NO x and SO 2 emissions. Particulate matter (PM 10 ) 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 pSO 4 and particulate ammonium (pNH 4 ), have shown dramatic decreases over the ten years at both the urban and rural sites; however, pNO 3 concentration increased by 95 % and 57 % in the urban and rural sites, respectively, despite significant NO x emission reductions. Our results demonstrate that changes in chemical mechanisms due to the decrease in SO 2 emissions contributed to decreases in pNH 4 , along with enhanced pNO 3 concentration. Furthermore, the change in SO 2 emissions has significantly impacted the atmospheric lifetime and transport distance of pNH 4 , favoring more localized contributions in recent years., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Oxygen stable isotopes (i.e., 16 O, 17 O, 18 O) of nitrite (NO 2 - ) 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 NO 2 - due to the lack of internationally recognized NO 2 - reference materials with a well-known Δ( 17 O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ( 17 O) of NO 2 - by reducing nitrate (NO 3 - ) materials with internationally recognized Δ( 17 O) values to NO 2 - using activated cadmium catalyzed by chloride in a basic solution while conserving Δ( 17 O). The NO 3 - reference materials reduced to NO 2 - and sample NO 2 - unknowns are converted to N 2 O using sodium azide/acetic acid reagent and decomposed to O 2 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( 17 O) quantification. The adapted method involves the following main points:•NO 3 - reference materials with internationally recognized oxygen isotope composition are reduced to NO 2 - under high pH conditions that conserve Δ( 17 O) values.•The NO 3 - reference materials reduced to NO 2 - and sample NO 2 - with unknown Δ( 17 O) values are reduced to N 2 O using chemical methods involving sodium azide/acetic acid.•The product N 2 O is extracted, purified, decomposed to O 2 , and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ( 17 O) quantification. The Δ( 17 O) of NO 2 - samples are calibrated with respect to the NO 3 - reference materials with known Δ( 17 O) values., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 The Author(s).)

The family of atmospheric oxides of nitrogen, NO y (e.g., nitrogen oxides (NO x ) + nitric acid (HNO 3 ) + nitrous acid (HONO) + peroxyacetyl nitrate (PAN) + particulate nitrate (pNO 3 - ) + other), have an influential role in atmospheric chemistry, climate, and the environment. The nitrogen (δ 15 N) and oxygen (δ 18 O and Δ 17 O) stable isotopes of NO y 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 NO y , to accurately quantify δ 15 N, δ 18 O, and Δ 17 O. This work presents controlled laboratory experiments and complex chamber α-pinene/NO x oxidation experiments of a sampling apparatus constructed for the simultaneous capture of multiple NO y species for isotope analysis using a series of coated denuders, with a focus on nitrogen dioxide (NO 2 • ). The laboratory tests indicate complete NO 2 • capture for the targeted concentration of 15 ppb v for at least 24 h collections at 10 liters per minute, with δ 15 N and δ 18 O precisions of ±1.3‰ and 1.0‰, respectively, and minimal (2.2% ± 0.1%) NO 2 • collection on upstream denuders utilized for the capture of HNO 3 and other acidic gases. The multispecies NO y collection system showed excellent concentration correlations with online instrumentation for both HNO 3 and NO 2 • and isotope reproducibility of ±1.7‰, ±1.8‰, and ±0.7‰ for δ 15 N, δ 18 O, and Δ 17 O, respectively, for replicate experiments and highly time-resolved collections. This work demonstrates a new method that can enable the simultaneous collection of HNO 3 and NO 2 • for accurate quantification of concentration and isotopic composition.

Atmospheric ammonia (NH 3 ) and ammonium (NH 4 + ) can substantially influence air quality, ecosystems, and climate. NH 3 volatilization from fertilizers and wastes (v-NH 3 ) has long been assumed to be the primary NH 3 source, but the contribution of combustion-related NH 3 (c-NH 3 , mainly fossil fuels and biomass burning) remains unconstrained. Here, we collated nitrogen isotopes of atmospheric NH 3 and NH 4 + and established a robust method to differentiate v-NH 3 and c-NH 3 . We found that the relative contribution of the c-NH 3 in the total NH 3 emissions reached up to 40 ± 21% (6.6 ± 3.4 Tg N yr -1 ), 49 ± 16% (2.8 ± 0.9 Tg N yr -1 ), and 44 ± 19% (2.8 ± 1.3 Tg N yr -1 ) in East Asia, North America, and Europe, respectively, though its fractions and amounts in these regions generally decreased over the past decades. Given its importance, c-NH 3 emission should be considered in making emission inventories, dispersion modeling, mitigation strategies, budgeting deposition fluxes, and evaluating the ecological effects of atmospheric NH 3 loading., (© 2022. The Author(s).)

It was previously believed that ammonia (NH 3 ) has a short residence time in the atmosphere and cannot be transported far from its sources. In late March, however, this study observed a severe NH 3 episode in urban Beijing when fertilizer was intensively applied on the North China Plain, with the highest hourly concentrations of 66.9 μg m -3 throughout the year. The stable nitrogen isotopic composition of NH 3 (δ 15 N-NH 3 ) during this episode (-37.0 to -20.0‰) fell in the range of endmembers of fertilizer and livestock, suggesting the long-range transport of NH 3 from agricultural to urban regions. Based on a Bayesian isotope mixing model, the contribution of agriculture (fertilization) to urban NH 3 concentrations was apportioned as 43.5% (26.0%) on polluted days. However, these contributions were reduced to 29.1% (12.8%) when nitrogen isotope fractionation between NH 3 and ammonium was considered. In contrast to the limited contribution of agricultural sources, we found that nonagricultural emissions, particularly vehicles, dominate the source of NH 3 in urban Beijing, even during the fertilization period. This finding indicated that nonagricultural sources should be considered when designing a control strategy for NH 3 to reduce haze pollution in the urban atmosphere., Competing Interests: Declaration of competing interest The authors declare no competing financial interest., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Effects of human activities on atmospheric nitrate (NO 3 - ) 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 NO 3 - oxidation processes in urban and rural atmospheres based upon the oxygen stable isotope composition of NO 3 - (Δ 17 O-NO 3 - ). Here we compared the Δ 17 O-NO 3 - 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 Δ 17 O 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 Δ 17 O-NO 3 - 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 Δ 17 O, wintertime conditions led to a contribution of O 3 related pathways (NO 3  + DMS/HC and N 2 O 5 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 Δ 17 O-NO 3 - 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., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

A general feature in the diurnal cycle of atmospheric ammonia (NH 3 ) concentrations is a morning spike that typically occurs around 07:00 to 10:00 (LST). Current hypotheses to explain this morning's NH 3 increase remain elusive, and there is still no consensus whether traffic emissions are among the major sources of urban NH 3 . Here, we confirmed that the NH 3 morning pulse in urban Beijing is a universal feature, with an annual occurrence frequency of 73.0% and a rapid growth rate (>20%) in winter. The stable nitrogen isotopic composition of NH 3 (δ 15 N-NH 3 ) in winter also exhibited a significant diurnal variation with an obvious morning peak at 07:00 to 10:00 (-18.6‰, mass-weighted mean), higher than other times of the day (-26.3‰). This diurnal pattern suggests that a large fraction of NH 3 in the morning originated from nonagricultural sources, for example, power plants, vehicles, and coal combustion that tend to have higher δ 15 N-NH 3 emission signatures relative to agricultural emissions. In particular, the contribution from vehicular emissions increased from 18% (00:00 to 07:00) to 40% (07:00 to 10:00), while the contribution of fertilizer sources to NH 3 was reduced from 15.8% at 00:00 to 07:00 to 5.2% at 07:00 to 10:00. We concluded that NH 3 concentrations in winter mornings in urban Beijing were indeed enhanced by vehicle emissions, which should be considered in air pollution regulations.

A top-down approach was employed to estimate the influence of lockdown measures implemented during the COVID-19 pandemic on NO x emissions and subsequent influence on surface PM 2.5 and ozone in China. The nation-wide NO x emission reduction of 53.4% due to the lockdown in 2020 quarter one in China may represent the current upper limit of China's NO x emission control. During the Chinese New Year Holiday (P2), NO x emission intensity in China declined by 44.7% compared to the preceding 3 weeks (P1). NO x emission intensity increased by 20.3% during the 4 weeks after P2 (P3), despite the unchanged NO 2 column. It recovered to 2019 level at the end of March (P4). The East China (22°N - 42°N, 102°E - 122°E) received greater influence from COVID-19. Overall NO x emission from East China for 2020 first quarter is 40.5% lower than 2019, and in P4 it is still 22.9% below the same period in 2019. The 40.5% decrease of NO x emission in 2020 first quarter in East China lead to 36.5% increase of surface O 3 and 12.5% decrease of surface PM 2.5 . The elevated O 3 promotes the secondary aerosol formation through heterogeneous pathways. We recommend that the complicated interaction between PM 2.5 and O 3 should be considered in the emission control strategy making process in the future., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Since the industrial revolution, it has been assumed that fossil-fuel combustions dominate increasing nitrogen oxide (NO x ) emissions. However, it remains uncertain to the actual contribution of the non-fossil fuel NO x to total NO x emissions. Natural N isotopes of NO 3 - in precipitation (δ 15 N w-NO3- ) have been widely employed for tracing atmospheric NO x sources. Here, we compiled global δ 15 N w-NO3- observations to evaluate the relative importance of fossil and non-fossil fuel NO x emissions. We found that regional differences in human activities directly influenced spatial-temporal patterns of δ 15 N w-NO3- variations. Further, isotope mass-balance and bottom-up calculations suggest that the non-fossil fuel NO x accounts for 55 ± 7% of total NO x emissions, reaching up to 21.6 ± 16.6Mt yr -1 in East Asia, 7.4 ± 5.5Mt yr -1 in Europe, and 21.8 ± 18.5Mt yr -1 in North America, respectively. These results reveal the importance of non-fossil fuel NO x emissions and provide direct evidence for making strategies on mitigating atmospheric NO x pollution.

Stable isotopic composition of atmospheric nitrate (nitric acid (HNO 3 ) + particulate nitrate (pNO 3 - )) 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 (δ 15 N) and oxygen (δ 18 O and Δ 17 O) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO 3(g) and pNO 3 - for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or Na 2 CO 3 solutions were used to collect HNO 3(g) . Laboratory experiments found that both coating solutions quantitatively collected HNO 3(g) , with the Na 2 CO 3 solution demonstrating a higher operative capacity (>1470 μg of HNO 3 ; n = 25) compared to the NaCl solution (∼750 μg of HNO 3 ; n = 25). The precision values for laboratory-tested HNO 3(g) collections are ±0.6‰ and ±1.2‰ for δ 15 N and δ 18 O for the NaCl solution and ± 0.8‰ and ±1.2‰ for the Na 2 CO 3 solution. Replicate (urban) samples indicate that the Na 2 CO 3 solution is significantly less selective for HNO 3(g) collection than the NaCl solution. Nylon filters were found to collect efficiently and retain laboratory-generated NaNO 3 and NH 4 NO 3 particles, with maximum standard deviations for δ 15 N and δ 18 O of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ 15 N and δ 18 O of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO 3(g) and pNO 3 - for isotopic measurements would best utilize the NaCl solution and Nylon filters.

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 (δ 15 N-NO 3 - ) 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 δ 15 N-NO 3 - decreased from 8.7 ± 3.7‰ to 4.2 ± 3.1‰, with much larger interannual variation in δ 15 N-NO 3 - 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 δ 15 N-NO 3 - . The large interannual variability of ice core δ 15 N-NO 3 - 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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Nitrogen stable isotope analysis (δ 15 N) of particulate ammonium (NH 4 + ) 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 δ 15 N(NH 4 + ). This is a critical point due to the difficulty of quantitative NH 4 + collection and possible sampling artifacts. Here, we report on δ 15 N(NH 4 + ) 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 NH 4 + collection in both laboratory-controlled environments and ambient air samples. Laboratory NH 4 + 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 NH 4 + was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δ 15 N(NH 4 + ) 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 δ 15 N(NH 4 + ) to be reproducible within 1.1‰, consistent with the laboratory findings. This work represents the first established method for speciated NH 4 + collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.

Nitrogen stable isotope analysis (δ 15 N) of ammonia (NH 3 ) has shown potential to be a useful tool for characterizing emission sources and sink processes. However, to properly evaluate NH 3 emission sources and sink processes under ambient conditions, it is necessary to collect and characterize the chemical speciation between NH 3 and particulate ammonium (p-NH 4 + ), together referred to as NH x . Current NH 3 collection methods have not been verified for their ability to accurately characterize δ 15 N-NH 3 and/or provide necessary chemical speciation (i.e., δ 15 N-NH 3 and δ 15 N-NH 4 + ). 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 δ 15 N-NH 3 under a variety of laboratory-controlled conditions and field collections. The collection method was tested under varying NH 3 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 δ 15 N-NH 3 analysis is found to have excellent accuracy and precision of ±1.6‰ (2σ), with an operative capacity of ∼400 μg of collected NH 3 for concentrations ≤207 ppb v . This work presents the first laboratory verified method for δ 15 N-NH 3 analysis and will be useful in future air quality studies.

Ab initio calculations have been carried out to investigate nitrogen (k 15 /k 14 ) and position-specific oxygen (k 17 /k 16 O & k 18 /k 16 ) kinetic isotope effects (KIEs) for the reaction between NO and O 3 using CCSD(T)/6-31G(d) and CCSD(T)/6-311G(d) derived frequencies in the complete Bigeleisen equations. Isotopic enrichment factors are calculated to be -6.7‰, -1.3‰, -44.7‰, -14.1‰, and -0.3‰ at 298 K for the reactions involving the 15 N 16 O, 14 N 18 O, 18 O 16 O 16 O, 16 O 18 O 16 O, and 16 O 16 O 18 O isotopologues relative to the 14N 16 O and 16O 3 isotopologues, respectively (CCSD(T)/6-311G(d)). Using our oxygen position-specific KIEs, a kinetic model was constructed using Kintecus, which estimates the overall isotopic enrichment factors associated with unreacted O 3 and the oxygen transferred to NO 2 to be -19.6‰  and -22.8‰, respectively, (CCSD(T)/6-311G(d)) which tends to be in agreement with previously reported experimental data. While this result may be fortuitous, this agreement suggests that our model is capturing the most important features of the underlying physics of the KIE associated with this reaction (i.e., shifts in zero-point energies). The calculated KIEs will useful in future NO x isotopic modeling studies aimed at understanding the processes responsible for the observed tropospheric isotopic variations of NO x as well as for tropospheric nitrate.

Although most models suggest continental Antarctica was covered by ice during the Last Glacial Maximum (LGM) it has been speculated that endemic species of soil invertebrates could have survived the Pleistocene at high elevation habitats protruding above the ice sheets. We analyzed a series of soil samples from different elevations at three locations along the Beardmore Glacier in the Transantarctic Mountains (in order of increasing elevation): Ebony Ridge (ER), Cloudmaker (CM), and Meyer Desert (MD). Geochemical analyses show the MD soils, which were exposed during the LGM, were the least weathered compared to lower elevations, and also had the highest total dissolved solids (TDS). MD soils are dominated by nitrate salts (NO3/Cl ratios >10) that can be observed in SEM images. High δ(17)O and δ(18)O values of the nitrate indicate that its source is solely of atmospheric origin. It is suggested that nitrate concentrations in the soil may be utilized to determine a relative "wetting age" to better assess invertebrate habitat suitability. The highest elevation sites at MD have been exposed and accumulating salts for the longest times, and because of the salt accumulations, they were not suitable as invertebrate refugia during the LGM.

The nitrogen stable isotope composition of NOx (δ(15)N-NOx) may be a useful indicator for NOx source partitioning, which would help constrain NOx source contributions in nitrogen deposition studies. However, there is large uncertainty in the δ(15)N-NOx values for anthropogenic sources other than on-road vehicles and coal-fired energy generating units. To this end, this study presents a broad analysis of δ(15)N-NOx from several fossil-fuel combustion sources that includes: airplanes, gasoline-powered vehicles not equipped with a three-way catalytic converter, lawn equipment, utility vehicles, urban buses, semitrucks, residential gas furnaces, and natural-gas-fired power plants. A relatively large range of δ(15)N-NOx values was measured from -28.1‰ to 8.5‰ for individual exhaust/flue samples that generally tended to be negative due to the kinetic isotope effect associated with thermal NOx production. A negative correlation between NOx concentrations and δ(15)N-NOx for fossil-fuel combustion sources equipped with selective catalytic reducers was observed, suggesting that the catalytic reduction of NOx increases δ(15)N-NOx values relative to the NOx produced through fossil-fuel combustion processes. Combining the δ(15)N-NOx measured in this study with previous published values, a δ(15)N-NOx regional and seasonal isoscape was constructed for the contiguous U.S., which demonstrates seasonal and regional importance of various NOx sources.

The nitrogen stable isotope ratio of NOx (δ(15)N-NOx) has been proposed as a regional indicator for NOx source partitioning; however, knowledge of δ(15)N values from various NOx emission sources is limited. This study presents a detailed analysis of δ(15)N-NOx emitted from vehicle exhaust, the largest source of anthropogenic NOx. To accomplish this, NOx was collected from 26 different vehicles, including gasoline and diesel-powered engines, using a modification of a NOx collection method used by the United States Environmental Protection Agency, and δ(15)N-NOx was analyzed. The vehicles sampled in this study emitted δ(15)N-NOx values ranging from -19.1 to 9.8‰ that negatively correlated with the emitted NOx concentrations (8.5 to 286 ppm) and vehicle run time because of kinetic isotope fractionation effects associated with the catalytic reduction of NOx. A model for determining the mass-weighted δ(15)N-NOx from vehicle exhaust was constructed on the basis of average commute times, and the model estimates an average value of -2.5 ± 1.5‰, with slight regional variations. As technology improvements in catalytic converters reduce cold-start emissions in the future, it is likely to increase current δ(15)N-NOx values emitted from vehicles.

We report on a mid-IR optical parametric oscillator (OPO)-based high resolution transient absorption spectrometer for state-resolved collisional energy transfer. Transient Doppler-broadened line profiles at λ = 3.3 μm are reported for HCl R7 transitions following gas-phase collisions with vibrationally excited pyrazine. The instrument noise, analyzed as a function of IR wavelength across the absorption line, is as much as 10 times smaller than in diode laser-based measurements. The reduced noise is attributed to larger intensity IR light that has greater intensity stability, which in turn leads to reduced detector noise and better frequency locking for the OPO.