Novel Method for Nitrogen Isotopic Analysis of Soil-Emitted Nitric Oxide

High-Resolution Measurements of Multi-Isotopic Signatures (δ15N, δ18O, and Δ17O) of Winter NO2 in a Megacity in Central China

Isotopic compositions (δ15N, δ18O, and Δ17O) of nitrogen dioxide (NO2) are crucial proxies for elucidating the sources and transformations of atmospheric NO2, yet observations remain scarce. Here, we reported high-resolution (2 h interval) measurements of NO2 isotopic compositions in a megacity in Central China. Δ17O(NO2) exhibited a clear diurnal cycle with elevated values during the day versus night. Daytime Δ17O(NO2) varying from 20.2 to 40.1‰ can be explained by variations in ozone (O3) versus peroxyl radical oxidation of nitric oxide (NO). The estimated peroxyl radicals were positively correlated with photochemical oxidants (Ox = O3 + NO2), revealing the ability of Δ17O(NO2) to reflect changes in the atmospheric oxidation environment. Nighttime Δ17O(NO2) variations (12.9 to 29.8‰) can be categorized into three regimes: the first regime when Δ17O(NO2) stabilized at approximately 20‰ can be well explained by Δ17O transfer from O3; the increases of Δ17O(NO2) above 20‰ were attributed to the vertical mixing of residual layer air with elevated Δ17O(NO2) and the exchange process between NO2 and dinitrogen pentoxide; and as low as 12.9‰ Δ17O(NO2) can be explained by primary NO2 emissions and the exchange process between NO and NO2. The NO-NO2 exchange also elevated δ15N(NO2), consistent with the observed negative relationships between Δ17O(NO2) and δ15N(NO2) at night.

Refining δ15N isotopic fingerprints of local NOx for accurate source identification of nitrate in PM2.5

Stable nitrogen isotopic composition (δ15N) has proven to be a valuable tool for identifying sources of nitrates (NO3-) in PM2.5. However, the absence of a systematic study on the δ15N values of domestic NOx sources hinders accurate identification of NO3- sources in China. Here, we systematically determined and refined δ15N values for six categories of NOx sources in Tianjin using an active sampling method. Moreover, the δ15N values of NO3- in PM2.5 were measured during pre-heating, mid-heating and late-heating periods, which are the most heavily polluted in Tianjin. The results indicate that the isotopic fingerprints of the six types of NOx sources in Tianjin are indicative of the regional characteristics of China, particularly the North China Plain. The Bayesian isotope mixing (MixSIAR) model demonstrated that coal combustion, biomass burning, and vehicle exhaust collectively contributed more than 60 %, dominating the sources of NO3- during sampling periods in Tianjin. However, failure to consider the isotopic signatures of local NOx sources could result in an overestimation of the contribution from natural gas combustion. Additionally, the absence of industrial sources, an uncharacterized source in previous studies, may directly result in the contribution fraction of other sources being overestimated by the model more than 10 %. Notably, as the number of sources input to the model increased, the contribution of various NOx sources was becoming more stable, and the inter-influence between various sources significantly reduced. This study demonstrated that the refined isotopic fingerprint in China could more effectively distinguish source of NO3-, thereby providing valuable insights for controlling NO3- pollution.

Nitrogen isotopes reveal high NOx emissions from arid agricultural soils in the Salton Sea Air Basin

Air quality management commonly aims to mitigate nitrogen oxide (NOx) emissions from combustion, reducing ozone (O3) 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 O3 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 NOx (an important precursor to both O3 and PM) using nitrogen stable isotopes of ambient NO2, which revealed a significant contribution from soil-emitted NOx to the regional budget. The soil source strength was estimated based on the mean δ15N-NOx from each emission category in the California Air Resources Board's NOx inventory. Our annual average soil emission estimate for the air basin was 11.4 ± 4 tons/d, representing ~ 30% of the extant NOx 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 NOx 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 NOx 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.

Nitrogen isotope characteristics and importance of NOx from biomass burning in China

Biomass burning is a primary source of atmospheric nitrogen oxide (NOx), however, the lack of isotopic fingerprints from biomass burning limits their use in tracing atmospheric nitrate (NO3-) and NOx. A total of 25 biomass fuels from 10 provinces and regions in China were collected, and the δ15N values of biomass fuels (δ15N-biomass) and δ15N-NOx values of biomass burning (δ15N-NOx values of BB, open burning, and rural cooking stove burning) were determined. The δ15N-NOx values of open burning and rural cooking stove burning ranged from -0.8 ‰ to 11.6 ‰ and 0.8 ‰ to 9.5 ‰, respectively, indicating a significant linear relation with δ15N-biomass. Based on the measured δ15N-NOx values of BB and biomass burning emission inventory data, the δ15N-NOx values of BB in different provinces and regions of China were calculated using the δ15N-NOx model, with a mean value of 5.0 ± 1.8 ‰. The spatial variations in the estimated δ15N-NOx values of BB in China were mainly controlled by the differences in the δ15N-NOx values and the proportions of NOx emissions from various straw burning activities in provinces and regions of China. Furthermore, by using the combined local emissions of biomass burning with regional transportations of NOx based on air-mass backward trajectories, we established an improved δ15N-NOx model and obtained more accurate δ15N-NOx values of BB in regions (2.3 ‰ to 8.4 ‰). By utilising the reported δ15N-NOx values of precipitation and particulate matter from 21 cities in China and the more accurate δ15N-NOx values of BB, the NOx contributions from four sources (mobile sources, coal combustion, biomass burning, and microbial N cycle) at the national scale were estimated using a Bayesian model. The significant contributions of biomass burning (20.9 % to 44.3 %) to NOx emissions were revealed, which is vital for controlling NOx emissions in China.

Quantification of NOx sources contribution to ambient nitrate aerosol, uncertainty analysis and sensitivity analysis in a megacity

Dual isotopes of nitrogen and oxygen of NO3- are crucial tools for quantifying the formation pathways and precursor NOx sources contributing to atmospheric nitrate. However, further research is needed to reduce the uncertainty associated with NOx proportional contributions. The acquisition of nitrogen isotopic composition from NOx emission sources lacks regulation, and its impact on the accuracy of contribution results remains unexplored. This study identifies key influencing factors of source isotopic composition through statistical methods, based on a detailed summary of δ15N-NOx values from various sources. NOx emission sources are classified considering these factors, and representative means, standard deviations, and 95 % confidence intervals are determined using the bootstrap method. During the sampling period in Tianjin in 2022, the proportional nitrate formation pathways varied between sites. For suburban and coastal sites, the ranking was [Formula: see text] (NO2 + OH radical) > [Formula: see text] (N2O5 + H2O) > [Formula: see text] (NO3 + DMS/HC), while the rural site exhibited similar fractional contributions from all three formation pathways. Fossil fuel NOx sources consistently contributed more than non-fossil NOx sources in each season among three sites. The uncertainties in proportional contributions varied among different sources, with coal combustion and biogenic soil emission showing lower uncertainties, suggesting more stable proportional contributions than other sources. The sensitivity analysis clearly identifies that the isotopic composition of 15N-enriched and 15N-reduced sources significantly influences source contribution results, emphasizing the importance of accurately characterizing the localized and time-efficient nitrogen isotopic composition of NOx emission sources. In conclusion, this research sheds light on the importance of addressing uncertainties in NOx proportional contributions and emphasizes the need for further exploration of nitrogen isotopic composition from NOx emission sources for accurate atmospheric nitrate studies.

Microbial nitrogen transformations tracked by natural abundance isotope studies and microbiological methods: A review

Nitrogen is an essential nutrient in the environment that exists in multiple oxidation states in nature. Numerous microbial processes are involved in its transformation. Knowledge about very complex N cycling has been growing rapidly in recent years, with new information about associated isotope effects and about the microbes involved in particular processes. Furthermore, molecular methods that are able to detect and quantify particular processes are being developed, applied and combined with other analytical approaches, which opens up new opportunities to enhance understanding of nitrogen transformation pathways. This review presents a summary of the microbial nitrogen transformation, including the respective isotope effects of nitrogen and oxygen on different nitrogen-bearing compounds (including nitrates, nitrites, ammonia and nitrous oxide), and the microbiological characteristics of these processes. It is supplemented by an overview of molecular methods applied for detecting and quantifying the activity of particular enzymes involved in N transformation pathways. This summary should help in the planning and interpretation of complex research studies applying isotope analyses of different N compounds and combining microbiological and isotopic methods in tracking complex N cycling, and in the integration of these results in modelling approaches.

Nitrogen Isotopes Reveal High NOx Emissions from Arid Agricultural Soils in the Salton Sea Air Basin

Air quality management commonly aims to mitigate emissions of oxides of nitrogen (NOx) from combustion, reducing ozone and particulate matter pollution. Despite such efforts, regulations have recently proven ineffective in rural areas like the Salton Sea Air Basin of Southern California, which routinely violates air quality standards. With $2 billion in annual agricultural sales and low population density, air quality in the region is likely influenced by year-round farming. We conducted NOx source apportionment using nitrogen stable isotopes of ambient NO2, which indicate a substantial contribution of soil-emitted NOx. The soil source strength was estimated based on the mean δ15N-NOx from each emission category in the California Air Resources Board's NOx inventory. Our annual average soil emission estimate for the air basin was 11.4 ± 4 tons/d, representing ~30% of the extant NOx inventory, 10× larger than the state's inventory. Therefore, the impact of soil NOx in agricultural regions must be re-evaluated.

Nitrogen and oxygen isotope characteristics, formation mechanism, and source apportionment of nitrate aerosols in Wuhan, Central China

Understanding the sources and formation mechanisms of nitrate in PM2.5 is important for effective and precise prevention and control of particulate matter pollution. In this study, we detected stable nitrogen and oxygen isotope signatures of NO- 3 (expressed as δ15N-NO- 3 and δ18O-NO3-) in PM2.5 samples in Wuhan, the largest city in central China. The sources and formation pathways of NO3- were quantitatively analyzed using the modified version of the Bayesian isotope mixing (MixSIR) model, and the regional transport characteristics of NO3- were analyzed using the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model and concentration-weighted trajectory (CWT) method. The results showed that NO3- significantly contributed to the ambient PM2.5 pollution and its driving effect increased with the gradient of pollution level. The average δ15N-NO3- and δ18O-NO3- values were 4.7 ± 0.9 ‰ and 79.7 ± 2.9 ‰, respectively. δ15N-NO3- and δ18O-NO3- were more enriched in winter and increased dramatically in heavily polluted days. The reaction pathway of NO2 + OH dominated nitrate formation in summer, while the reaction pathway of N2O5+ H2O dominated in other seasons and contributed more in polluted days than clean days. The contributions of vehicle emission, coal combustion, biomass burning, biogenic soil emission, and ship emission sources to NO3- were 26.4 %, 23.4 %, 22.8 %, 15.3 %, and 12.1 %, respectively. In addition to local emissions, air mass transport from the northern China had a significant impact on particulate NO3- in Wuhan. Overall, we should pay special attention to vehicle and ship emissions and winter coal combustion emissions in future policymaking.

Low blank sampling method for measurement of the nitrogen isotopic composition of atmospheric NOx

The nitrogen isotopic composition of nitrogen oxide (NOx) is useful for estimating its sources and sinks. Several methods have been developed to convert atmospheric nitric oxide (NO) and/or nitrogen dioxide (NO2) to nitrites and/or nitrates for collection. However, the collection efficiency and blanks are poorly evaluated for many collection methods. Here, we present a method for collecting ambient NOx (NO and NO2 simultaneously) with over 90% efficiency collection of NOx and low blank (approximately 0.5 μM) using a 3 wt% hydrogen peroxide (H2O2) and 0.5 M sodium hydride (NaOH) solution. The 1σ uncertainty of the nitrogen isotopic composition was ± 1.2 ‰. The advantages of this method include its portability, simplicity, and the ability to collect the required amount of sample to analyze the nitrogen isotopic composition of ambient NOx in a short period of time. Using this method, we observed the nitrogen isotopic compositions of NOx at the Tsukuba and Yoyogi sites in Japan. The averaged δ15N(NOx) value and standard deviation (1σ) in the Yoyogi site was (-2.7 ± 1.8) ‰ and in the Tsukuba site was (-1.7 ± 0.9) ‰ during the sampling period. The main NOx source appears to be the vehicle exhaust in the two sites.

Increasing Nonfossil Fuel Contributions to Atmospheric Nitrate in Urban China from Observation to Prediction

China's nitrogen oxide (NOx) emissions have undergone significant changes over the past few decades. However, nonfossil fuel NOx emissions are not yet well constrained in urban environments, resulting in a substantial underestimation of their importance relative to the known fossil fuel NOx emissions. We developed an approach using machine learning that is accurate enough to generate a long time series of the nitrogen isotopic composition (δ15N) of atmospheric nitrate using high-level accuracies of air pollutants and meteorology data. Air temperature was found to be the critical driver of the variation of nitrate δ15N at daily resolution based on this approach, while significant reductions of aerosol and its precursor emissions played a key role in the change of nitrate δ15N on the yearly scale. Predictions from this model found a significant decrease in nitrate δ15N in Chinese megacities (Beijing and Guangzhou as representative cities in the north and south, respectively) since 2013, implying an enhanced contribution of nonfossil fuel NOx emissions to nitrate aerosols (up to 22%-26% in 2021 from 18%-22% in 2013 quantified by an isotope mixing model), as confirmed by the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem) simulation. Meanwhile, the declining contribution in coal combustion (34%-39% in 2013 to 31%-34% in 2021) and increasing contribution of natural gas combustion (11%-14% in 2013 to 14%-17% in 2021) demonstrated the transformation of China's energy structure from coal to natural gas. This approach provides missing records for exploring long-term variability in the nitrogen isotope system and may contribute to the study of the global reactive nitrogen biogeochemical cycle.

Vertical measurements of stable nitrogen and oxygen isotope composition of fine particulate nitrate aerosol in Guangzhou city: Source apportionment and oxidation pathway

Nowadays, the emission source and formation mechanism of fine particulate nitrate (pNO₃^-) in China are mired in controversy. In this study, the stable nitrogen isotope (δ¹⁵N-NO₃^-) and triple oxygen isotope (Δ¹⁷O-NO₃^-) were determined for the pNO₃^- samples collected at three heights under different atmospheric oxidation capacity (AOC) (O_x = O₃ + NO₂: 107 ± 29 μg m^-3 at ground, 102 ± 28 μg m^-3 at 118 m, 122 ± 23 μg m^-3 at 488 m) conditions during the sampling period based on the Canton Tower, Guangzhou, China. The Bayesian mixing model showed that coal combustion was the largest contributor to pNO₃^- in this city, followed by biomass burning, vehicle exhaust, and soil emission. Interestingly, we found that vertical NO_x and pNO₃^- concentrations displayed an opposite pattern owing to the different formation mechanisms among heights. The average contributions of oxidation pathways for (NO₂ + OH, P1), (NO₃ + DMS/HC, P2), and (N₂O₅ + H₂O, P3) were 61 %, 12 %, and 27 % at the ground, respectively, and these values would vary greatly among heights. These results implied that both AOC and NO_x loading played an important role in pNO₃^- production. The pNO₃^- displayed a positive correlation with NO_x (r = 0.95) with an enhanced contribution of the P1 pathway under the relatively high AOC condition. However, pNO₃^- has a negative correlation with NO_x (r = -0.99) with a rise of heterogeneous reaction (P2 and P3) under the relatively low AOC condition. Therefore, the current emission control strategy for air pollution in China needs to consider the AOC conditions among regions to effectively mitigate particulate air pollution.

Nitrate dynamics and source identification of rainwater in Beijing during rainy season: Insight from dual isotopes and Bayesian model

Anthropogenic reactive nitrogen emissions have a significant impact on atmospheric chemical composition and earth surface ecosystem. As one of the most important sinks of atmospheric nitrogen, the wet deposition of nitrate (rainwater NO₃^-) has been widely concerned. Yet, the sources and transformation processes of wet deposited NO₃^- were not well revealed in megacity during rainy season in the context of global climate change. Here, we investigated the concentrations of nitrogen components and dual isotopes of rainwater nitrate collected in Beijing during July to August 2021 (rainy season). The main findings showed that the concentrations of NH₄⁺-N, NO₃^--N, and NO₂^--N ranged 0.5- 6.7 mg L^-1, 0.3- 4.5 mg L^-1, and 0.05- 0.18 mg L^-1, respectively, with the average relative percentages of 69 %, 29 %, and 2 %. The stoichiometry analysis of characteristic ion ratios indicated that the contribution of municipal wastes and agricultural sources to rainwater NH₄⁺-N is relatively significant, while traffics were the major contributor of NO₃^--N instead of the fixed emission. Rainwater δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- presented slightly ¹⁵N-depleted characteristic compared to previous studies with the average values of -3.9 ± 3.1 ‰ and 58.7 ± 12.6 ‰. These isotope compositions suggesting an origin of rainwater NO₃^- from the mixing of multi-sources and was mainly generated via the pathway of OH radical oxidization. Further source apportionment of rainwater NO₃^- by Bayesian mixing model evaluated that traffic (30.3 %) and soil (30.3 %) emissions contributed mostly to NO₃^-, while the contribution of biomass burning (18.8 %) and coal combustion (20.6 %) were relatively lower. This study highlighted the important role of dual isotopes in rainwater nitrate source identification and formation processes in megacity.

Seasonal differences in sources and formation processes of PM2.5 nitrate in an urban environment of North China

Nitrate (NO₃^-) has been the dominant ion of secondary inorganic aerosols (SIAs) in PM_2.5 in North China. Tracking the formation mechanisms and sources of particulate nitrate are vital to mitigate air pollution. In this study, PM_2.5 samples in winter (January 2020) and in summer (June 2020) were collected in Jiaozuo, China, and water-soluble ions and (δ¹⁵N, δ¹⁸O)-NO₃^- were analyzed. The results showed that the increase of NO₃^- concentrations was the most remarkable with increasing PM_2.5 pollution level. δ¹⁸O-NO₃^- values for winter samples (82.7‰ to 103.9‰) were close to calculated δ¹⁸O-HNO₃ (103‰ ± 0.8‰) values by N₂O₅ pathway, while δ¹⁸O-NO₃^- values (67.8‰ to 85.7‰) for summer samples were close to calculated δ¹⁸O-HNO₃ values (61‰ ± 0.8‰) by OH oxidation pathway, suggesting that PM_2.5 nitrate is largely from N₂O₅ pathway in winter, while is largely from OH pathway in summer. Averaged fractional contributions of P_N2O5+H2O were 70% and 39% in winter and summer sampling periods, respectively, those of P_OH were 30% and 61%, respectively. Higher δ¹⁵N-NO₃^- values for winter samples (3.0‰ to 14.4‰) than those for summer samples (-3.7‰ to 8.6‰) might be due to more contributions from coal combustion in winter. Coal combustion (31% ± 9%, 25% ± 9% in winter and summer, respectively) and biomass burning (30% ± 12%, 36% ± 12% in winter and summer, respectively) were the main sources using Bayesian mixing model. These results provided clear evidence of particulate nitrate formation and sources under different PM_2.5 levels, and aided in reducing atmospheric nitrate in urban environments.

Fast and Efficient Atmospheric NO2 Collection for Isotopic Analysis by a 3D-Printed Denuder System

Being major species of atmospheric reactive nitrogen, nitrogen oxides (NO_x = NO + NO₂) have important implications for ozone and OH radical formation in addition to nitrogen cycles. Stable nitrogen isotopes (δ¹⁵N) of NO_x have been sought to track NO_x emissions and NO_x chemical reactivities in the atmosphere. The current atmospheric NO_x collection methods for isotopic analysis, however, largely suffer from unverified collection efficiency and/or low collection speed (<10 L/min). The latter makes it difficult to study δ¹⁵N(NO_x) in pristine regions with low NO_x concentrations. Here, we present a three-dimensional (3D)-printed honeycomb denuder (3DP-HCD) system, which can effectively collect atmospheric NO₂ (a major part of NO_x) under a variety of laboratory and field conditions. With a coating solution consisting of 10% potassium hydroxide (KOH) and 25% guaiacol in methanol, the denuder system can collect NO₂ with nearly 100% efficiency at flow rates of up to 70 L/min, which is 7 times higher than that of the existing method and allows high-resolution (e.g., diurnal or finer resolution) NO₂ collection even in pristine sites. Besides, the δ¹⁵N of NO₂ collected by the 3DP-HCD system shows good reproducibility and consistency with the previously tested method. Preliminary results of online NO oxidation by a chrome trioxide (CrO₃) oxidizer for simultaneous NO and NO₂ collection are also presented and discussed.

Nitrate sources and its formation in precipitation during typhoons (In-fa and Chanthu) in multiple cities, East China

A clear understanding of the factors governing dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in typhoons is essential for understanding their NO₃^- sources and its formation mechanisms. In this study, sequential precipitation samples during typhoons, including In-fa and Chanthu, were collected from Ningbo, Hangzhou and Huzhou. The chemical compositions, nitrogen and oxygen isotopes of NO₃^- and oxygen isotopes of H₂O (δ¹⁸O-H₂O) were measured. The results showed that the δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values ranged from -6.3‰ to 6.0‰, and 38.0‰ to 66.5‰, respectively. The lower δ¹⁸O-NO₃^- values (less than 52‰) indicated the importance of peroxy radicals (RO₂ or HO₂) in NO_x oxidation to NO₃^- formation pathways. By the Monte Carlo simulation of δ¹⁸O-NO₃^- values of typhoons, the calculated oxidation proportions of NO by RO₂ (or HO₂) during the OH· pathway ranged from 0% to 27% of In-fa and from 0% to 32% of Chanthu, respectively, in the three cities. More NO_x emissions from marine microbial processes caused the lower δ¹⁵N-NO₃^- values of typhoons in Ningbo than those in Hangzhou and Huzhou. The variation in δ¹⁵N-NO₃^- values in sequential samples in In-fa reflected the decreased marine sources (lightning) and the increased anthropogenic sources in land (coal combustion and microbial N cycle) from Phrase I to Phrase II and III. Based on the improved Bayesian model with nitrogen isotopic fractionation, the contributions of lightning + biomass burning, coal combustion, mobile sources and the microbial N cycle were 35.7%, 22.5%, 27.1% and 14.7% in In-fa, and 28.3%, 32.3%, 28.0% and 11.4% in Chanthu, respectively, in the three cities, emphasizing the influence of marine NO_x sources (lightning). The results highlight the importance of RO₂ (or HO₂) in NO_x oxidation pathways in typhoons and provide valuable insight into the NO_x sources of typhoons.

The excellent photocatalytic NO removal performance relates to the synergistic effect between the prepositive NaOH solution and the g-C3N4 photocatalysis

Photocatalysis technology is used to remove the low concentration NO in recent years. However, the effect of this process is not very satisfactory. In this study, it was found that the prepositive NaOH solution could significantly improve the photocatalytic NO removal activity of g-C₃N₄. The apparent quantum yield of g-C₃N₄ in the NO removal process was increased 3.5 times by the prepositive NaOH solution. The reason is that there was a synergistic effect formed between the prepositive NaOH solution and the photocatalytic NO removal process. The prepositive NaOH solution not only could increase the humidity and pH value in the photocatalytic unit, but also could improve the adsorption ability of g-C₃N₄ for the H₂O, NO, and O₂. Moreover, the prepositive NaOH solution reduced the difficulty of the photogenerated carriers' transport and the ·OH generation. This study provided a new idea for the removal of low-concentration NOx.

Stable nitrogen isotope composition of NOx of biomass burning in China

Owing to the local characteristics of stable nitrogen isotopes in nitrogen oxides (δ¹⁵N-NO_x) emitted from biomass burning, the lack of data on δ¹⁵N-NO_x values associated with biomass burning in China limits the use of this parameter in identifying and quantifying the sources of atmospheric nitrate (NO₃^-) and NO_x. The results showed that the δ¹⁵N-NO_x values of open burning and rural cooking stoves in China ranged from -3.7‰ to 3.1‰ and -11.9‰ to 1.5‰, respectively. The δ¹⁵N values of nine biomass fuel sources (δ¹⁵N-biomass) ranged from 0.1‰ to 4.1‰. Significant linear relationships between the δ¹⁵N-biomass values and δ¹⁵N-NO_x values of open burning (δ¹⁵N-NO_x = 1.1δ¹⁵N-biomass - 2.7; r² = 0.63; p < 0.05) and rural cooking stoves (δ¹⁵N-NO_x = 1.7δ¹⁵N-biomass - 9.8; r² = 0.72; p < 0.01) suggested that the variations in δ¹⁵N-NO_x values from biomass burning were mainly controlled by the biomass fuel source. The isotopic fractionation of nitrogen during the biomass burning process might have led to the higher δ¹⁵N-NO_x values from open burning in comparison to rural cooking stoves. By combining the δ¹⁵N-NO_x values of biomass burning with biomass burning emission inventory data, a model for calculating the δ¹⁵N-NO_x values of biomass burning in different regions of China was established, and the estimated δ¹⁵N-NO_x value of biomass burning at the national scale was -0.8 ± 1.2‰. But the limited δ¹⁵N-biomass values increase the uncertainty of model in national scale.

Stable isotopic characterization of nitrate wet deposition in the tropical urban atmosphere of Costa Rica

Increasing energy consumption and food production worldwide results in anthropogenic emissions of reactive nitrogen into the atmosphere. To date, however, little information is available on tropical urban environments where inorganic nitrogen is vastly transported and deposited through precipitation on terrestrial and aquatic ecosystems. To fill this gap, we present compositions of water stable isotopes in precipitation and atmospheric nitrate (δ¹⁸O-H₂O, δ²H-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-) collected daily between August 2018 and November 2019 in a tropical urban atmosphere of central Costa Rica. Rainfall generation processes (convective and stratiform rainfall fractions) were identified using stable isotopes in precipitation coupled with air mass back trajectory analysis. A Bayesian isotope mixing model using δ¹⁵N-NO₃^- compositions and corrected for potential ¹⁵N fractionation effects revealed the contribution of lightning (25.9 ± 7.1%), biomass burning (21.8 ± 6.6%), gasoline (19.1 ± 6.4%), diesel (18.4 ± 6.0%), and soil biogenic emissions (15.0 ± 2.6%) to nitrate wet deposition. δ¹⁸O-NO₃^- values reflect the oxidation of NO_x sources via the ·OH + RO₂ pathways. These findings provide necessary baseline information about the combination of water and nitrogen stable isotopes with atmospheric chemistry and hydrometeorological techniques to better understand wet deposition processes and to characterize the origin and magnitude of inorganic nitrogen loadings in tropical regions.

Multi isotope systematics of precipitation to trace the sources of air pollutants in Seoul, Korea

Multiple sulfur (δ³⁴S_sulfate, Δ³³S_sulfate, & Δ³⁶S_sulfate), nitrogen and oxygen (δ¹⁵N_nitrate & δ¹⁸O_nitrate) and strontium (⁸⁷Sr/⁸⁶Sr) isotope compositions of precipitation collected from Seoul, South Korea were analyzed to study the sources, transportation and deposition of air pollutants in East Asia. The δ³⁴S_sulfate values (from 1.9 to 14.6‰ with a median of 4.7‰) and the δ¹⁵N_nitrate values (from -2.0 to 13.3‰ with a median of 1.0‰) suggest that fossil fuel use (emission from coal combustion and vehicle exhaust) is a predominant source for sulfur and nitrogen, but other natural sources including biogenic contributions of DMS also add to their total budget. The seasonal variations are observed in δ³⁴S_sulfate and δ¹⁵N_nitrate values (both higher in winter season), which is likely to result from the increase of coal use for domestic heating in China. The δ¹⁸O_nitrate values also varied seasonally depending on the NO_x oxidation pathway, being about 20‰ higher in winter than in summer season. The Δ³³S_sulfate and Δ³⁶S_sulfate values are not anomalous, showing the absence of MIF signals in precipitation of Seoul. The ⁸⁷Sr/⁸⁶Sr ratio of the precipitation samples range from 0.70988 to 0.71487 with a median of 0.71073, indicating the influence of at least three end-member (silicate dust, carbonate dust and anthropogenic emission). Ionic ratios such as (K⁺+NH₄⁺)/(Ca²⁺+Mg²⁺) and Cl^-/Na⁺ suggest the overwhelming effect of anthropogenic input rather than carbonate dust on the end-member with lower ⁸⁷Sr/⁸⁶Sr ratios.

Combining dual isotopes and a Bayesian isotope mixing model to quantify the nitrate sources of precipitation in Ningbo, East China

Nitrate (NO₃^-) is becoming a significant contributor to acid deposition in many cities in China. Based on the chemical compositions and stable isotopes of NO₃^- in precipitation (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), the NO₃^- sources and their formation pathways were determined to aid in reducing NO_x emissions in Ningbo, an important port city. The acid rain frequency in Ningbo was 67%, and the mean SO₄^2-/NO₃^- ratio was 1.07. The δ¹⁸O-NO₃^- (49.5‰-82.8‰) and δ¹⁵N-NO₃^- values (-4.3‰-2.7‰) both varied seasonally, with high values during the cold season and low values during the warm season. The seasonal variations in the δ¹⁸O-NO₃^- values were mainly controlled by the NO₃^- formation pathways, following the OH· pathway during the warm season and N₂O₅ pathway during the cold season. The Monte Carlo simulation results indicated that the contributions of the OH· pathway ranged from 28.3% to 75.4%, with the remainder contributed by the N₂O₅ pathway. The improved Bayesian model incorporating nitrogen (N) isotopic fractionation (Ԑ = 4‰) indicated that mobile sources, including ship emissions (35.0%) > coal combustion (26.0%) > biomass burning (20.0%) > soil emissions (19.0%), were the major sources of NO_x emissions in Ningbo. The results indicate that the influence of isotopic fractionation on source apportionment must be considered in a Bayesian model.

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 HNO_3(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 HNO_3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO_3(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 HNO_3(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 HNO_3(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 HNO_3(g) and pNO₃^- for isotopic measurements would best utilize the NaCl solution and Nylon filters.

Isotopic constraints on the formation pathways and sources of atmospheric nitrate in the Mt. Everest region

Inorganic particulate nitrate (p-NO₃^-), gaseous nitric acid (HNO_3(g)) and nitrogen oxides (NO_x = NO + NO₂), as main atmospheric pollutants, have detrimental effects on human health and aquatic/terrestrial ecosystems. Referred to as the 'Third Pole' and the 'Water Tower of Asia', the Tibetan Plateau (TP) has attracted wide attention on its environmental changes. Here, we evaluated the oxidation processes of atmospheric nitrate as well as traced its potential sources by analyzing the isotopic compositions of nitrate (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) in the aerosols collected from the Mt. Everest region during April to September 2018. Over the entire sampling campaigns, the average of δ¹⁵N(NO₃^-), δ¹⁸O(NO₃^-), and Δ¹⁷O(NO₃^-) was -5.1 ± 2.3‰, 66.7 ± 10.2‰, and 24.1 ± 3.9‰, respectively. The seasonal variation in Δ¹⁷O(NO₃^-) indicates the relative importance of O₃ and HO₂/RO₂/OH in NO_x oxidation processes among different seasons. A significant correlation between NO₃^- and Ca²⁺ and frequent dust storms in the Mt. Everest region indicate that initially, the atmospheric nitrate in this region might have undergone a process of settling; subsequently, it got re-suspended in the dust. Compared with the Δ¹⁷O(NO₃^-) values in the northern TP, our observed significantly higher values suggest that spatial variations in atmospheric Δ¹⁷O(NO₃^-) exist within the TP, and this might result from the spatial variations of the atmospheric O₃ levels, especially the stratospheric O₃, over the TP. The observed δ¹⁵N(NO₃^-) values predicted remarkably low δ¹⁵N values in the NO_x of the sources and the N isotopic fractionation plays a crucial role in the seasonal changes of δ¹⁵N(NO₃^-). Combined with the results from the backward trajectory analysis of air mass, we suggest that the vehicle exhausts and agricultural activities in South Asia play a dominant role in determining the nitrate levels in the Mt. Everest region.

Insight into the Variability of the Nitrogen Isotope Composition of Vehicular NOx in China

Nitrogen isotope (δ¹⁵N) monitoring is a potentially powerful tool in tracing atmospheric nitrogen oxides (NO_x); however, the isotopic fingerprint of vehicle exhaust remains poorly interpreted. This deficiency limits our understanding of the origin of atmospheric haze pollution, especially in China. In this study, we systemically explored the δ¹⁵N-NO_x fingerprints of various vehicle exhausts (n = 137) in China. The δ¹⁵N-NO_x values of vehicle exhausts ranged from -18.8‰ to +6.4‰, presenting a significant correlation with NO_x concentrations (p < 0.01). The highest δ¹⁵N-NO_x values were observed for liquefied petroleum gas vehicles (-0.1 ± 1.8‰), followed by gasoline vehicles (-7.0 ± 4.8‰) and diesel vehicles (-12.7 ± 3.4‰), all of which displayed a rising trend as emissions standards were continuously updated. The δ¹⁵N-NO_x values under working conditions followed the trend warm start (-5.9 ± 5.0‰) > driving (-7.3 ± 5.9‰) > cold start (-9.2 ± 2.7‰). By establishing a suitable model for assessing representative δ¹⁵N-NO_x values, the δ¹⁵N-NO_x values of various vehicles, including different fuel types with different emission standards, were evaluated. A model of δ¹⁵N-NO_x associated with motor vehicle data was developed, which estimated the national δ¹⁵N-NO_x value of vehicle emissions to be -12.6 ± 2.2‰, but there was considerable variation among different target areas in China.

Fossil-driven secondary inorganic PM2.5 enhancement in the North China Plain: Evidence from carbon and nitrogen isotopes

Measuring isotopic ratios in aerosol particles is a powerful tool for identifying major sources, particularly in separating fossil from non-fossil sources and investigating aerosol formation processes. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM_2.5 in Beijing (BJ) and Changdao (CD) in the North China Plain (NCP) from May to mid-June 2016. The mean PM_2.5 concentrations were 48.6 ± 28.2 μg m^-3 and 71.2 ± 29.0 μg m^-3 in BJ and CD, respectively, with a high contribution (∼66%) from secondary inorganic aerosol (SIA; NO₃^-, NH₄⁺, and SO₄^2-). The mean δ¹³C of total carbon (TC) and δ¹⁵N of total nitrogen (TN) values differed significantly between the two sites (p-value of <0.001): -25.1 ± 0.3‰ in BJ and -24.5 ± 0.4‰ in CD and 10.6 ± 1.8‰ in BJ and 5.0 ± 3.1‰ in CD, respectively. In BJ, the average δ¹⁵N (NH₄⁺) and δ¹⁵N (NO₃^-) values were 12.9 ± 2.3‰ and 5.2 ± 3.5‰, respectively. The ionic molar ratios and isotopic ratios suggest that NO₃^- in BJ was formed through the phase-equilibrium reaction of NH₄NO₃ under sufficient NH_{3 (g)} conditions, promoted by fossil-derived NH_{3 (g)} transported with southerly winds. In BJ, fossil fuel sources comprised 52 ± 7% of TC and 45 ± 28% of NH₄⁺ on average, estimated from radiocarbon (¹⁴C) analysis and the δ¹⁵N and isotope mixing model, respectively. These multiple-isotopic composition results emphasize that PM_2.5 enhancement is derived from fossil sources, in which vehicle emissions are a key contributor. The impact of the coal source was sporadically noticeable. Under regional influences, the fossil fuel-driven SIA led to the PM_2.5 enhancements. Our findings demonstrate that the multiple-isotope approach is highly advantageous to elucidate the key sources and limiting factors of secondary inorganic PM_2.5 aerosols.

Isotopic Interpretation of Particulate Nitrate in the Metropolitan City of Karachi, Pakistan: Insight into the Oceanic Contribution to NOx

Nitrogen oxide (NO_x) abatement has become the focus of air quality management strategies. In this study, we examined NO_x sources and the atmospheric conversion of NO_x in Karachi, Pakistan, a megacity in South Asia with serious particulate pollution problems. Oceanic contributions to NO_x were quantified for the first time based on a novel approach using nitrogen/oxygen isotopic analysis in nitrate (δ¹⁵N-NO₃^-; δ¹⁸O-NO₃^-) and a Bayesian model. Our results showed that δ¹⁵N-NO₃^- in Karachi varied between -10.2‰ and +12.4‰. As indicated by the δ¹⁸O-NO₃^- findings (+66.2 ± 7.8‰), the •OH pathway dominated NO_x conversion throughout the nearly two-year observation, but high NO₃^- events were attributed to the O₃ pathway. Coal combustion was the most significant source (32.0 ± 9.8%) of NO_x in Karachi, with higher contributions in the autumn and winter; a similar situation occurred for biomass burning + lightning (30.3 ± 6.5%). However, mobile sources (25.2 ± 6.4%) and microbial processes (12.5 ± 7.5%) exhibited opposite seasonal trends. The oceanic contributions to NO_x in Karachi were estimated to be 16.8%, of which lightning, shipping emissions, and microbial processes accounted for 20.3%, 46.3%, and 33.4%, respectively, emphasizing the dominance of shipping emissions as an oceanic NO_x source.

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.

The observation of isotopic compositions of atmospheric nitrate in Shanghai China and its implication for reactive nitrogen chemistry

The occurrence of PM_2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NO_X = NO + NO₂). The oxygen-17 excess of nitrate (Δ¹⁷O(NO₃^-)) can be used to reveal the relative importance of nitrate formation pathways and get more insight into reactive nitrogen chemistry. Here we present the observation of isotopic composition of atmospheric nitrate (Δ¹⁷O and δ¹⁵N) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m^-3 with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m^-3 in winter, spring and summer respectively. Δ¹⁷O(NO₃^-) varied from 20.5‰ to 31.9‰ with the mean value being (26.9 ± 2.8) ‰ in winter, followed by (26.6 ± 1.7) ‰ in spring and the lowest (23.2 ± 1.6) ‰ in summer. Δ¹⁷O(NO₃^-)-constrained estimates suggest that the conversion of NO_X to nitrate is dominated by NO₂ + OH and/or NO₂ + H₂O, with the mean possible contribution of 55-77% in total and even higher (84-92%) in summer. A diurnal variation of Δ¹⁷O(NO₃^-) featured by high values at daytime (28.6 ± 1.2‰) and low values (25.4 ± 2.8‰) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (τ) and calculations indicate τ is around 15 h during the diurnal sampling period. In terms of δ¹⁵N(NO₃^-), it changed largely in our observation, from -2.9‰ to 18.1‰ with a mean of (6.4 ± 4.4) ‰. Correlation analysis implies that the combined effect of NO_X emission sources and isotopic fractionation processes are responsible for δ¹⁵N(NO₃^-) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions.

Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing

Recent advances in stable isotope measurements now allow for detailed investigations of the sources, transformations, and deposition of reactive nitrogen (N) species. Stable isotopes show promise as a complementary tool for apportioning emissions sources that contribute to deposition and also for developing a more robust understanding of the transformations that can influence these isotope ratios. Methodological advances have facilitated the unprecedented examination of the isotopic composition of reactive N species in the atmosphere and in precipitation including nitrogen oxides (NOx = nitric oxide (NO) + nitrogen dioxide (NO2)), atmospheric nitrate (NO3-), nitric acid (HNO3), ammonia (NH3), and ammonium (NH4+). This isotopic information provides new insight into the mechanisms of transformation and cycling of reactive N in the atmosphere and moreover helps resolve the contribution of multiple NOx and NH3 emission sources to deposition across landscapes, regions, and continents. Here, we highlight the current state of knowledge regarding the isotopic ratios of NOx and NH3 emission sources and chemical alterations of isotopic ratios during atmospheric transformations. We also highlight illustrative examples where isotopic approaches are used and review recent methodological advances. While these highlights are not an exhaustive review of the literature, we hope they provide a glimpse of the potential for these methods to help resolve knowledge gaps regarding total N deposition to Earth surfaces. We conclude with promising opportunities for future research in the short-, medium-, and long-term.