First Measurements of the Nitrogen Isotopic Composition of NOx from Biomass Burning

Changes in source composition of wet nitrate deposition after air pollution control in a typical area of Southeast China

In recent years, China has adopted numerous policies and regulations to control NOx emissions to further alleviate the adverse impacts of NO3--N deposition. However, the variation in wet NO3--N deposition under such policies is not clear. In this study, the southeastern area, with highly developed industries and traditional agriculture, was selected to explore the variation in NO3--N deposition and its sources changes after such air pollution control through field observation and isotope tracing. Results showed that the annual mean concentrations of NO3--N in precipitation were 0.67 mg L-1 and 0.54 mg L-1 in 2014-2015 and 2021-2022, respectively. The average wet NO3--N depositions in 2014-2015 and 2021-2022 was 7.76 kg N ha-1 yr-1 and 5.03 kg N ha-1 yr-1, respectively, indicating a 35% decrease. The δ15N-NO3- and δ18O-NO3- values were lower in warm seasons and higher in cold seasons, and both showed a lower trend in 2021-2022 compared with 2014-2015. The Bayesian model results showed that the NOx emitted from coal-powered plants contributed 53.6% to wet NO3--N deposition, followed by vehicle exhaust (22.9%), other sources (17.1%), and soil emissions (6.4%) during 2014-2015. However, the contribution of vehicle exhaust (33.3%) overpassed the coal combustion (32.3%) and followed by other sources (25.4%) and soil emissions (9.0%) in 2021-2022. Apart from the control of air pollution, meteorological factors such as temperature, precipitation, and solar radiation are closely related to the changes in atmospheric N transformation and deposition. The results suggest phased achievements in air pollution control and that more attention should be paid to the control of motor vehicle exhaust pollution in the future, at the same time maintaining current actions and supervision of coal-powered plants.

Insights into anthropogenic impact on atmospheric inorganic aerosols in the largest city of the Tibetan Plateau through multidimensional isotope analysis

Particulate inorganic nitrogen aerosols (PIN) significantly influence air pollution and pose health risks worldwide. Despite extensive observations on ammonium (pNH4+) and nitrate (pNO3-) aerosols in various regions, their key sources and mechanisms in the Tibetan Plateau remain poorly understood. To bridge this gap, this study conducted a sampling campaign in Lhasa, the Tibetan Plateau's largest city, with a focus on analyzing the multiple isotopic signatures (δ15N, ∆17O). These isotopes were integrated into a Bayesian mixing model to quantify the source contributions and oxidation pathways for pNH4+ and pNO3-. Our results showed that traffic was the largest contributor to pNH4+ (31.8 %), followed by livestock (25.4 %), waste (21.8 %), and fertilizer (21.0 %), underscoring the impact of vehicular emissions on urban NH3 levels in Lhasa. For pNO3-, coal combustion emerged as the largest contributor (27.3 %), succeeded by biomass burning (26.3 %), traffic emission (25.3 %), and soil emission (21.1 %). In addition, the ∆17O-based model indicated a dominant role of NO2 + OH (52.9 %) in pNO3- production in Lhasa, which was similar to previous observations. However, it should be noted that the NO3 + volatile organic component (VOC) contributed up to 18.5 % to pNO3- production, which was four times higher than the Tibetan Plateau's background regions. Taken together, the multidimensional isotope analysis performed in this study elucidates the pronounced influence of anthropogenic activities on PIN in the atmospheric environment of Lhasa.

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.

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 (NO x ) 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 NO x source apportionment using nitrogen stable isotopes of ambient NO 2 , which indicate a substantial contribution of soil-emitted NO x . 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. Therefore, the impact of soil NO x in agricultural regions must be re-evaluated.

Increase in Agricultural-Derived NHx and Decrease in Coal Combustion-Derived NOx Result in Atmospheric Particulate N-NH4+ Surpassing N-NO3- in the South China Sea

The atmospheric deposition of anthropogenic active nitrogen significantly influences marine primary productivity and contributes to eutrophication. The form of nitrogen deposition has been evolving annually, alongside changes in human activities. A disparity arises between observation results and simulation conclusions due to the limited field observation and research in the ocean. To address this gap, our study undertook three field cruises in the South China Sea in 2021, the largest marginal sea of China. The objective was to investigate the latest atmospheric particulate inorganic nitrogen deposition pattern and changes in nitrogen sources, employing nitrogen-stable isotopes of nitrate (δ15N-NO3-) and ammonia (δ15N-NH4+) linked to a mixing model. The findings reveal that the N-NH4+ deposition generally surpasses N-NO3- deposition, attributed to a decline in the level of NOx emission from coal combustion and an upswing in the level of NHx emission from agricultural sources. The disparity in deposition between N-NH4+ and N-NO3- intensifies from the coast to the offshore, establishing N-NH4+ as the primary contributor to oceanic nitrogen deposition, particularly in ocean background regions. Fertilizer (33 ± 21%) and livestock (20 ± 6%) emerge as the primary sources of N-NH4+. While coal combustion continues to be a significant contributor to marine atmospheric N-NO3-, its proportion has diminished to 22 (Northern Coast)-35% (background area) due to effective NOx emission controls by the countries surrounding the South China Sea, especially the Chinese Government. As coal combustion's contribution dwindles, the significance of vessel and marine biogenic emissions grows. The daytime higher atmospheric N-NO3- concentration and lower δ15N-NO3- compared with nighttime further underscore the substantial role of marine biogenic emissions.

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.

Estimating atmospheric nitrogen deposition within a large river basin using moss nitrogen and isotope signatures

Assessing flux and primary sources of the atmospheric nitrogen (N) deposition with high spatial resolution remained challenging. The epilithic moss is considered a suitable biological monitor to explore N deposition. Our study presented a detailed analysis of flux and major source contributions of ammonium (NH4+) and nitrate (NO3-) deposition using N and δ15N signatures of epilithic moss collected densely from the Yangtze River basin. The results showed a more negative δ15N and higher N concentration of the moss in cropland and urban area than in forest and grassland of the basin. A gradient of the estimated N deposition (9.6-34.0 kg ha-1 yr-1) occurred from the Tibetan Plateau to lower reaches, with amount of NH4+ was approximately three times higher than NO3- deposition. The contribution from volatilization to NH4+ deposition (33.28 ± 8.10%) was less than the contribution from combustion (66.72 ± 8.10%), inconsistent with the traditional findings that N fertilizer and livestock waste are the principal sources of NH3 emissions. Fossil fuel was the dominant sources of NO3- deposition, accounted for 70.22 ± 18.67%. From 2006 to 2019, the source contribution of N deposition in forest remained unchanged, while NH3 volatilization and fossil fuel emitted NOx in urban areas have increased. Our findings highlighted the importance of combustion sources to N deposition in the Yangtze River basin.

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.

Nitrate isotopes (δ15N, δ18O) in precipitation: best practices from an international coordinated research project

Stable isotope ratios of nitrogen and oxygen (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) of nitrate (NO₃^-) are excellent tracers for developing systematic understanding of sources, conversions, and deposition of reactive atmospheric nitrogen (N_r) in the environment. Despite recent analytical advances, standardized sampling of NO₃^-) isotopes in precipitation is still lacking. To advance atmospheric studies on N_r species, we propose best-practice guidelines for accurate and precise sampling and analysis of NO₃^- isotopes in precipitation based on the experience obtained from an international research project coordinated by the International Atomic Energy Agency (IAEA). The precipitation sampling and preservation strategies yielded a good agreement between the NO₃^- concentrations measured at the laboratories of 16 countries and at the IAEA. Compared to conventional methods (e.g., bacterial denitrification), we confirmed the accurate performance of the lower cost Ti(III) reduction method for isotope analyses (¹⁵N and ¹⁸O) of NO₃^- in precipitation samples. These isotopic data depict different origins and oxidation pathways of inorganic nitrogen. This work emphasized the capability of NO₃^- isotopes to assess the origin and atmospheric oxidation of N_r and outlined a pathway to improve laboratory capability and expertise at a global scale. The incorporation of other isotopes like ¹⁷O in N_r is recommended in future studies.

Investigations of Vacancy-Assisted Selective Detection of NO2 Molecules in Vertically Aligned SnS2

Two important methods for enhancing gas sensing performance are vacancy/defect and interlayer engineering. Tin sulfide (SnS₂) has recently attracted much attention for sensing of the NO₂ gas due to its active surface sites and tunable electronic structure. Herein, SnS₂ has been synthesized by the chemical vapor deposition (CVD) method followed by nitrogen plasma treatment with different exposure times for fast detection of NO₂ molecules. Plasma treatment created a substantial number of surface vacancies on SnS₂ flakes, which were controlled by the exposure period to modify the surface of flakes. After 12 min of nitrogen plasma treatment, SnS₂ nanoflakes show considerable improvement in NO₂ sensing characteristics, including a high sensing response of ∼264% toward 100 ppm NO₂ at 120°C. The enhancement in the relative response of the sensor is due to the electronic interaction between NO₂ molecules and the S vacancies on the surface of SnS₂. Density functional theory (DFT) computations indicate that the S-vacancy defects on the surface dominate the effective NO₂ detection and the NO₂ adsorption mechanism transition from physisorption to chemisorption. Adsorption kinetics of the NO₂ gas over SnS₂ nanoflake-based chemiresistor sensors were studied using the Lee and Strano model [ Langmuir 2005, 21(11), 5192-5196]. The irreversible rate of the reaction for various NO₂ concentrations exposed to the gas sensor is extracted using this model, which also appropriately describes the response curves. The forward rate constant of the irreversible gas sensor increased with the increase of the N₂ plasma treatment time and reached the maximum in the 12 min plasma-treated sample. Through defect engineering, this research may open up new vistas for the design and synthesis of 2D materials with enhanced sensing properties.

Quantifying the source and formation of nitrate in PM2.5 using dual isotopes combined with Bayesian mixing model: A case study in an inland city of southeast China

Atmospheric nitrate has been attracting increasing attention because it is one of the important components of PM_2.5. Understanding the sources and formation mechanism of nitrate in PM_2.5 is essential to take effective measures to prevent and control the emission of nitrogen oxides in the atmosphere and reduce the formation of haze. In this study, PM_2.5 samples were collected from Ganzhou, an inland city in southeast China, during summer and winter. The concentrations of PM_2.5 and NO₃^- were determined, and the isotopic compositions of NO₃^- (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were analyzed in order to quantify the relative contributions of different emission sources and formation pathways of nitrate in PM_2.5. The results showed that PM_2.5 and NO₃^- concentrations were lower in summer (39.80 ± 11.10 μg·m^-3 and 1.79 ± 0.70 μg·m^-3) while higher in winter (69.85 ± 29.58 μg·m^-3 and 10.83 ± 9.89 μg·m^-3). The values of δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- ranged from 42.84‰ to 56.80‰ and from -11.17‰ to -2.08‰ in summer, while from 55.86‰ to 78.66‰ and from -10.63‰ to -1.88‰ in winter, respectively. The results of δ¹⁵N-NO₃^- combined with Bayesian isotope mixing model showed that the relative contributions of vehicle exhaust, soil microbial activity, biomass combustion and coal fired power plants were 59.3%, 28.5%, 8.7% and 3.4% in summer, while 65.1%, 20.1%, 10.6% and 4.1% in winter, respectively. The results of δ¹⁸O-NO₃^- combined with Bayesian isotope mixing model showed that the possible relative contributions of pathway 1 (P1) (NO₂ + ·OH), pathway 2 (P2) (NO₃ + HC) and pathway 3 (P3) (N₂O₅ + H₂O) were 73.5%, 12.4% and 14.1% in summer, while 41.6%, 28.9% and 29.5% in winter, respectively. Moreover, P2 and P3 contributed more when NO₃^- concentrations were higher, suggesting that P2 and P3 were of significance to the formation of nitrate in PM_2.5, especially during winter.

Application of Stable Isotope Techniques in Tracing the Sources of Atmospheric NOX and Nitrate

Nitrate is an important component of PM2.5, and its dry deposition and wet deposition can have an impact on ecosystems. Nitrate in the atmosphere is mainly transformed by nitrogen oxides (NOX = NO + NO2) through a number of photochemical processes. For effective management of the atmosphere’s environment, it is crucial to understand the sources of atmospheric NOX and the processes that produce atmospheric nitrate. The stable isotope method is an effective analytical method for exploring the sources of NO3− in the atmosphere. This study discusses the range and causes of δ15N data from various sources of NOX emissions, provides the concepts of stable isotope techniques applied to NOX traceability, and introduces the use of Bayesian mixture models for the investigation of NOX sources. The combined application of δ15N and δ18O to determine the pathways of nitrate formation is summarized, and the contribution of Δ17O to the atmospheric nitrate formation pathway and the progress of combining Δ17O simulations to reveal the atmospheric oxidation characteristics of different regions are discussed, respectively. This paper highlights the application results and development trend of stable isotope techniques in nitrate traceability, discusses the advantages and disadvantages of stable isotope techniques in atmospheric NOX traceability, and looks forward to its future application in atmospheric nitrate pollution. The research results could provide data support for regional air pollution control measures.

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.

Nitrogen isotopic composition of NOx from residential biomass burning and coal combustion in North China

Stable nitrogen isotope (δ¹⁵N) technology has often been used as a powerful tool to separate nitrogen oxides (NO_x) produced by residential combustion (i.e., biomass burning and coal combustion) from other sources. However, the insufficient measurement of δ¹⁵N-NO_x fingerprints of these emissions limits its application, especially in North China where residential emissions are significant. This study conducted combustion experiments to determine the δ¹⁵N-NO_x of typical residential fuels in North China, including ten biomass fuels and five types of coal. The results showed that the δ¹⁵N of biomass varied between -6.9‰ and 2.3‰, which was lower than the δ¹⁵N of residential coal (-0.2‰-4.6‰). After combustion, the δ¹⁵N of biomass residues increased greatly, while that of coal residues showed no significant upward trend (p > 0.05). The δ¹⁵N-NO_x produced by biomass burning ranged from -5.6‰ to 3.2‰ (-0.4‰ ± 2.4‰), showing a significant linear relation with δ¹⁵N-biomass. Comparatively, the δ¹⁵N-NO_x derived from residential coal combustion was much higher (16.1‰ ± 3.3‰), ranging from 11.7‰ to 19.7‰. It was not well correlated with δ¹⁵N-coal, and only slightly lower than the estimated δ¹⁵N-NO_x of industrial coal combustion (17.9‰, p > 0.05). These observations indicate that the δ¹⁵N-NO_x of residential coal combustion is a result of the mixture of thermal- and fuel-released NO_x. Based on the isotopic characteristics observed in this study, we analyzed the reported δ¹⁵N-NO_x, and provided more statistically robust δ¹⁵N-NO_x distributions for biomass burning (1.3‰ ± 4.3‰; n = 101) and coal combustion (17.9‰ ± 3.1‰; n = 26), which could provide guidance for scientific studies aiming to quantify the origin of NO_x in North China and in other regions.

The isotopic patterns and source apportionment of nitrate and ammonium in atmospheric aerosol

Nitrate (NO₃^-) and ammonium (NH₄⁺) are the major components in inorganic aerosol. However, their sources and formation processes remain unclear. This study conducted a year-round field measurement of TSP, PM_2.5 and PM_1.0 in five different sites in the Beijing-Tianjin-Hebei (BTH) region to determine the concentrations of water-soluble inorganic ions (WSIIs) and the isotopic compositions of inorganic nitrogen (δ¹⁵N-NH₄⁺, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-). The results showed the highest concentration of WSIIs in winter and lowest in summer. δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁵N-NH₄⁺ were in the range of -6.1-18.2, 52.2-103.8, and -28.7-36.2‰, respectively. The seasonal variations of δ¹⁵N-NO₃^- and δ¹⁵N-NH₄⁺ were an indication of relative contributions of the main sources and effects of meteorological conditions. The source apportionment identified fossil fuel combustion (38.2-50.6%), agricultural emissions (18-24.7%), biomass burning (16.3-22.7%), and road dust/soil (8.7-23.4%) were the main sources of inorganic aerosols. The local sources and regional migration contributed to the level of inorganic aerosol pollution. In winter, the aerosol in the BTH region was affected by the air mass from the northwest. While in spring and summer, the air mass was mainly from the South China. The low temperature and high relative humidity favored to the formation of inorganic nitrogen aerosol, and solar radiation affected the formation processes of inorganic aerosols by changing the oxidation pathway of NO₃^- and accelerating the volatilization and dissociation of ammonium nitrate (NH₄NO₃). This study discovered the main source contributions of inorganic nitrogen aerosol using N and O isotopes composition, and the obtained information has a great importance in understanding the effects of meteorological conditions on formation and the contribution of regional transport.

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.

The use of stable oxygen and nitrogen isotopic signatures to reveal variations in the nitrate formation pathways and sources in different seasons and regions in China

Nitrate (NO₃^-) is one of the most important inorganic ions in fine particulate (PM_2.5) and drives regional haze formation; however, the NO₃^- sources and formation mechanisms in different seasons and regions are still debated. Here, PM_2.5 samples were collected from Kunming and Nanning in southwestern China from September 1, 2017, to February 28, 2018 (spanning warm and cold months). We measured the daily O and N isotopic compositions of NO₃^- (δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^-), estimated the δ¹⁸O-HNO₃ values produced by different oxidation pathways, and quantified the NO₃^- formation pathways based on the isotope mass-balance equation. Our results showed that the δ¹⁸O-NO₃^- values in Kunming (65.3 ± 7.6‰) and Nanning (67.7 ± 10.1‰) are close to the δ¹⁸O-HNO₃ values arising from the OH radical pathway (P_OH, 54.7 ± 1.2‰ to 61.2 ± 1.8‰), suggesting that the δ¹⁸O-NO₃^- values are mainly influenced by P_OH, which showed a contribution greater than 74%. Stronger surface solar radiation and higher air temperatures in low-latitude regions and warm months increased the amount of HNO₃ produced by P_OH and reduced the amount of HNO₃ produced by P_N2O5, which produced low δ¹⁸O-NO₃^- values. Increased air pollution emissions decreased the contribution from P_OH and increased the contribution from N₂O₅ and NO₃ pathways (P_N2O5+NO3). The δ¹⁵N-NO₃^- values of PM_2.5 in Kunming (7.3 ± 2.8‰) were slightly higher than those in Nanning (2.8 ± 2.7‰). The increased NO_x emissions with positive isotopic values led to high δ¹⁵N-NO₃^- values in northern China and during cold months. A higher f_NO2 (f_NO2 = NO₂/(NO + NO₂), temperature, and contribution of P_OH produced lower N isotope fractionation between NO_x and δ¹⁵N-NO₃^-, which was found to further decrease the δ¹⁵N-NO₃^- values in southwestern China and during warm months.

Formation Mechanisms and Source Apportionments of Airborne Nitrate Aerosols at a Himalayan-Tibetan Plateau Site: Insights from Nitrogen and Oxygen Isotopic Compositions

Formation pathways and sources of atmosphere nitrate (NO₃^-) have attracted much attention as NO₃^- had detrimental effects on Earth's ecosystem and climate change. Here, we measured nitrogen (δ¹⁵N-NO₃^-) and oxygen (δ¹⁸O-NO₃^- and Δ¹⁷O-NO₃^-) isotope compositions in nitrate aerosols at the Qomolangma station (QOMS) over the Himalayan-Tibetan Plateau (HTP) to quantify the formation mechanisms and emission sources of nitrate at the background site. At QOMS, the enhanced NO₃^- concentrations were observed in the springtime. The average δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and Δ¹⁷O-NO₃^- values were 0.4 ± 4.9, 64.7 ± 11.5 and 27.6 ± 6.9‰, respectively. Seasonal variations of isotope ratios at QOMS can be explained by the different emissions and formation pathways to nitrate. The average fractions of NO₂ + OH and N₂O₅ + H₂O to nitrate production were estimated to be 43 and 52%, respectively, when the NO₃ + hydrocarbon (HC)/dimethyl sulfide (DMS) (NO₃ + HC/DMS) pathway was assumed to be 5%. Using stable isotope analysis in the R (SIAR) model, the relative contributions of biomass burning (BB), biogenic soil emission, traffic, and coal combustion to nitrate were estimated to be 28, 25, 24, and 23%, respectively, on yearly basis. By FLEXible PARTicle (FLEXPART) dispersion model, we highlighted that NO_x from BB emission over South Asia that had undergone N₂O₅ + H₂O processes enhanced the nitrate concentrations in the springtime over the HTP region.

Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate

Nitrate (NO₃^-) has become recognized as the most important water-soluble ion in fine particulate (PM_2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM_2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO₃^-concentration, δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- values in PM_2.5 were analyzed. NO₃^- concentrations increased as PM_2.5 levels increased during both polluted and non-polluted days over the entire year. δ¹⁸O-NO₃^- values during cold months (63.5-103‰) were higher than those during warm months (50.3-85.4‰), these results suggested that the nitrate formation pathways shifted from the NO₂ + OH (P_OH) in warm months to the N₂O₅ + H₂O (P_N2O5) and NO₃ + VOCs (P_NO3) pathways in cold months. Especially during cold months, δ¹⁸O-NO₃^- values increased from 65.2-79.9‰ to 80.7-96.2‰ when PM_2.5 increased from ∼25 to >100 μg/m³, but when PM_2.5 > 100 μg/m³, there were relatively small variations in δ¹⁸O-NO₃^-. These results suggested that nitrate formation pathways changed from P_OH to P_N2O5 and P_NO3 pathways when PM_2.5 < 100 μg/m³, but that P_N2O5 and P_NO3 dominated nitrate production when PM_2.5 > 100 μg/m³. Higher δ¹⁵N-NO₃^- values in warm months (-11.8-13.8‰) than in cold months (-0.7-22.6‰) may be attributed to differences in NO_x emission sources and nitrogen isotopic fractionation among NO_x and NO₃^-. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM_2.5 levels.

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.

Chemical and isotopic characteristics of PM10 over the Bay of Bengal: Effects of continental outflow on a marine environment

Pollutants transport from South and Southeast Asia can profoundly affect the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB). This study presents chemical and stable isotopic composition of PM₁₀ collected at Port Blair Island (11.6°N, 92.7°E) located in the middle of the BoB during the late northeast monsoon (February-April), a period when the BoB receives considerable continental outflow. These samples (n = 50) were analysed for major ions, carbonaceous species, trace metals, and isotopic composition of total C, N, and S components. Mass concentration of PM₁₀ ranged from 24 to 65 μg m^-3 during the study period. The dominance of continental inputs over a marine realm was evident by a significant amount of non-sea-salt (nss)-SO₄^2- (range: 1.8 to 16.9 μg m^-3), which accounts for ~65% of the total water-soluble inorganic constituents. The impact of anthropogenic emissions was further evident from the widespread depletion of chloride (range: 57-100%, avg.: 98 ± 7%) from sea-salt aerosols. Carbonaceous species (elemental carbon and organic matter) contributed nearly 35% to PM₁₀. Further, average δ¹³C (-25.6‰ ± 0.5) and δ³⁴S (4.5‰ ± 1.3) values observed over the marine study region were similar to those found in typical urban environments. δ¹⁵N values (13.7‰ ± 5.1) show the significant presence of combustion sources along with the effect of atmospheric processing. Aerosol δ¹³C values correlate positively with the ratio of water-soluble organic carbon to total organic carbon, indicating the aging of organic aerosols during the transport. Chemical and isotopic data suggest that both biomass burning (BB) and fossil fuel burning (FFB) contributed to ambient PM₁₀ with relatively more contribution of BB during February to early March and that of FFB during late March to middle of April. In aggregate, this study provides newer insights into sources of carbonaceous species and their chemical processing in MABL of BoB.

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.

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.

Impact of Coal Replacing Project on atmospheric fine aerosol nitrate loading and formation pathways in urban Tianjin: Insights from chemical composition and 15N and 18O isotope ratios

The 'Coal Replacing Project' (CRP), replacing coal with cleaner energy like natural gas and electricity, was implemented in North China to curb PM_2.5 pollution; therefore, it is important to explore the sources and transformation mechanisms of PM_2.5 nitrate under this strategy for examining its effectiveness. In this study, daytime and nighttime PM_2.5 samples of one summer (Jul-2016, C1) and two winters (Jan-2017, C2 and Jan-2018, C3, before and during the CRP, respectively) were collected in urban Tianjin. Concentrations of PM_2.5 and water-soluble inorganic ions were analyzed, and δ¹⁵N and δ¹⁸O were used to calculate the contributions of different NO_X sources to nitrate based on a Bayesian mixing model. The results showed that the average concentrations of PM_2.5 and its dominant inorganic ions (SO₄^2-, NO₃^-, NH₄⁺) in C3 during the CRP, compared to C2, decreased by 62.13%, 79.69%, 55.14% and 38.84%, respectively, attesting the improvement of air quality during the CRP. According to the correlation between [NO₃^-/SO₄^2-] and [NH₄⁺/SO₄^2-] as well as δ¹⁸O variations, the homogeneous formation pathway might be dominant in C1, while the heterogeneous pathway would be primary in C2 and C3 during the formation of nitrate. Moreover, the heterogeneous pathway contributed more in C3 than in C2. The dominant sources in C1 were biogenic soil emission (37.0% ± 9.9%) and mobile emission (25.7% ± 19.1%), while coal combustion (42.4% ± 13.8% in C2 and 34.9% ± 14.4% in C3) and biomass burning (31.0% ± 21.2% and 34.7% ± 22.7%) were the main sources in C2 and C3. In the winter, the contribution of coal combustion dropped by about 8% during the CRP (C3) in comparison with that in C2, suggesting the implementation of CRP played an important role in reducing NO_X from coal combustion.

Rayleigh based concept to track NOx emission sources in urban areas of China

Despite the implementation of some effective measures to control emissions of nitrogen oxides (NOx = NO + NO₂) in recent years, the ambient NOx concentration in urban cities of China remains high. Therefore, a quantitative understanding of NOx emission sources is critical to developing effective mediation policies. In the present study, the dual isotopic compositions of nitrate (p-NO₃^-) in fine-particle aerosol (PM_2.5) collected daily at a regional scale (Beijing-Tianjin-Shijiazhuang) were measured to better constrain the NOx emission sources. The specific focus was on a typical haze episode that occurred simultaneously in the three urban cities (October 22-29, 2017). It was found that the nitrogen isotopic values of nitrate in PM_2.5 (hereafter as δ¹⁵N-NO₃^-) ranged widely from -3.1‰ to + 11.4‰, with a mean value of 3.5 ± 3.7‰. Furthermore, a negative relationship between the δ¹⁵N-NO₃^- values and the corresponded p-NO₃^- concentrations during the haze period was observed. This implied the preferential formation of ¹⁵N-enriched NO₃^- into a fine-particle aerosol. Taking a different approach to previous publications, the Rayleigh fractionation model was used to characterize the initial isotopic signatures of ambient NOx. After accounting for the δ¹⁵N difference between p-NO₃^- and the source NOx, the estimated initial δ¹⁵N-NO_x ranged from -20‰ to 0‰, which indicated a cosniderable contribution of non-fossil fuel emissions. The individual contributions of potential sources were further quantified using the Bayesian mixing model, revealing that NOx from coal or natural gas combustion, vehicle exhausts, biomass burning, and the microbial activity contributed 17.9 ± 11.4%, 29.4 ± 19.6%, 29.1 ± 18.9% and 23.5 ± 12.7% to NO₃^- in PM_2.5, respectively. These results highlighted that tightening controls of gaseous NOx emissions from non-fossil sources may represent an opportunity to mitigate PM_2.5 pollution in urban cities of China.

Nitrogen isotope differences between atmospheric nitrate and corresponding nitrogen oxides: A new constraint using oxygen isotopes

Tracking of reactive nitrogen (N) sources is important for the effective mitigation of N emissions. By combining the N and oxygen (O) isotopes of atmospheric NO₃^-, stable isotope mixing models were recently applied to evaluate the relative contributions of major NO_x sources. However, it has long been unresolved how to accurately constrain the δ¹⁵N differences between NO₃^- and corresponding NO_x (ε_(NO2→NO3-) values). Here, we first incorporated the HC oxidation (NO₂ → NO₃^-) pathway by using Δ¹⁷O values to evaluate the ε_(NO2→NO3-) values, performed on NO₃^- in PM_2.5 collected during the day and at night from January 4-13, 2015 at an urban site in Beijing. We found that the Δ¹⁷O-based ε values (ε_{17O-based(NO2→NO3-)}) (15.6 ± 7.4‰) differed distinctly from δ¹⁸O-based ε values (ε_{18O-based(NO2→NO3-)}) (33.0 ± 9.5‰) so did not properly incorporate the isotopic effects of the HC oxidation (NO₂ → NO₃^-) pathway. Based on the ε_(NO2→NO3-) values, δ¹⁵N values of NO_x from coal combustion (CC), vehicle exhausts (VE), biomass burning (BB), and the microbial N cycle (MC), as well as NO₃^- in PM_2.5, we further quantified the source contributions by using Stable Isotope Analysis in R (the SIAR model). We found that the respective fractional contributions of CC-NO_x and MC-NO_x were underestimated by 64% and were overestimated by 216% by using ε_{18O-based(NO2→NO3-)} values. We concluded that the new ε_{17O-based(NO2→NO3-)} values reduced uncertainties in contribution analysis and the evaluation method for atmospheric NO₃^- sources.

Isotopic evaluation on relative contributions of major NOx sources to nitrate of PM2.5 in Beijing

Nitrate (NO₃^-) is a key component of secondary inorganic aerosols and PM_2.5. However, the contributions of nitrogen oxides (NO_x) emission sources to NO₃^- in PM_2.5 remain poorly constrained. This study measured nitrogen (N) isotopes of NO₃^- (hereafter as δ¹⁵N-NO₃^-) in PM_2.5 collected at Beijing in 2014. We observed that δ¹⁵N-NO₃^- values in PM_2.5 (-2.3‰ - 19.7‰; 7.3 ± 5.4‰ annually) were significantly higher in winter (11.9 ± 4.4‰) than in summer (2.2 ± 2.5‰). The δ¹⁵N differences between source NO_x and NO₃^- in PM_2.5 (hereafter as Δ values) were estimated by a computation module as 7.8 ± 2.2‰ - 10.4 ± 1.6‰ (8.8 ± 2.4‰). Using the Δ values and δ¹⁵N values of NO_x from major fossil (coal combustion, vehicle exhausts) and non-fossil sources (biomass burning, microbial N cycle), contributions of major NO_x sources to NO₃^- in PM_2.5 were further estimated by the SIAR model. We found that seasonal variations of δ¹⁵N-NO₃^- values in PM_2.5 of Beijing were mainly caused by those of NO_x contributions from coal combustion (38 ± 10% in winter, 20 ± 9% in summer). Annually, NO_x from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 ± 12%, 29 ± 17%, 27 ± 15%, and 16 ± 7% to NO₃^- in PM_2.5, respectively, showing actually comparable contributions between non-fossil NO_x (43 ± 16%) and fossil NO_x (57 ± 21%). These results are useful for planning the reduction of NO_x emissions in city environments and for elucidating relationships between regional NO_x emissions and atmospheric NO₃^- pollution or deposition.

Enhance the Discrimination Precision of Graphene Gas Sensors with a Hidden Markov Model

Sensors are the key element to enable smart electronics and will play an important role in the emerging big data era. In this work, we reported an experimental study and a data-analytical characterization method to enhance the precision of discriminating chemically and structurally similar gases. Graphene sensors were fabricated by conventional photolithography and measured with feature analysis against different chemicals. A new hidden Markov model assisted with frequency spectral analysis, and the Gaussian mixture model (K-GMM-HMM) is developed to discriminate similar gases. The results indicated that the new method achieved a high prediction accuracy of 94%, 27% higher than the maximum value obtained by the conventional methods or other feature transient analysis methods. This study indicated that graphene gas sensors with the new K-GMM-HMM analysis are very attractive for chemical discrimination used in future smart electronics.

Assessment and quantification of NOx sources at a regional background site in North China: Comparative results from a Bayesian isotopic mixing model and a positive matrix factorization model

Regional sources of nitrogen oxides (NO_x) in North China during summer were explored using both a Bayesian isotopic mixing model and a positive matrix factorization (PMF) model. Results showed that the nitrogen isotope (δ¹⁵N) composition of particulate nitrate (NO₃^-) varied between -8.9‰ and +14.1‰, while the oxygen isotope (δ¹⁸O) composition ranged from +57.4‰ to +93.8‰. Based on results from the Bayesian isotopic mixing model, the contribution of the hydroxyl radical (•OH) NO_x conversion pathway showed clear diurnal fluctuation; values were higher during the day (0.53 ± 0.16) and lower overnight (0.42 ± 0.17). Values peaked at 06:00-12:00 and then decreased gradually until 00:00-06:00 the next day. Coal combustion (31.34 ± 9.04%) was the most significant source of NO_x followed by biomass burning (25.74 ± 2.58%), mobile sources (23.83 ± 3.66%), and microbial processes (19.09 ± 5.21%). PMF results indicated that the contribution from mobile sources was 19.83%, slightly lower as compared to the Bayesian model (23.83%). The PMF model also reported a lower contribution from coal combustion (28.65%) as compared to the Bayesian model (31.34%); however, the sum of biomass burning and microbial processes in the Bayesian model (44.83%) was lower than the aggregate of secondary inorganic aerosol, sea salt, and soil dust in PMF model (51.52%). Overall, differences between the two models were minor, suggesting that this study provided a reasonable source quantification for NO_x in North China during summer.

A comparison of various approaches used in source apportionments for precipitation nitrogen in a mountain region of southwest China

Six different approaches are applied in the present study to apportion the sources of precipitation nitrogen making use of precipitation data of dissolved inorganic nitrogen (DIN, including NO₃^- and NH₄⁺), dissolved organic nitrogen (DON) and δ¹⁵N signatures of DIN collected at six sampling sites in the mountain region of Southwest China. These approaches include one quantitative approach running a Bayesian isotope mixing model (SIAR model) and five qualitative approaches based on in-situ survey (ISS), ratio of NH₄⁺/NO₃^- (R_N), principal component analysis (PCA), canonical-correlation analysis (CCA) and stable isotope approach (SIA). Biomass burning, coal combustion and mobile exhausts in the mountain region are identified as major sources for precipitation DIN while biomass burning and volatilization sources such as animal husbandries are major ones for DON. SIAR model results suggest that mobile exhausts, biomass burning and coal combustion contributed 25.1 ± 14.0%, 26.0 ± 14.1% and 27.0 ± 12.6%, respectively, to NO₃^- on the regional scale. Higher contributions of both biomass burning and coal combustion appeared at rural and urban sites with a significant difference between Houba (rural) and the wetland site (p < 0.05). The R_N method fails to properly identify sources of DIN, the ISS and SIA approach only respectively identifies DON and DIN sources, the PCA only tracks source types for precipitation N, while the CCA identify sources of both DIN and DON in precipitation. SIAR quantified the contributions of major sources to precipitation NO₃^- but failed for precipitation NH₄⁺ and DON. It is recommended to use ISS and SIAR in combination with one or more approaches from PCA, CCA and SIA to apportion precipitation NO₃^- sources. As for apportioning precipitation NH₄⁺ sources, more knowledge is needed for local ¹⁵N databases of NH₃ and DON and ¹⁵N fractional mechanisms among gaseous, liquid and particulate surfaces in this mountain region and similar environments.

Stable isotope analyses of precipitation nitrogen sources in Guiyang, southwestern China

To constrain sources of anthropogenic nitrogen (N) deposition is critical for effective reduction of reactive N emissions and better evaluation of N deposition effects. This study measured δ¹⁵N signatures of nitrate (NO₃^-), ammonium (NH₄⁺) and total dissolved N (TDN) in precipitation at Guiyang, southwestern China and estimated contributions of dominant N sources using a Bayesian isotope mixing model. For NO₃^-, the contribution of non-fossil N oxides (NO_x, mainly from biomass burning (24 ± 12%) and microbial N cycle (26 ± 5%)) equals that of fossil NO_x, to which vehicle exhausts (31 ± 19%) contributed more than coal combustion (19 ± 9%). For NH₄⁺, ammonia (NH₃) from volatilization sources (mainly animal wastes (22 ± 12%) and fertilizers (22 ± 10%)) contributed less than NH₃ from combustion sources (mainly biomass burning (17 ± 8%), vehicle exhausts (19 ± 11%) and coal combustions (19 ± 12%)). Dissolved organic N (DON) accounted for 41% in precipitation TDN deposition during the study period. Precipitation DON had higher δ¹⁵N values in cooler months (13.1‰) than in warmer months (-7.0‰), indicating the dominance of primary and secondary ON sources, respectively. These results newly underscored the importance of non-fossil NO_x, fossil NH₃ and organic N in precipitation N inputs of urban environments.

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

The global inventory of NO_x (NO_x = NO + NO₂) emissions is poorly constrained, with a large portion of the uncertainty attributed to soil NO emissions that result from soil abiotic and microbial processes. While natural abundance stable N isotopes (δ¹⁵N) in various soil N-containing compounds have proven to be a robust tracer of soil N cycling, soil δ¹⁵N-NO is rarely quantified due to the measurement difficulties. Here, we present a new method that collects soil-emitted NO through NO conversion to NO₂ in excess ozone (O₃) and subsequent NO₂ collection in a 20% triethanolamine (TEA) solution as nitrite and nitrate for δ¹⁵N analysis using the denitrifier method. The precision and accuracy of the method were quantified through repeated collection of an analytical NO tank and intercalibration with a modified EPA NO_x collection method. The results show that the efficiency of NO conversion to NO₂ and subsequent NO₂ collection in the TEA solution is >98% under a variety of controlled conditions. The method precision (1σ) and accuracy across the entire analytical procedure are ±1.1‰. We report the first analyses of soil δ¹⁵N-NO (-59.8‰ to -23.4‰) from wetting-induced NO pulses at both laboratory and field scales that have important implications for understanding soil NO dynamics.