Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing

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.

Sources and processes affecting the abundances of atmospheric NHx using δ15N over northwestern Indo-Gangetic plain

Ammonia (NH3) is the major constituent among all the reactive nitrogen species present in the atmosphere, and the most essential species for secondary inorganic aerosol formation. Recent satellite-based observations have identified the Indo-Gangetic Plain (IGP) as a major hotspot of global NH3 emission; however, the major sources and atmospheric processes affecting its abundance are poorly understood. The present study aims to understand the wintertime sources of NH3 over a semi-urban site (Patiala, 30.3°N, 76.4°E, 249 m amsl) located in the IGP using species specific δ15N in PM2.5. A distinct diurnal variation in the stable isotopic signature of total nitrogen (δ15N-TN) and ammonium (δ15N-NH4+) were observed; although, average day and night time concentrations of TN and NH4+ were similar. Mixing model results using δ15N-NH3 reveal the dominance of non-agricultural emissions (NH3 slip: 47 ± 24%) over agricultural emissions (24 ± 11%), combustion sources (19 ± 14 %), and biomass burning (10 ± 8%) for atmospheric NH3. Diurnal variability in source contributions to NH3 was insignificant. Further, significantly negative correlations of δ15N-NH4+ with ambient relative humidity (RH) and daytime NO3--N concentration were observed, and attributed to the possibility of NH4NO3 volatilization during day-time owing to lower RH and higher temperature, resulting in isotopic enrichment of the remaining NH4+ in aerosol phase. This study, a first of its type from India, highlights the importance of non-agricultural NH3 emissions over the agriculture dominated IGP region, and the role of local meteorology on the isotopic fractionation of δ15N in aerosol NH4+.

Moss differentiating the fluxes and sources of nitrogen deposition between 1984 and 2021 in a mountain area of Northern China

Anthropogenic reactive nitrogen (N) deposition has increased significantly since the industrial revolution. Northern China has become a global hotspot for N deposition. However, few studies have been conducted to quantify the historical changes of atmospheric N deposition fluxes and source contributions in Northern China. By investigating N contents and δ15N values of mosses at Mount Tai (Northern China) in 1984 and 2021, we reconstructed fluxes and source contributions of wet inorganic N deposition and evaluated their historical changes. Compared with 1984, moss N contents (from 1.7 ± 0.3% to 2.1 ± 0.4%) showed a significant increase in 2021, which was mainly attributed to a significant increase in nitrate N deposition fluxes at Mount Tai. Moss δ15N values (from -5.9 ± 0.9‰ to -5.2 ± 2.4‰) showed a slight increase from 1984 to 2021 at Mount Tai. The importance of combustion-related NH3 (including vehicle exhaust, coal combustion, and biomass burning) in 2021 (51.2%) were higher than those in 1984 (43.9%), while the importance of volatilization NH3 sources (including waste and fertilizers) in 2021 (48.8%) were lower than those in 1984 (56.1%). It was fossil-fuel NOx (from vehicle exhaust and coal combustion) (54.1%) rather than non-fossil fuel NOx (from biomass burning and microbial N cycles) (45.9%) dominated NOx emissions in both 1984 and 2021. Our results revealed significant contributions of combustion-related NH3 and fossil-fuel NOx sources emissions to the elevation of N deposition at Mount Tai in Northern China, which are beneficial for mitigating N emissions and conducting ecological benefit assessments in Northern 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.

Significant Increase in Ammonia Emissions in China: Considering Nonagricultural Sectors Based on Isotopic Source Apportionment

Isotopic source apportionment results revealed that nonagricultural sectors are significant sources of ammonia (NH3) emissions, particularly in urban areas. Unfortunately, nonagricultural sources have been substantially underrepresented in the current anthropogenic NH3 emission inventories (EIs). Here, we propose a novel approach to develop a gridded EI of nonagricultural NH3 in China for 2016 using a combination of isotopic source apportionment results and the emission ratios of carbon monoxide (CO) and NH3. We estimated that isotope-corrected nonagricultural NH3 emissions were 4370 Gg in China in 2016, accounting for an increase in the total NH3 emissions from 7 to 31%. As a result, compared to the original NH3 EI, the annual emissions of total NH3 increased by 35%. Thus, in comparison to the simulation driven by the original NH3 EI, the WRF-Chem model driven by the isotope-corrected NH3 EI has reduced the model biases in the surface concentrations and dry deposition flux of reduced nitrogen (NHx = gaseous NH3 + particulate NH4+) by 23 and 31%, respectively. This study may have wide-ranging implications for formulating targeted strategies for nonagricultural NH3 emissions controls, making it facilitate the achievement of simultaneously alleviating nitrogen deposition and atmospheric pollution in the future.

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

Biomonitoring of dairy farm emitted ammonia in surface waters using phytoplankton and periphyton

The increasing environmental abundance of reactive N ('Nr') entails many adverse effects for society such as soil degradation and eutrophication. In addressing the global surplus of N, there is a pressing need to quantify local sources and dynamics of Nr. Although quantified as an important anthropogenic source of Nr, the spatiotemporal patterns of ammonia ('NH3') emitted by dairy farming and its resulting pressure on local surface waters lacks quantification. Quantification could optimize farm management with minimized losses of valuable nitrogen and protection of freshwater ecology. This study aimed to unravel spatiotemporal dynamics of ammonia nitrogen emitted by a dairy farm in the atmospheric and aquatic geo-ecosphere. Atmospheric NH3 and aqueous ammonium ('NH4+') were determined over time, together with meteorological variables. Aquatic biomonitors (periphyton and phytoplankton) were employed to monitor the spatial impacts of cattle-stable emitted NH3. Atmospheric NH3 on the farm was significantly regulated by wind, sharply declining over increasing distances from the stable (average decrease in the dominant wind direction from 55.5 μg/m3 at 20 m to 5.8 μg/m3 at 500 m, in the other wind directions values decreased from 38.3 μg/m3 to 6.0 μg/m3). This was also reflected in local surface water concentrations of NH4+, with average concentrations decreasing from 37.0 mg [NH4+-N]/L at 65 m to 4.8 mg [NH4+-N]/L in the dominant wind direction, and from 1.2 to 0.7 in other directions. Periphyton biomass, total N ("TN") and δ15N all significantly reflected spatiotemporal dynamics of atmospheric NH3 and aqueous NH4+, as did phytoplankton TN. The cattle stable significantly influenced local water quality through atmospheric spreading of NH3, and both aquatic biomonitors were influenced by and reflected dairy farm emitted NH3 with a sharp dilution over distance. This study strongly underlines the importance of atmospheric transport of dairy farm emitted NH3 and its effects on local water quality.

Traceability of atmospheric ammonia in a suburban area of the Beijing-Tianjin-Hebei region

Ammonia (NH3) is one of the most important sources that have been linked to the formation of PM2.5. Therefore, it is important to study the source contributions to atmospheric NH3 for air pollution control. Here we used 15N natural abundance (expressed by δ15N) values to quantify the source contributions to atmospheric NH3 in the Beijing-Tianjin-Hebei (BTH) region, which suffers from the country's worst air pollution. Results showed that from 2017 to 2019, the annual mean δ15N-NH3 value at the livestock site (-27.5 ± 6.0 ‰) was lower than at cropland (-20.7 ± 6.0 ‰) and rural residential sites (-22.1 ± 7.4 ‰), while their concentrations were the opposite. Seasonal mean δ15N-NH3 values were the highest in winter and lowest in summer, whereas monthly mean δ15N-NH3 values were the highest in January and lowest in June. The isotope mixing model results showed that agricultural sources account for 64.5 ± 13.5 % of year-round total NH3 emissions, while industrial and other sources contributed 27.4 and 8.1 %, respectively. However, the contribution of industrial sources was higher than that of agricultural sources in January. Our results indicated that the contribution of agricultural sources has decreased after the implementation of air pollution control policies in this region suggesting that NH3 abatement from agricultural sources is effective. However, further refinement of agricultural emission abatement measures will be required, accompanied by a greater focus on controlling winter non-agricultural sources.

Isotopic comparison of ammonium between two summertime field campaigns in 2013 and 2021 at a background site of North China

Ammonia (NH3) is the primary atmospheric alkaline gas, playing a crucial role in the atmospheric chemistry. Recently, non-agricultural emissions have been identified as the dominant sources of NH3 in urban areas. However, few studies have quantified the contributions of different sources to regional NH3. This study conducted two summertime field observations in 2013 and 2021 at a background site of North China to comprehensively explore the regional variations in concentration, nitrogen isotope composition (δ15N), and sources of ammonium (NH4+). The results indicate that NHx (NHx = NH3 + NH4+) concentration has increased in 2021, but the fNH4+ (NH4+/ NHx) has decreased significantly. The δ15N-NH4+ values show a significant increase, ranging from -4.7 ± 8.1 ‰ to +12.0 ± 2.4 ‰. The increase can be attributed to two primary factors: changes in fNH4+ resulting from the reduction of atmospheric acid gases and alterations in the sources of NH3. Bayesian simulation analysis reveals substantial variations in NH3 sources between 2013 and 2021 observations. Non-agricultural sources have significantly increased their contribution to NHx concentration, with vehicle exhaust and NH3 slip experiencing growth rates of 187 % and 104 %, respectively. Our results confirm the dominate contribution of non-agricultural sources to regional NH3 at the present stage and propose relevant mitigation strategies, which would provide essential insights for reducing NH3 emissions in North China.

Quantifying the Nitrogen Sources and Secondary Formation of Ambient HONO with a Stable Isotopic Method

Nitrous acid (HONO) is a reactive gas that plays an important role in atmospheric chemistry. However, accurately quantifying its direct emissions and secondary formation in the atmosphere as well as attributing it to specific nitrogen sources remains a significant challenge. In this study, we developed a novel method using stable nitrogen and oxygen isotopes (δ15N; δ18O) for apportioning ambient HONO in an urban area in North China. The results show that secondary formation was the dominant HONO formation processes during both day and night, with the NO2 heterogeneous reaction contributing 59.0 ± 14.6% in daytime and 64.4 ± 10.8% at nighttime. A Bayesian simulation demonstrated that the average contributions of coal combustion, biomass burning, vehicle exhaust, and soil emissions to HONO were 22.2 ± 13.1, 26.0 ± 5.7, 28.6 ± 6.7, and 23.2 ± 8.1%, respectively. We propose that the isotopic method presents a promising approach for identifying nitrogen sources and the secondary formation of HONO, which could contribute to mitigating HONO and its adverse effects on air quality.

A distinctive latitudinal trend of nitrogen isotope signature across urban forests in eastern China

Rapid urbanization has greatly altered nitrogen (N) cycling from regional to global scales. Compared to natural forests, urban forests receive much more external N inputs with distinctive abundances of stable N isotope (δ15 N). However, the large-scale pattern of soil δ15 N and its imprint on plant δ15 N remain less well understood in urban forests. By collecting topsoil (0-20 cm) and leaf samples from urban forest patches in nine large cities across a north-south transect in eastern China, we analyzed the latitudinal trends of topsoil C:N ratio and δ15 N as well as the correlations between tree leaf δ15 N and topsoil δ15 N. We further explored the spatial variation of topsoil δ15 N explained by corresponding climatic, edaphic, vegetation-associated, and anthropogenic drivers. Our results showed a significant increase of topsoil C:N ratio towards higher latitudes, suggesting lower N availability at higher latitudes. Topsoil δ15 N also increased significantly at higher latitudes, being opposite to the latitudinal trend of soil N availability. The latitudinal trend of topsoil δ15 N was mainly explained by mean annual temperature, mean annual precipitation, and atmospheric deposition of both ammonium and nitrate. Consequently, tree leaf δ15 N showed significant positive correlations with topsoil δ15 N across all sampled plant species and functional types. Our findings reveal a distinctive latitudinal trend of δ15 N in urban forests and highlight an important role of anthropogenic N sources in shaping the large-scale pattern of urban forest 15 N signature.

Precipitation frequency controls nitrogenous aerosol in a tropical coastal city and its implications for plant carbon sequestration

The concentration of nitrogenous aerosols is influenced by air mass transition, local meteorological conditions, local emissions, and the wet removal effect driven by precipitation. Deposited nitrogenous aerosols influence nitrogen availability in the canopy, affecting the amount of plant carbon sequestration. However, the factors controlling nitrogenous aerosol concentrations and their implications for plant carbon sequestration remain unclear. In this study, multiple stable nitrogen isotopes in atmospheric aerosols (δ¹⁵N-TN, δ¹⁵N-NO₃^-, and δ¹⁵N-NH₄⁺) and rainwater (rainwater δ¹⁵N-NO₃^- and rainwater δ¹⁵N-NH₄⁺) in one-year observations were analyzed to explore the main factors controlling nitrogenous aerosol concentrations. The results showed that NO₃^- and NH₄⁺ were the major components of TN, and their concentrations in seasonal patterns were sensitive to frequent rainfall rather than local emissions or external contributions. The concentrations of nitrogenous aerosols were negatively correlated with precipitation frequency, indicating that increased precipitation frequency induced low concentrations of nitrogenous aerosols. Moreover, the positive matrix factorization (PMF) analysis showed that coarse mode NO₃^- was generated in the wet season but not in the dry season, reflecting the removal of precipitation. With the increased precipitation frequency from May to July, 42.4% of aerosol NO₃^- was scavenged into rainwater, indicated by the variations in the δ¹⁵N values of nitrogenous aerosols and rainwater. This result prompted us to calculate the loss of 12.1 ± 3.9 Gg carbon/yr plant carbon sequestration. Our study suggests that nitrogenous aerosols are captured by the high precipitation frequency in tropical areas, decreasing nitrogen availability in the canopy, which might decrease plant carbon sequestration.

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.

Ammonia in urban atmosphere can be substantially reduced by vehicle emission control: A case study in Shanghai, China

To investigate the impact of emission controls on ammonia (NH₃) pollution in urban atmosphere, observation on NH₃ (1 hr interval) was performed in Shanghai before, during and after the 2019 China International Import Expo (CIIE) event, along with measurements on inorganic ions, organic tracers and stable nitrogen isotope compositions of ammonium in PM_2.5. NH₃ during the CIIE period was 6.5±1.0 µg/m³, which is 41% and 32% lower than that before and after the event, respectively. Such a decrease was largely ascribed to the emission controls in nonagricultural sources, of which contribution for measured NH₃ in control phase abated by ∼20% compared to that during uncontrol period. Molecular compositions of PAHs and hopanes further suggested a dominant role of the reduced vehicle emissions in the urban NH₃ abatement during the CIIE period. Our results revealed that vehicle exhaust emission control is an effective way to mitigate NH₃ pollution and improve air quality in Chinese urban areas.

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

The family of atmospheric oxides of nitrogen, NO_y (e.g., nitrogen oxides (NO_x) + nitric acid (HNO₃) + nitrous acid (HONO) + peroxyacetyl nitrate (PAN) + particulate nitrate (pNO₃^-) + other), have an influential role in atmospheric chemistry, climate, and the environment. The nitrogen (δ¹⁵N) and oxygen (δ¹⁸O and Δ¹⁷O) stable isotopes of NO_y are novel tools for potentially tracking emission sources and quantifying oxidation chemistry. However, there is a lack of well-established methods, particularly for speciated gas-phase components of NO_y, to accurately quantify δ¹⁵N, δ¹⁸O, and Δ¹⁷O. This work presents controlled laboratory experiments and complex chamber α-pinene/NO_x oxidation experiments of a sampling apparatus constructed for the simultaneous capture of multiple NO_y species for isotope analysis using a series of coated denuders, with a focus on nitrogen dioxide (NO_2^•). The laboratory tests indicate complete NO₂^• capture for the targeted concentration of 15 ppb_v for at least 24 h collections at 10 liters per minute, with δ¹⁵N and δ¹⁸O precisions of ±1.3‰ and 1.0‰, respectively, and minimal (2.2% ± 0.1%) NO₂^• collection on upstream denuders utilized for the capture of HNO₃ and other acidic gases. The multispecies NO_y collection system showed excellent concentration correlations with online instrumentation for both HNO₃ and NO₂^• and isotope reproducibility of ±1.7‰, ±1.8‰, and ±0.7‰ for δ¹⁵N, δ¹⁸O, and Δ¹⁷O, respectively, for replicate experiments and highly time-resolved collections. This work demonstrates a new method that can enable the simultaneous collection of HNO₃ and NO₂^• for accurate quantification of concentration and isotopic composition.

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.

Nitrogen isotopes indicate vehicle emissions and biomass burning dominate ambient ammonia across Colorado's Front Range urban corridor

Urban ammonia (NH₃) emissions contribute to poor local air quality and can be transported to rural landscapes, impacting sensitive ecosystems. The Colorado Front Range urban corridor encompasses the Denver Metropolitan Area, rural farmland/rangeland and montane forest between the city and the Rocky Mountains. Reactive nitrogen emissions from the corridor are partly responsible for increased N deposition to the wildland-urban interface (WUI) in this region. To determine the significance of individual NH₃ sources to WUI ecosystems, we measured the concentration and isotopic composition (δ¹⁵N-NH₃) of ambient NH_3(g) from April to October 2018 across a five-site urban to rural gradient in the corridor. The urban sites had higher NH₃ concentrations and δ¹⁵N-NH₃ values than the rural/suburban sites. Based on isotope mixing models, NH₃ emission source contributions for all sites were fertilizer (12 ± 5.7%), livestock waste (18 ± 12%), vehicles (37 ± 23%), and biomass burning (34 ± 20%). Vehicle contributions were consistent across all months with an average of 35% and summer months showed a peak in biomass burning contributions (40%). As wildfires are projected to increase due to climate change, we stress a need for constraints on the isotopic signature of NH₃ emitted from wildfires. Vehicle emissions contributed the greatest amount of NH₃ (40%) at the urban sites while rural/suburban sites had higher agricultural contributions (41%). Had 2018 not had an anomalously high wildfire season, 46% and 60% of the NH₃ would have been attributed to vehicle emissions at the WUI site and urban sites, respectively. NH₃ emissions have historically been ascribed to agricultural activities but these findings illustrate the universal significance of vehicle emissions and the potential for sustained wildfire activity to be a primary contributor to NH₃. Air quality (e.g., particulate matter) and nitrogen deposition reduction plans may benefit by including management practices that address vehicle NH₃ emissions.

Non-agricultural source dominates the ammonium aerosol in the largest city of South China based on the vertical δ15N measurements

Ammonia (NH₃) is the most prevalent alkaline gas in the atmosphere and plays a critical role in air pollution and public health. However, scientific debate remains over whether agricultural emissions (e.g., livestock and fertilizer application) dominate NH₃ in urban atmosphere in China, which is one of the largest NH₃ emitters in the world. In this study, we first simultaneously collected the fine atmospheric particles (PM_2.5) at two heights (ground and 488 m) using the atmospheric observatories in Canton Tower, Guangzhou city, China for the measurements of stable nitrogen isotope composition in ammonium (δ¹⁵N-NH₄⁺). Our results showed that the average δ¹⁵N-NH₄⁺ value at the ground and the 488 m observatory were 16.9 ‰ and 3.8 ‰, respectively, implying that NH₄⁺ aerosols between the two heights probably have different sources. Moreover, we found that the δ¹⁵N-NH₄⁺ value would sharply decrease to -16.7 ‰ when the air masses came from western Guangzhou, where the urbanization is limited compared to other surrounding areas. The Bayesian mixing model indicated that NH₄⁺ aerosol at the ground observatory was mainly derived from non-agricultural activities (76 %, e.g., vehicular exhaust), with the rest from agricultural sources (24 %). As for the 488 m observatory, the contribution of non-agricultural sources was 53 %, which is lower than the ground observatory. This is expected as the lower air receives more impacts from the local urban emission. However, the current "bottom-up" emission inventory illustrates that only ~20 % NH₃ in Guangzhou is associated with non-agricultural emissions, which is significantly lower than our δ¹⁵N-based results. Overall, our findings strongly imply that non-agricultural sources dominate the urban NH₃ in Guangzhou or maybe in adjacent cities of the Pearl River Delta region as well, suggesting that the emission inventory of NH₃ in this region probably is urgently needed to be revisited in future studies.

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.

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.

Seasonal source analysis of nitrogen and carbon aerosols of PM2.5 in typical cities of Zhejiang, China

Fine particulate matter (PM_2.5) significantly impacts global air quality and human health due to its smaller particle size and larger specific surface area. Nitrogen and carbon aerosols, as the main components of PM_2.5, play key roles in air pollution. This study identified the sources and seasonal variation of nitrogen and carbon aerosols in PM_2.5 in typical cities of Zhejiang. The annual average PM_2.5 concentrations of Hangzhou (HZ), Ningbo (NB), and Huzhou (HUZ) were 39.8 ± 19.1 μg m^-‍3, 40.0 ± 21.5 μg m^-3, and 50.1 ± 22.6 μg m^-3, respectively, which exceeded the Chinese air quality limit of 35.0 μg m^-3. The results showed that the concentrations of nitrogen aerosols (NO₃^- and NH₄⁺) in water-soluble inorganic ions were higher at 9.6 ± 4.6 μg m^-3, 9.0 ± 4.5 μg m^-3 and 11.5 ± 5.4 μg m^-3 in HZ, NB and HUZ, respectively, especially in winter, accounting for over 60% of the total. The annual average δ¹⁵N values of PM_2.5 were 6.2 ± 1.9‰, 6.4 ± 2.2‰ and 6.7 ± 1.9‰ in HZ, NB and HZ, respectively; the δ¹⁵N values in winter were relatively low. A Bayesian isotopic mixing model was employed to analyse the sources of nitrogen aerosols in winter; the results showed that nitrogen concentration was mainly affected by NH₃ and NO_X emitted by motor vehicle exhaust, coal combustion, biomass combustion, biogenic soil emissions, animal wastes and ocean evaporation (NB). In addition, the carbon component analysis of PM_2.5 showed that the annual average mass concentration of TC accounted for 18.7%, 16.4% and 20.1% of PM_2.5 in HZ, HUZ and NB, respectively. The same isotope model was used to analyse the sources of carbon aerosols; the results showed that carbon aerosols were mainly affected by the sources of motor vehicle exhaust, coal combustion, biomass combustion and dust. In the PM_2.5 in Zhejiang, the most contributory sources of nitrogenous aerosols and carbon aerosols were motor vehicle exhaust sources.

Elucidating sources of atmospheric NOX pollution in a heavily urbanized environment using multiple stable isotopes

Atmospheric deposition of nitrogen (N) from rain and aerosols can be a significant non-point source - particularly in urbanized coastal areas and contribute to coastal eutrophication and hypoxia. Here, we present geochemical and isotopic data from surface waters coupled with an 18-month time series of geochemical and isotopic data measured on wet and dry deposition over Hong Kong from June 2018. Dual stable isotopes of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) of rain and total suspended particulates (TSP) were analyzed to trace the sources and understand seasonal pattern of atmospheric nitrate. The δ¹⁵N of TSP, δ¹⁵N-NO₃ in rain and TSP ranged from +0.94 to +17.6‰, -4.1 to +3.0‰ and -1.3 to +9.0‰ respectively. δ¹⁵N varied seasonally with higher values in winter and lower values in summer. This variation can be explained by a change in the sources of atmospheric NO_x driven by the East Asian Monsoon. It was found that most NO_x comes from coal burning in winter and a mix of vehicle emissions, fossil fuel combustion and lightning in summer. Moreover, the estimated dry and wet deposition of nitrate and ammonium in Hong Kong is around 18 kg N ha^-1 annually, which is of the same order of magnitude as N released by sewage effluents and groundwater. This implies that atmospheric N deposition over the N-limited waters of the eastern side of Hong Kong could contribute significantly to the N budget. Therefore, atmospheric N deposition may alter the local N marine cycling, thus monitoring its impact is crucial for water quality in Southern China.

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.

Unraveling the sources of atmospheric organic aerosols over the Arabian Sea: Insights from the stable carbon and nitrogen isotopic composition

The isotopic composition of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) in marine aerosols influenced by the continental outflows are useful proxies for understanding the aging and secondary formation processes. Every winter, the haze pollutants transported from South Asia significantly affect the chemical composition of marine atmospheric boundary layer of the Arabian Sea. Here, we assessed the δ¹³C of total carbon (TC) and δ¹⁵N of total nitrogen (TN) in marine aerosols collected over the Arabian Sea during a winter cruise (6-24 December 2018). TC (2.1-13.4 μg m^-3) is strongly correlated with TN (0.9-5.0 μg m^-3), likely because of their common source-emissions, biomass burning and fossil-fuel combustion in the Indo-Gangetic Plain and South Asia (corroborated by backward-air mass trajectories and satellite fire counts). Besides, the linear relationship between the mass ratios of water-soluble organic carbon (WSOC) to TC (0.04-0.65) and δ¹³C_TC (-25.1‰ to -22.9‰) underscores the importance of aging process. This means oxidation of organic aerosols during transport not only influences the WSOC levels but also affects their δ¹³C_TC. Likewise, the prevalent inverse linear relationship between the equivalent mass ratio of (NH₄⁺/non-sea-salt- or nss-SO₄^2-) and δ¹⁵N_TN (+15.3‰ to +25.1‰) emphasizes the overall significance of neutralization reactions between major acidic ([nss-SO₄^2-] ≫ [NO₃^-]) and alkaline species (NH₄⁺) in aerosols. Higher δ¹⁵N_TN values in winter than the spring inter-monsoon clearly emphasizes the significance of the anthropogenic combustion sources (i.e., biomass burning) in the South Asian outflow. A comparison of δ¹³C_TC and δ¹⁵N_TN with the source emissions revealed that crop-residue burning emissions followed by the coal fired power plants mostly dictate the atmospheric abundance of organic aerosols in the wider South Asian outflow.

Atmospheric Organic Nitrogen Deposition in Strategic Water Sources of China after COVID-19 Lockdown

Atmospheric nitrogen deposition (AND) may lead to water acidification and eutrophication. In the five months after December 2019, China took strict isolation and COVID-19 prevention measures, thereby causing lockdowns for approximately 1.4 billion people. The Danjiangkou Reservoir refers to the water source in the middle route of South-to-North Water Diversion Project in China, where the AND has increased significantly; thus, the human activities during the COVID-19 period is a unique case to study the influence of AND to water quality. This work monitored the AND distribution around the Danjiangkou Reservoir, including agricultural, urban, traffic, yard, and forest areas. After lockdown, the DTN, DON, and Urea-N were 1.99 kg · hm⁻² · month⁻¹, 0.80 kg · hm⁻² · month⁻¹, and 0.15 kg · hm⁻² · month⁻¹, respectively. The detected values for DTN, DON, and Urea-N in the lockdown period decreased by 9.6%, 30.4%, and 28.97%, respectively, compared to 2019. The reduction in human activities is the reason for the decrease. The urban travel intensity in Nanyang city reduced from 6 to 1 during the lockdown period; the 3 million population which should normally travel out from city were in isolation at home before May. The fertilization action to wheat and orange were also delayed.

A review on moss nitrogen and isotope signatures evidence for atmospheric nitrogen deposition

Moss nitrogen (N) concentration and isotopic composition (δ¹⁵N) values can reveal a better understanding of atmospheric N deposition patterns. Here, we summarize the moss N content and δ¹⁵N signatures using data compiled from 104 papers. Based on the dataset, we summarize the models for assessing the level and reduced (NH_x): oxidised compounds (NO_x) ratio of atmospheric N deposition. Results showed a historical increase in N concentration and ¹⁵N depletion of specimen mosses close to anthropogenic N sources from intensive animal production and agricultural activities (NH_x emission) since the 1800s. However, an increase of moss N with a less negative ¹⁵N observed in the last three decades could be due to a substantial fossil fuel combustion contributed NO_x emission. Spatially, N deposition in Europe decreased due to successful control actions, but Asia has become a hotspot for NH_x emission from agriculture. The present results highlight the importance of moss N and δ¹⁵N values for estimating atmospheric N deposition patterns at spatio-temporal trends.

High atmospheric wet nitrogen deposition and major sources in two cities of Yangtze River Delta: Combustion-related NH3 and non-fossil fuel NOx

High ammonia (NH₃) and nitrogen oxide (NO_x) emissions are related to serious air pollution in urban areas and the negative impacts of excessive reactive nitrogen (N) deposition on many ecosystems. However, whether there is a relationship between N deposition rates and their sources with urbanization or not remains unclear in many areas. Here, we investigated the deposition rates of ammonium (NH₄⁺), nitrate (NO₃^-), dissolved organic N, and water-insoluble particular N from July 2017 to June 2018 at two urban and two suburban sites in the Yangtze River Delta (YRD). The δ¹⁵N values of precipitation NH₄⁺ and NO₃^- were measured, and major sources were analyzed using a Bayesian isotope mixing model. Wet N deposition rates were higher in Yangzhou (developing city, 20.3-22.7 kg N ha^-1 yr^-1) than those in Nanjing (developed city, 19.4-20.5 kg N ha^-1 yr^-1), and were higher at urban sites (20.4-22.5 kg N ha^-1 yr^-1) than those at suburban sites (18.7-20.3 kg N ha^-1 yr^-1). δ¹⁵N values of precipitation NH₄⁺ increased with an increase in precipitation pH because ambient acidity affects the equilibrium isotope fractionation between NH₃ and NH₄⁺ and wet scavenging coefficients of NH₃ and particulate NH₄⁺. For NH₄⁺, combustion-related NH₃ sources (62%-65% with 5.5-6.4 kg N ha^-1 yr^-1, including coal combustion, vehicle exhaust, and biomass burning) contributed more than volatilization NH₃ sources (35%-38% with 2.9-3.9 kg N ha^-1 yr^-1, including fertilizer application and waste volatilization). For NO₃^-, non-fossil fuel NO_x sources (50%-63% with 3.4-4.1 kg N ha^-1 yr^-1, including biomass burning and microbial N cycle) were comparable to fossil fuel NO_x sources (37%-50% with 2.4-3.4 kg N ha^-1 yr^-1, including coal combustion and vehicle exhaust). This study evidenced high N deposition rates and the importance of combustion-related NH₃ emissions and non-fossil fuel NO_x emissions in city areas of the YRD.

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.

Long-term ecosystem nitrogen limitation from foliar δ15 N data and a land surface model

The effect of nutrient availability on plant growth and the terrestrial carbon sink under climate change and elevated CO₂ remains one of the main uncertainties of the terrestrial carbon cycle. This is partially due to the difficulty of assessing nutrient limitation at large scales over long periods of time. Consistent declines in leaf nitrogen (N) content and leaf δ¹⁵ N have been used to suggest that nitrogen limitation has increased in recent decades, most likely due to the concurrent increase in atmospheric CO₂ . However, such data sets are often not straightforward to interpret due to the complex factors that contribute to the spatial and temporal variation in leaf N and isotope concentration. We use the land surface model (LSM) QUINCY, which has the unique capacity to represent N isotopic processes, in conjunction with two large data sets of foliar N and N isotope content. We run the model with different scenarios to test whether foliar δ¹⁵ N isotopic data can be used to infer large-scale N limitation and if the observed trends are caused by increasing atmospheric CO₂ , changes in climate or changes in sources and magnitude of anthropogenic N deposition. We show that while the model can capture the observed change in leaf N content and predict widespread increases in N limitation, it does not capture the pronounced, but very spatially heterogeneous, decrease in foliar δ¹⁵ N observed in the data across the globe. The addition of an observation-based temporal trend in isotopic composition of N deposition leads to a more pronounced decrease in simulated leaf δ¹⁵ N. Our results show that leaf δ¹⁵ N observations cannot, on their own, be used to assess global-scale N limitation and that using such a data set in conjunction with an LSM can reveal the drivers behind the observed patterns.

Estimating NOx removal capacity of urban trees using stable isotope method: A case study of Beijing, China

It is widely recognized that green infrastructures in urban ecosystems provides important ecosystem services, including air purification. The potential absorption of nitrogen oxides (NO_x) by urban trees has not been fully quantified, although it is important for air pollution mitigation and the well-being of urban residents. In this study, four common tree species (Sophora japonica L., Fraxinus chinensis Roxb., Populus tomentosa Carrière, Sabina chinensis (L.)) in Beijing, China, were studied. The dual stable isotopes (¹⁵N and ¹⁸O) and a Bayesian isotope mixing model were applied to estimate the sources contributions of potential nitrogen sources to the roadside trees based on leaf and soil sampling in urban regions. The following order of sources contributions was determined: soil > dry deposition > traffic-related NO_x. The capacity of urban trees for NO_x removal in the city was estimated using a remote sensing and GIS approach, and the removal capacity was found to range from 0.79 to 1.11 g m^-2 a^-1 across administrative regions, indicating that 1304 tons of NO_x could be potentially removed by urban trees in 2019. Our finding qualified the potential NO_x removal by urban trees in terms of atmospheric pollution mitigation, highlighting the role of green infrastructure in air purification, which should be taken into account by stakeholders to manage green infrastructure as the basis of a nature-based approach.

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.

Important contribution of N2O5 hydrolysis to the daytime nitrate in Xi'an, China during haze periods: Isotopic analysis and WRF-Chem model simulation

Nitrate, as one of the major components of tropospheric aerosols, plays a crucial role in winter haze formation. While, the formation mechanism of the high production of nitrate in Chinese megacities is still not fully understood. To quantify the contributions of major formation pathways to nitrate, airborne particles in Xi'an, inland China during the winter of 2017 were measured and analyzed for the water-soluble ions and stable nitrogen/oxygen isotope compositions of nitrate in PM_2.5, followed by a WRF-Chem model simulation. The oxygen isotopic results indicated that N₂O₅ hydrolysis was an important formation pathway for the daytime nitrate in the haze episodes. The model simulation further revealed that N₂O₅ hydrolysis contribution increased from 8.2% to 20.5% of the total nitrate over 14:00-16:00 p.m., clearly showing that N₂O₅ formation followed by a heterogeneous hydrolysis to nitrate can effectively proceed in daytime under the abundantly co-existing O₃, NO₂ and NH₃ conditions.

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.

Significant contributions of combustion-related NH3 and non-fossil fuel NOx to elevation of nitrogen deposition in southwestern China over past five decades

Anthropogenic nitrogen (N) emissions and deposition have been increasing over past decades. However, spatiotemporal variations of N deposition levels and major sources remain unclear in many regions, which hinders making strategies of emission mitigation and evaluating effects of elevated N deposition. By investigating moss N contents and δ¹⁵ N values in southwestern (SW) China in 1954-1964, 1970-1994, and 2005-2015, we reconstructed fluxes and source contributions of atmospheric ammonium ( NH 4 + ) and nitrate ( NO 3 - ) deposition and evaluated their historical changes. For urban and non-urban sites, averaged moss N contents did not differ between 1954-1964 and 1970-1994 (1.2%-1.3%) but increased distinctly in 2005-2015 (1.6%-2.3%), and averaged moss δ¹⁵ N values decreased from +0.4‰ to +3.3‰ in 1954-1964 to -1.9‰ to -0.7‰ in 1974-1990, and to -4.8‰ to -3.6‰ in 2005-2015. Based on quantitative estimations, N deposition levels from the 1950s to the 2000s did not change in the earlier 20 years but were elevated substantially in the later 30 years. Moreover, the elevation of NH 4 + deposition (by 12.2 kg-N/ha/year at urban sites and 4.6 kg-N/ha/year at non-urban sties) was higher than that of NO 3 - deposition (by 6.0 and 2.9 kg-N/ha/year, respectively) in the later 30 years. This caused a shifted dominance from NO 3 - to NH 4 + in N deposition. Based on isotope source apportionments, contributions of combustion-related NH₃ sources (vehicle exhausts, coal combustion, and biomass burning) to the elevation of NH 4 + deposition were two times higher than volatilization NH₃ sources (wastes and fertilizers) in the later 30 years. Meanwhile, non-fossil fuel NO_x sources (biomass burning, microbial N cycles) contributed generally more than fossil fuel NO_x sources (vehicle exhausts and coal combustion) to the elevation of NO 3 - deposition. These results revealed significant contributions of combustion-related NH₃ and non-fossil fuel NO_x emissions to the historical elevation of N deposition in SW China, which is useful for emission mitigation and ecological effect evaluation of atmospheric N loading.

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.

Oxidation and sources of atmospheric NOx during winter in Beijing based on δ18O-δ15N space of particulate nitrate

The determination of both stable nitrogen (δ¹⁵N-NO₃^-) and stable oxygen (δ¹⁸O-NO₃^-) isotopic signatures of nitrate in PM_2.5 has shown potential for an approach of assessing the sources and oxidation pathways of atmospheric NOx (NO+NO₂). In the present study, daily PM_2.5 samples were collected in the megacity of Beijing, China during the winter of 2017-2018, and this new approach was used to reveal the origin and oxidation pathways of atmospheric NOx. Specifically, the potential of field δ¹⁵N-NO₃^- signatures for determining the NOx oxidation chemistry was explored. Positive correlations between δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- were observed (with R² between 0.51 and 0.66, p < 0.01), and the underlying environmental significance was discussed. The results showed that the pathway-specific contributions to NO₃^- formation were approximately 45.3% from the OH pathway, 46.5% from N₂O₅ hydrolysis, and 8.2% from the NO₃+HC channel based on the δ¹⁸O-δ¹⁵N space of NO₃^-. The overall nitrogen isotopic fractionation factor (εN) from NOx to NO₃^- on a daily scale, under winter conditions, was approximately +16.1‰±1.8‰ (consistent with previous reports). Two independent approaches were used to simulate the daily and monthly ambient NOx mixtures (δ¹⁵N-NOx), respectively. Results indicated that the monthly mean values of δ¹⁵N-NOx compared well based on the two approaches, with values of -5.5‰ ± 2.6‰, -2.7‰ ± 1.9‰, and -3.2‰ ± 2.2‰ for November, December, and January (2017-2018), respectively. The uncertainty was in the order of 5%, 5‰ and 5.2‰ for the pathway-specific contributions, the εN, and δ¹⁵N-NOx, respectively. Results also indicated that vehicular exhaust was the key contributor to the wintertime atmospheric NOx in Beijing (2017-2018). Our advanced isotopic perspective will support the future assessment of the origin and oxidation of urban atmospheric NOx.

Chemical characterization and stable nitrogen isotope composition of nitrogenous component of ambient aerosols from Kanpur in the Indo-Gangetic Plains

Measurements of water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), water-soluble organic nitrogen (WSON) and ẟ¹⁵N_TN (total N) was carried out on PM_2.5 aerosol samples during wintertime to understand the major sources of ambient nitrogenous species at a heavily polluted location of Kanpur in north India. During the nighttime sampling campaign, WSON and NH₄⁺_N contributed dominantly to the WSTN. Ammonium-rich condition persisted during sampling (NH₄⁺/SO₄^2- average equivalent mass ratio = 3.1 ± 0.7), suggesting complete neutralization of SO₄^2- and formation of NH₄NO₃, which is stable in winter due to low temperature and high relative humidity (RH). Stagnant atmospheric conditions during wintertime enhanced concentrations of ionic species (SO₄^2-, NH₄⁺, and NO₃^-) at this location. Good correlations between NO₃^-_N, NH₄⁺_N and biomass burning tracer K⁺_BB (and also between NO₃^-_N, NH₄⁺_N and SO₄^2-) suggests a strong impact of biomass burning activities. Multi-linear regression (MLR) analysis shows a strong dependence of ẟ¹⁵N on NO₃^-_N, SO₄^2- and WSON in night-1 (10:00 pm to 2:00 am) and on NO₃^-_N and SO₄^2- in night-2 (2:00 am to 6:00 am) depicting different formation and removal mechanism of aerosols during both the time-periods. ẟ¹⁵N_TN in PM_2.5 varied from +8.8 to +15.5‰ (10.8 ± 1.3), similar to the variability observed for many urban locations in India and elsewhere. NH₄⁺_N and WSON control the final ẟ¹⁵N value of nitrogenous aerosols. High relative humidity during nighttime enhanced the secondary organic aerosols formation due to aqueous-phase formation and gas to particle-phase partitioning. Isotopic fractionations associated with multi-phase reactions during gas to particle conversion of NH₃ would result in an increase in ẟ¹⁵N by ~48‰ to 51‰ (at T of 5.4 °C to 15.4 °C) than that of the emission source(s), which indicates the most likely N-emission sources at Kanpur to be from agriculture activities and waste generation.

A First Approach to Aerosol Classification Using Space-Borne Measurement Data: Machine Learning-Based Algorithm and Evaluation

A new method was developed for classifying aerosol types involving a machine-learning approach to the use of satellite data. An Aerosol Robotic NETwork (AERONET)-based aerosol-type dataset was used as a target variable in a random forest (RF) model. The contributions of satellite input variables to the RF-based model were quantified to determine an optimal set of input variables. The new method, based on inputs of satellite variables, allows the classification of seven aerosol types: pure dust, dust-dominant mixed, pollution-dominant mixed aerosols, and pollution aerosols (strongly, moderately, weakly, and non-absorbing). The performance of the model was statistically evaluated using AERONET data excluded from the model training dataset. Model accuracy for classifying the seven aerosol types was 59%, improving to 72% for four types (pure dust, dust-dominant mixed, strongly absorbing, and non-absorbing). The performance of the model was evaluated against an earlier aerosol classification method based on the wavelength dependence of single-scattering albedo (SSA) and fine-mode-fraction values from AERONET. Typical wavelength dependences of SSA for individual aerosol types are consistent with those obtained for aerosol types by the new method. This study demonstrates that an RF-based model is capable of satellite aerosol classification with sensitivity to the contribution of non-spherical particles.

Changes of δ15N values during the volatilization process after applying urea on soil

Ammonia (NH₃) volatilized from soils plays an important role in N cycle and air pollution, thus it is important to trace the emission source and predict source contributions to development strategies mitigating the environmental harmful of soil NH₃ volatilization. The measurements of ¹⁵N natural abundance (δ¹⁵N) could be used as a complementary tool for apportioning emissions sources to resolve the contribution of multiple NH₃ emission sources to air NH₃ pollution. However, information of the changes of δ¹⁵N-NH₃ values during the whole volatilization process under different N application rates are currently lacking. Hence, to fill this gap, we conducted a 15-day incubation experiment included different urea-N application rates to determine δ¹⁵N values of NH₃ during volatilization process. Results showed that volatilization process depleted ¹⁵N in NH₃. The average δ¹⁵N value of NH₃ volatilized from the 0, 20, 180, and 360 kg N ha^-1 treatment was -16.2 ± 7.3‰, -26.0 ± 5.4‰, -34.8 ± 4.8‰, and -40.6 ± 5.7‰. Overall, δ¹⁵N-NH₃ values ranged from -46.0‰ to -4.7‰ during the whole volatilization process, with lower in higher urea-N application treatments than those in control. δ¹⁵N-NH₃ values during the NH₃ volatilization process were much lower than those of the primary sources, soil (-3.4 ± 0.1‰) and urea (-3.6 ± 0.1‰). Therefore, large isotopic fractionation may occur during soil volatilization process. Moreover, negative relationships between soil NH₄⁺-N and NH₃ volatilization rate and δ¹⁵N-NH₃ values were observed in this study. Our results could be used as evidences of NH₃ source apportionments and N cycle.

Atmospheric Ammonia in Beijing during the COVID-19 Outbreak: Concentrations, Sources, and Implications

This study investigates the concentrations and δ15N values of NH3 in Beijing during and after the 2020 COVID-19 lockdown. Higher NH3 concentrations and lower δ15N-NH3(measured) were observed at most sites in 2020 compared to 2017. Except for a site inside a tunnel, NH3 concentrations did not increase significantly after the lockdown had ended compared to those during the lockdown, while δ15N-NH3(measured) increased by 2.1-9.9‰. Nonagricultural sources (fossil fuel and urban waste) overall contributed 81% and 62% of NH3 at on-road (tunnel interior) and nonroad (CAU) sites in 2020, respectively, comparable to those in 2017 (without significant difference). The contribution of nonagricultural sources slightly increased after the lockdown compared to the contribution during the lockdown at the nonroad site and hardly changed at the tunnel interior site. Our results suggest that (1) unfavorable meteorological conditions, especially lower boundary layer heights and changes in regional transport patterns, might play a more important role than reduced anthropogenic emissions in the temporal variations of Beijing NH3 and (2) the effect of reduced anthropogenic emissions, during the COVID-19 outbreak or with the future implementation of emission control strategies, on atmospheric NH3 can be better demonstrated by isotope-based source apportionment of NH3, rather than only by changes in NH3 concentrations.

δ15N-stable isotope analysis of NH x : An overview on analytical measurements, source sampling and its source apportionment

Agricultural sources and non-agricultural emissions contribute to gaseous ammonia (NH₃) that plays a vital role in severe haze formation. Qualitative and quantitative contributions of these sources to ambient PM_2.5 (particulate matter with an aerodynamic equivalent diameter below 2.5 µm) concentrations remains uncertain. Stable nitrogen isotopic composition (δ¹⁵N) of NH₃ and NH₄ ⁺ (δ¹⁵N(NH₃) and δ¹⁵N(NH₄ ⁺), respectively) can yield valuable information about its sources and associated processes. This review provides an overview of the recent progress in analytical techniques for δ¹⁵N(NH₃) and δ¹⁵N(NH₄ ⁺) measurement, sampling of atmospheric NH₃ and NH₄ ⁺ in the ambient air and their sources signature (e.g., agricultural vs. fossil fuel), and isotope-based source apportionment of NH₃ in urban atmosphere. This study highlights that collecting sample that are fully representative of emission sources remains a challenge in fingerprinting δ¹⁵N(NH₃) values of NH₃ emission sources. Furthermore, isotopic fractionation during NH₃ gas-to-particle conversion under varying ambient field conditions (e.g., relative humidity, particle pH, temperature) remains unclear, which indicates more field and laboratory studies to validate theoretically predicted isotopic fractionation are required. Thus, this study concludes that lack of refined δ¹⁵N(NH₃) fingerprints and full understanding of isotopic fractionation during aerosol formation in a laboratory and field conditions is a limitation for isotope-based source apportionment of NH₃. More experimental work (in chamber studies) and theoretical estimations in combinations of field verification are necessary in characterizing isotopic fractionation under various environmental and atmospheric neutralization conditions, which would help to better interpret isotopic data and our understanding on NH _x (NH₃ + NH₄ ⁺) dynamics in the atmosphere.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available in the online version of this article at 10.1007/s11783-021-1414-6 and is accessible for authorized users. Supplementary material includes supplementary tables on summary of recent isotope-based source apportionment studies on ambient NH₃ derived from δ¹⁵N(NH₃) values (Table A1); and summary of recent isotope-based source apportionment studies on particulate NH₄ ⁺ derived from δ¹⁵N(NH₄ ⁺) values (Table A2).

Sources of gaseous NH3 in urban Beijing from parallel sampling of NH3 and NH4+, their nitrogen isotope measurement and modeling

Atmospheric gaseous ammonia (NH₃) is the most abundant alkaline gas in the atmosphere while aerosol ammonium (NH₄⁺) constitutes a majority of the inorganic cation concentration in total PM_2.5 mass and plays a vital role in severe haze formation. This study tried to shed some light on sources of gaseous NH₃ through integrating the parallel measurements of δ¹⁵N values in NH₄⁺ and ambient NH₃, NH₃ source signature measurement, IsoSource model, and chemistry and transport model (CTM). As a result, predicted initial δ¹⁵N (NH₃) values ranging from -42.0‰ to -4.9‰ were derived from daily δ¹⁵N(NH₄⁺) values of fine particulate NH₄⁺, and δ¹⁵N(NH₃) values ranging from -26.8‰ to -17.2‰ were obtained from weekday/weekend δ¹⁵N(NH₃) values, respectively. During summer, non-agricultural sources (e.g. fossil fuel sources, urban waste) contributed 63% to ambient NH₃ in urban Beijing, derived from δ¹⁵N(NH₃) values whereas 64% to ambient NH₃, derived from δ¹⁵N(NH₄⁺) values. These results suggested that non-agricultural sources were main contributors to gaseous NH₃ even during summer and agricultural sources were not likely the main source of gaseous NH₃ in urban Beijing. To further reduce the uncertainty of isotope-based source apportionment results, more laboratory and field studies are necessary to refine the δ¹⁵N(NH₃) source profile of NH₃ using validated collection technique as overlapping exist between δ¹⁵N(NH₃) values in agricultural sources such as livestock breeding waste (-42.5‰ to -29.1‰) and fertilizer application (-51.5‰ to -35.0‰).

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.

Non-agricultural sources dominate the atmospheric NH3 in Xi'an, a megacity in the semi-arid region of China

Ammonia (NH₃), as a dominant alkaline gas in the atmosphere, plays a vital role in Chinese urban haze formation process, but its source in urban areas of China is controversial. To identify the sources of urban NH₃ in the semi-arid region of East Asia, real-time measurements of NH₃ and NH₄⁺ of PM_2.5 in the urban atmosphere of Xi'an, inland China during the winter and summer of 2017 were performed and their stable nitrogen isotope composition were analyzed. NH₃ was 38.0 ± 9.4 μg/m³ in the summer, which is 1.5 times higher than that in the winter. Concentration of NH₃ in both seasons well correlated with that of PAHs in PM_2.5 and the mass ratio of (BbF + BeP + IP + BghiP) to the total PAHs, suggesting that fossil fuel combustion is an important source of NH₃ in Xi'an. Moreover, diurnal variation pattern of NH₃ was consistent with that of CO in the summer, peaking in the morning and evening rush hours, respectively, further indicating an importance of the contribution of traffic emissions to NH₃ in the city. Based on the source apportionment by using isotope mixing model, we found that 66.4% and 62.5% of NH₃ in the urban atmosphere were contributed by non-agricultural sources in the summer and winter, respectively. Our work revealed that non-agricultural sources dominate the atmospheric NH₃ of Xi'an, where haze pollution is still severe, and suggested that emission controls of non-agricultural NH₃ could be an effective way to mitigate the air pollution problem in the semi-arid region of East Asia.

Hydrochemical Characteristic of Groundwater and Its Impact on Crop Yields in the Baojixia Irrigation Area, China

While irrigated crops produce much higher yields than rain-fed crops, the ionic components of irrigation water have important effects on crop yield. Groundwater is widely used for irrigation in the Baojixia irrigation area in China. The chemical characteristics and water quality of groundwater in the Baojixia irrigation area were analyzed and evaluated to study the impact of groundwater quality on crop yield. Results showed cations in the groundwater to mainly be Na+, Ca2+, and Mg2+, whereas the anions are mainly HCO3−, SO42−, and Cl−. Water-rock interaction and cation exchange were identified as the main factors affecting hydrogeochemical properties from west to east. The study found salinity and alkalinity of groundwater in the western region of the study area to be low, and therefore suitable for irrigation. Groundwater in the eastern part of the study area was found to have a medium to high salinity and alkalinity, and is therefore not recommended for long-term irrigation. The groundwater irrigated cultivation of wheat and corn in the research area over 2019, for example, would have resulted in a drop in the annual crop output and an economic loss of 0.489 tons and 0.741 × 104 yuan, respectively. Irrigation using groundwater was calculated to result in the cumulative loss of crop yields and an economic loss of 49.17 tons and 80.781 × 104 yuan, respectively, by 2119. Deterioration of groundwater quality will reduce crop yields. It is recommended that crop yields in the study area be increased by strengthening irrigation water management and improving groundwater quality.

Dual-modelling-based source apportionment of NOx in five Chinese megacities: Providing the isotopic footprint from 2013 to 2014

In China, nitrate (NO₃^-) becomes the main contributor to fine particles (PM_2.5) because the emissions of its precursor, nitrogen oxides (NO_x), were not recognized and controlled well in recent years. In this work, sources, conversion, and geographical origin of NO_x were interpreted combining the isotopic information (δ¹⁵N and δ¹⁸O) of NO₃^- and dual modelling at five Chinese megacities (Beijing, Shanghai, Guangzhou, Wuhan and Chengdu) during 2013-2014. Results showed that the δ¹⁵N-NO₃^- values (n = 512) ranged from -12.3‰ to +22.9‰, and the average δ¹⁸O-NO₃^- value was +83.4‰ ± 17.2‰. The isotopic compositions both had a rising tendency as ambient temperature dropped, attributing largely to the source changes. Bayesian model indicated the percentage for the OH pathway of NO_x conversion had a clear seasonal variation with a higher value during summer (58.0% ± 9.82%) and a lower value during winter (11.1% ± 3.99%); it was also significantly correlated with latitude (p < 0.01). Coal combustion was the most important source of NO_x (31.1%-41.0%), which was geographically derived from North China and other south-central developed regions implied by Potential Source Contribution Function (PSCF). Apart from Chengdu, mobile sources was the second largest contributor to NO_x. This source was extensive but uniformly distributed all around the typical urban agglomerations of China. Biomass burning and microbial processes shared similar source areas, mostly originating from the North China Plain and Sichuan Basin. Based on the NO_x features, we infer that residential coal combustion was the primary source of heavy PM_2.5 pollution in Chinese megacities. Controlling the source categories of these regional priorities would help mitigate atmospheric pollution in these areas.

Nitrate sources and formation of rainwater constrained by dual isotopes in Southeast Asia: Example from Singapore

Emission of reactive nitrogen species has a major impact on atmospheric chemistry, ecosystem and human health. The origin and formation mechanisms of wet-deposited nitrate are not well understood in Southeast Asia (SEA). In this study, we measured stable isotopes of nitrate (δ¹⁵N and δ¹⁸O) and chemical compositions of daily rainwater from May 2015 to July 2017 in Singapore. Our results showed that δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- varied seasonally with higher values during the Inter-monsoon period (April-May and October-November) than during Northeast (December-March) and Southwest monsoon (June-September). Bayesian mixing modeling, which took account of the isotope fractionation, indicated that traffic emission (47 ± 32%) and lightning (19 ± 20%) contributed the most to NO₃^- with increased traffic contribution (55 ± 37%) in the Northeast monsoon and lightning (24 ± 23%) during the Inter-monsoon period. Biomass burning and coal combustion, likely from transboundary transport, contributed ∼25% of nitrate in the rainwater. Monte Carlo simulation of δ¹⁸O-NO₃^- indicated that oxidation process by hydroxyl radical contributed 65 ± 14% of NO₃^-, with the rest from hydrolysis of N₂O₅. Wind speed had large effect on δ¹⁸O-NO₃^- variations in the atmosphere with more involvement of hydroxyl radical reactions when wind speed increased. Our study highlights the key role of isotopic fractionation in nitrate source apportionment, and the influence of meteorological conditions on nitrate formation processes in SEA.