Assessing Contributions of Agricultural and Nonagricultural Emissions to Atmospheric Ammonia in a Chinese Megacity

Significantly underestimated traffic-related ammonia emissions in Chinese megacities: Evidence from satellite observations during COVID-19 lockdowns

Ammonia (NH3) plays an important role in the formation of atmospheric particulate matter, but the contribution of traffic-related emissions remains unclear, particularly in megacities with a large number of vehicles. Taking the opportunity of the stringent COVID-19 lockdowns implemented in Beijing and Shanghai in 2022, this study aims to estimate the traffic-related NH3 emissions in these two megacities based on satellite observations. Differences between urban and suburban areas during the lockdown and non-lockdown periods are compared. It was found that despite different dominating sources, the overall NH3 concentrations in urban and suburban areas were at a similar level, and the lockdown resulted in a more prominent decrease in urban areas, where traffic activities were most heavily affected. The traffic-related contribution to the total emission was estimated to be ∼30% in megacities, and ∼40% in urban areas, which are about 2-10 times higher than that in previous studies. The findings indicate that the traffic-related NH3 emissions have been significantly underestimated in previous studies and may play a more critical role in the formation of air pollution in megacities, especially in winter, when agricultural emissions are relatively low. This study highlights the importance of traffic-related NH3 emissions in Chinese megacities and the need to reassess the emissions and their impacts on air quality.

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+.

Photocatalytic removal of ammonia nitrogen from water: investigations and challenges for enhanced activity

Ammonia nitrogen (NH3-N/NH4+-N) serves as a crucial chemical in biochemistry and fertilizer synthesis. However, it is also a toxic compound, posing risks from eutrophication to direct threats to human health. Ammonia nitrogen pollution pervades water sources, presenting a significant challenge. While several water treatment technologies exist, biological treatment, though widely used, has its limitations. Hence, green and efficient photocatalytic technology emerges as a promising solution. However, current monolithic semiconductor photocatalysts prove inadequate in controlling ammonia nitrogen pollution. Therefore, this review focuses on enhancing semiconductor photocatalysts' efficiency through modification, discussing four mechanisms: (1) mono-ionic modification; (2) metallic and non-metallic modification; (3) construct heterojunctions; and (4) enhancement of synergistic effects of multiple technologies. The influencing factors of photocatalytic ammonia nitrogen removal efficiency are also explored. Moreover, the review outlines the limitations of current photocatalytic pollution treatment and discusses future development trends and research challenges. Currently, the main products of ammonia nitrogen removal include NO3-, NO2-, and N2. To mitigate secondary pollution, the green process of converting ammonia nitrogen to N2 using photocatalysis emerges as a fundamental approach for future treatment. Overall, this review aims to deepen understanding of photocatalysis in ammonia nitrogen treatment and guide researchers toward widespread implementation of this endeavor.

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

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

Dominant contribution of combustion-related ammonium during haze pollution in Beijing

Aerosol ammonium (NH4+), mainly produced from the reactions of ammonia (NH3) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH4+ can provide scientific insights into air quality improvements. However, the sources of NH3 in urban areas are not well understood, and few studies focus on NH3/NH4+ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH4+. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH3 emission sources and vertically resolved NH4+ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH3 emissions, including fossil fuel sources, NH3 slip, and biomass burning, are important sources of aerosol NH4+ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH3 sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH3 is mostly local from Beijing, and biomass burning is likely an important NH3 source (∼15%-20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO2 and NH3) control policy should be considered to improve air quality.

Source apportionment of atmospheric ammonia in suburban Beijing revealed through 15N-stable isotopes

Addressing the urgent issue of atmospheric ammonia (NH3) emissions is crucial in combating poor air quality in megacities. Previous research has highlighted the significant contribution of nonagricultural sources, particularly fossil fuel emissions, to urban NH3 levels. However, there is limited assessment of NH3 dynamics in suburban areas. This study focuses on four suburban sites in Beijing, covering a 16 to 22-month observation period, to investigate spatial and temporal patterns of NH3 concentrations. The δ15N-stable isotope method is employed to identify NH3 sources and their contributions. Our results demonstrate that agricultural sources (53 %) dominate atmospheric NH3 emissions in suburban areas of Beijing, surpassing nonagricultural sources, and primarily emanate from local sources. Notably, fertilizer application (37 ± 11 %) and livestock breeding (32 ± 6 %) emerge as the primary contributors in summer and spring, respectively, leading to significantly elevated NH3 concentrations during these seasons. Even in autumn and winter, both agricultural (49 %) and nonagricultural (51 %) sources contribute almost equally to NH3 emissions. This study emphasizes the need for coordinated efforts to control atmospheric NH3 pollution in Beijing City, with particular attention to addressing both vehicular and agricultural emissions.

Enhanced photocatalytic ammonia oxidation activity and nitrogen selectivity over Ag/AgCl/N-TiO2 photocatalyst

The removal of ammonia (NH3) emitted from agricultural and industrial activities is of great significance to protect human health and ecological environment. Photocatalytic NH3 oxidation to N2 under mild conditions is a promising strategy. However, developing visible light photocatalysts for NH3 oxidation is still in its infancy. Here, we fabricate N-TiO2 and Ag/AgCl/N-TiO2 photocatalysts by sol-gel and photodeposition methods, respectively. The introduction of N not only endows TiO2 with visible light response (absorption edge at 460 nm) but also results in the formation of heterophase junction (anatase and rutile). Thus, N-TiO2 shows 2.0 and 1.8 times higher than those over anatase TiO2 and commercial TiO2 for NH3 oxidation under full spectrum irradiation. Meanwhile, surface modification of Ag can simultaneously enhance visible light absorption (generating localized surface plasmon resonance effect) and charge separation efficiency. Therefore, the photocatalytic activity of Ag/AgCl/N-TiO2 is further improved. Furthermore, the presence of N and Ag also enhances the selectivity of N2 product owing to the change of reaction pathway. This work simultaneously regulates photocatalytic conversion efficiency and product selectivity, providing some guidance for developing highly efficient photocatalysts for NH3 elimination.

High spatiotemporal resolution ammonia emission inventory from typical industrial and agricultural province of China from 2000 to 2020

As a typical industrial and agricultural province, Shandong is one of China's most seriously air-polluted regions. One comprehensive ammonia emission inventory with a high spatial resolution (1 km × 1 km) for 136 county-level administrative divisions in Shandong from 2000 to 2020 is developed based on county-level activity data with the corrected and updated emission factors of seventy-seven subcategories. Annual ammonia emissions decrease from 1003.3 Gg in 2000 to 795.9 Gg in 2020, with an annual decrease rate of 1.2 %. Therein, the ammonia emissions associated with livestock and farmland ecosystems in 2020 account for 50.8 % and 32.9 % of the provincial total ammonia emission, respectively. Laying hen and wheat are the livestock and crop with the highest ammonia emissions, accounting for 23.3 % and 36.3 % of ammonia emissions from livestock and the application of synthetic fertilizers, respectively. Furthermore, waste treatment, humans and vehicles are the top three ammonia emission sources in urban areas, accounting for 5.0 %, 4.7 % and 1.3 % of total ammonia emissions, respectively. The spatial distribution of grids with high ammonia emissions is consistent with the distribution of intensive farms. Significant emission intensity areas mainly concentrate in western Shandong (e.g., Caoxian of Heze, Qihe of Dezhou, Yanggu of Liaocheng, Liangshan of Jining) due to the large area of arable land and the high levels of agricultural activity. Overall, prominent seasonal variability characteristics of ammonia emission are observed. Ammonia emissions tend to be high in summer and low in winter, and the August to January-emission ratio is 5.6. The high temperature and fertilization for maize are primarily responsible for Shandong's increase in ammonia emissions in summer. Finally, the validity of the estimates is further evaluated using uncertainty analysis and comparison with previous studies. This study can provide information to determine preferentially effective PM2.5 control strategies.

Reduction potential of ammonia emissions and impact on PM2.5 in a megacity of central China

Ammonia control has attracted attention due to the possibility for fine particles (PM2.5) mitigation. Based on past decade ammonia emissions assessments and future predictions, this study seasonally evaluated the ammonia emissions reduction potential in 2025 and 2030 in Wuhan, a Central China megacity, according to the short-term and long-term predictable policies. Furthermore, combined with the reduction potential, PM2.5 components observation and thermodynamic model, the effectiveness of implementing ammonia emission control to reduce PM2.5 by 2025 and 2030 was explored seasonally. Results indicated that the total ammonia emissions are expected to decrease by 19.6-33.9% in 2025 and 2030 under positive reduction scenarios, or increase by 8.9-11.7% in the absence of any intervention. Livestock holds the largest potential for reducing ammonia emissions accounting for 46.4-52.5% of the total. Improvement of human excrement management in rural regions also contributes a 35-37% potential. Despite the implementation of exhaust requirements, ammonia emissions from vehicles in 2030 are expected to continue to increase by 55.3% and 23.5% under the regular (S1) and enhanced (S2) reduction strategy scenarios, respectively. Seasonally, the most potential source of ammonia reduction in spring, summer and fall remains livestock. While in winter, non-agricultural sources dominate the reduction potential. Further results indicated that by ammonia control is expected to decrease PM2.5 concentration up to 5% (less than 1 μg m-3) in 2025-2030. Despite the better effectiveness in winter, ammonia control won't be an effective way to reduce PM2.5 in Central China in future, from the management policies and areal ammonia-rich conditions.

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.

A new portable open-path instrument for ambient NH3 and on-road emission measurements

Increased attentions to vehicle emission of NH3 have been paid since it is generally regarded as an important source in urban areas. Here, we developed a movable instrument based on Differential Optical Absorption Spectroscopy (DOAS) principle for detecting on-road NH3, which can avoid the losses in the sampling process attributed to the non-sampling methods. For this mobile DOAS, the temporal resolution, detection limit and relative error for NH3 were 1 min, 2.29 ppbv and 4.57% ± 2.44%, respectively. By employed to the on-road measurements along the arterial highway in Shanghai, the spatial distributions of NH3 and NO were obtained, and their dependence of traffic and road conditions were studied. The slopes of linear regression between NH3 and NO were 0.40, 0.02 and 0.07 on the Middle Ring Road, Outer Ring Road and Chongming Island Ring Road. It indicates that light gasoline vehicles (LGVs) were found to be the main contributor to NH3 emissions, while heavy-duty diesel vehicles (HDVs) mainly emitted NO. Based on the measured NH3 in the tunnel, the mileage-based NH3 emission factor per vehicle was estimated to be 17.9 ± 6.3 mg/km. The reported open-path instrument can be broadly used in on-road pollutant monitoring or vehicle emissions, and the measurements can reveal the real situation of emission characteristics, even find the abnormal operations of vehicle catalyst system.

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.

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.

Roles of historical land use/cover and nitrogen fertilizer application changes on ammonia emissions in farmland ecosystem from 1990 to 2020 in China

In the past decades, China has witnessed significant changes in its land use/land cover (LULC) pattern. These changes have led to a direct impact on ammonia (NH3) emissions in soil background, and indirectly affected the total nitrogen fertilizer (N-fertilizer) application, crop planting amount and the resulting straw mass through the changes of cropland area. Great changes have also taken place in the amount and structure of fertilizer application in China, which affects the NH3 emissions from farmland ecosystems caused by N-fertilizer application. The aforementioned changes have led to significant alterations in NH3 emissions from China's farmland ecosystems over the past 30 years. The process of these changes remains to be analyzed, and the contributions of LULC changes and N-fertilizer application in this process are yet to be assessed. This study aims to investigate the NH3 emission changes and spatiotemporal variation characteristics from farmland ecosystems during 1990 and 2020 due to the LULC changes. Additionally, the study employs scenario analysis method to discuss the effects of LULC changes and N-fertilizer application changes on NH3 emissions in farmland ecosystems. Results indicate that there is evident spatiotemporal heterogeneity in China's LULC pattern, particularly in eastern China. The southeast region is predominantly characterized by the conversion of cropland into construction land. Moreover, some regions such as Northwest China and Northeast China have experienced the conversion of other land types into cropland, significantly influenced by national development policies. From 1990 to 2020, the national NH3 emissions from farmland ecosystem range from 3294.75 Gg to 4064.20 Gg. NH3 emissions and their interannual variation in farmland ecosystems exhibit significant differences across various regions. The regions with higher contributions to NH3 emissions in farmland ecosystems are East China, Central China, and North China, accounting for 25.32 %-37.26 %, 18.85 %-22.46 % and 11.24 %-18.50 % of the total emissions, respectively. NH3 emissions in each region are influenced by cropland area, N-fertilizer application, and regional development characteristics. Compared to LULC changes, changes in N-fertilizer application have a more pronounced impact on NH3 emission changes in farmland ecosystems. From 1990 to 2020, the contribution (increase or decrease) of N-fertilizer application changes to NH3 emission changes in farmland ecosystems in China ranges from 0.11 % to 16.61 %, while the contribution (increase or decrease) of LULC changes ranges from 0.47 % to 2.38 %. South China demonstrates a unique situation regarding the influence of LULC changes. This region has a relatively small cropland area, and fluctuations in cropland area significantly affect NH3 emissions in farmland ecosystems. The influence of policies is evident. From the changes in cropland area in Northwest China and Northeast China to changes in N-fertilizer application, policy changes have consistently impacted the changes in NH3 emissions in China's farmland ecosystems. From "soft policies" involving encouragement and guidance to "hard policies" encompassing the establishment of necessary targets, the degree of strictness in policy directly affects the timeliness of policies effectiveness. The results of this study indicate that reducing the application of N-fertilizers is the primary approach to reducing NH3 emissions in China's farmland ecosystems. In terms of policy guidance, compared to implementing structural and pathway adjustments, implementing clear total control of fertilizer usage is a timely and effective choice for reducing NH3 emissions.

Apportioning Atmospheric Ammonia Sources across Spatial and Seasonal Scales by Their Isotopic Fingerprint

Mitigating ammonia (NH3) emissions is a significant challenge, given its well-recognized role in the troposphere, contributing to secondary particle formation and impacting acid rain. The difficulty arises from the highly uncertain attribution of atmospheric NH3 to specific emission sources, especially when accounting for diverse environments and varying spatial and temporal scales. In this study, we established a refined δ15N fingerprint for eight emission sources, including three previously overlooked sources of potential importance. We applied this approach in a year-long case study conducted in urban and rural sites located only 40 km apart in the Shandong Peninsula, North China Plain. Our findings highlight that although atmospheric NH3 concentrations and seasonal trends exhibited similarities, their isotopic compositions revealed significant distinctions in the primary NH3 sources. In rural areas, although agriculture emerged as the dominant emission source (64.2 ± 19.5%), a previously underestimated household stove source also played a considerably greater role, particularly during cold seasons (36.5 ± 12.5%). In urban areas, industry and traffic (33.5 ± 15.6%) and, surprisingly, sewage treatment (27.7 ± 11.3%) associated with high population density were identified as the major contributors. Given the relatively short lifetime of atmospheric NH3, our findings highlight the significance of the isotope approach in offering a more comprehensive understanding of localized and seasonal influences of NH3 sources compared to emissions inventories. The refined isotopic fingerprint proves to be an effective tool in distinguishing source contributions across spatial and seasonal scales, thereby providing valuable insights for the development of emission mitigation policies aimed at addressing the increasing NH3 burden on the local atmosphere.

Summertime Urban Ammonia Emissions May Be Substantially Underestimated in Beijing, China

Ammonia (NH3) is critical to the nitrogen cycle and PM2.5 formation, yet a great deal of uncertainty exists in its urban emission quantifications. Model-underestimated NH3 concentrations have been reported for cities, yet few studies have provided an explanation. Here, we explore reasons for severe WRF-Chem model underestimations of NH3 concentrations in Beijing in August 2018, including simulated gas-particle partitioning, meteorology, regional transport, and emissions, using spatially refined (3 km resolution) NH3 emission estimates in the agricultural sector for Beijing-Tianjin-Hebei and in the traffic sector for Beijing. We find that simulated NH3 concentrations are significantly lower than ground-based and satellite observations during August in Beijing, while wintertime underestimations are much more moderate. Further analyses and sensitivity experiments show that such discrepancies cannot be attributed to factors other than biases in NH3 emissions. Using site measurements as constraints, we estimate that both agricultural and non-agricultural NH3 emission totals in Beijing shall increase by ∼5 times to match the observations. Future research should be performed to allocate underestimations to urban fertilizer, power, traffic, or residential sources. Dense and regular urban NH3 observations are necessary to constrain and validate bottom-up inventories and NHx simulation.

Flux of NH3 release from dew evaporation in downtown and suburban Changchun, China

Ammonia, as the only high-concentration alkaline gas in the atmosphere, plays an extremely important role in the initial nucleation process of aerosols. A rise in the concentration of NH3 after sunrise has been observed in many areas, known as the "morning peak phenomenon", which is likely related to the dew evaporation process because of the considerable amount of NH4+ present in dew. To investigate and compare the flux and rate of NH3 release from dew evaporation in downtown (WH) and suburban areas (SL), the dew amount and chemical makeup were measured and analyzed in Changchun, in northeastern China, from April to October 2021. The differences in the fraction of NH4+ released as NH3 gas and the NH3 emission flux and rate during the process of dew evaporation between SL and WH were identified. The results showed that the daily dew amount in WH (0.038 ± 0.017 mm) was lower than that in SL (0.065 ± 0.032 mm) (P < 0.01), and the pH in SL (6.58 ± 0.18) was approximately 1 pH unit higher than that in WH (5.60 ± 0.25). SO42-, NO3-, Ca2+ and NH4+ were the main ions in WH and SL. The ion concentration in WH was significantly higher than that in SL (P < 0.05), which was influenced by human activities and pollution sources. A total of 24%-48% NH4+ was released as NH3 gas during dew evaporation in WH, which was lower than the conversion fraction of SL dew (44%-57%). The evaporation rate of NH3 was 3.9-20.6 ng/m2·s (9.9 ± 5.7 ng/m2·s) in WH and 3.3-15.9 ng/m2·s (8.6 ± 4.2 ng/m2·s) in SL. The dew evaporation process makes an important contribution to the NH3 morning peak phenomenon, but it is not the only contributor.

Source Apportionment of Atmospheric Ammonia at 16 Sites in China Using a Bayesian Isotope Mixing Model Based on δ15N-NHx Signatures

Reducing atmospheric ammonia (NH₃) emissions is critical to mitigating poor air quality. However, the contributions of major agricultural and non-agricultural source emissions to NH₃ at receptor sites remain uncertain in many regions, hindering the assessment and implementation of effective NH₃ reduction strategies. This study conducted simultaneous measurements of the monthly concentrations and stable nitrogen isotopes of NH_x (gaseous NH₃ plus particulate NH₄⁺) at 16 sites across China. Ambient NH_x concentrations averaged 21.7 ± 19.6 μg m^-3 at rural sites, slightly higher than those at urban (19.2 ± 6.0 μg m^-3) and three times of those at background (7.0 ± 6.9 μg m^-3) sites. Based on revised δ¹⁵N values of the initial NH₃, source apportionment results indicated that non-agricultural sources (traffic and waste) and agricultural sources (fertilizer and livestock) contributed 54 and 46% to NH₃ at urban sites, 51 and 49% at rural sites, and 61 and 39% at background sites, respectively. Non-agricultural sources contributed more to NH₃ at rural and background sites in cold than warm seasons, arising from traffic and waste, but were similar across seasons at urban sites. We concluded that non-agricultural sources need to be addressed when reducing ambient NH₃ across China, even in rural regions.

Heat Treatment Improves the Activity and Water Tolerance of Pt/Al2O3 Catalysts in Ammonia Catalytic Oxidation

Ammonia selective catalytic oxidation (NH₃-SCO) is a commercial technology applied to diesel vehicles to eliminate ammonia leakage. In this study, a series of Pt/Al₂O₃ catalysts were synthesized by an impregnation method, and the state of Pt species was carefully adjusted by heat treatment. These Pt/Al₂O₃ catalysts were further systematically characterized by Brunauer-Emmett-Teller, X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, UV-vis, H₂-tempertaure-programmed reduction, and NH₃-temperature-programmed desorption. The characterization results showed that dispersed oxidized Pt species were present on conventional Pt/Al₂O₃ samples, while high-temperature treatment induced the aggregation of platinum species to form metallic Pt nanoparticles. The Pt/Al₂O₃ catalysts treated at high temperatures showed superior activity and water tolerance in the NH₃-SCO reaction. Diffuse reflectance infrared Fourier-transform spectroscopy combined with mass spectrometry experiments revealed that the Lewis acid sites were more reactive than the Brønsted acid sites. Moreover, compared to oxidized Pt species, metallic Pt nanoparticles were beneficial for oxygen activation and were less affected by water vapor, thus contributing to the superior activity and water tolerance of Pt/Al-800.

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.

Atmospheric ammonia in the rural North China Plain during wintertime: Variations, sources, and implications for HONO heterogeneous formation

Atmospheric ammonia (NH₃) plays an important role in secondary inorganic aerosol formation. Understanding the temporal variations, sources, and environmental influences of NH₃ is conducive to better formulate PM_2.5 pollution control strategies for policy-makers. Here, we performed a comprehensive field campaign with the measurements of NH₃ and related parameters at a rural site of the North China Plain (NCP) in winter of 2017. The results showed that residential coal combustion contributed dominantly to NH₃ during the entire observation period, resulting in the obviously high average concentration of NH₃ (31.2 ± 24.6 ppbv). The sensitivity tests of pH-NH_x during the three different pollution periods suggested that the rural site was always in the NH_x-rich atmosphere where high levels of NH_x increased the particle pH inefficiently. Nevertheless, the particle pH still elevated by 1.5-2.2 units at the excessive NH_x levels during the three pollution periods. In addition, the HONO/NO₂ ratios were found to correlate linearly with NH₃ concentrations, implying the acceleration effect of NH₃ on HONO production from NO₂ heterogeneous reactions. After considering the NH₃-enhanced uptake coefficient of NO₂ in the nocturnal HONO budget, the unknown source of HONO could be fully explained. Therefore, more attentions should be given for effective emission control of NH₃ to improve air quality throughout the NCP, especially in the rural areas.

Vehicular Ammonia Emissions Significantly Contribute to Urban PM2.5 Pollution in Two Chinese Megacities

Ammonia (NH₃) plays a vital role in the formation of fine particulate matter (PM_2.5). Prior studies have primarily focused on the control of agricultural NH₃ emissions, the dominant source of anthropogenic NH₃ emissions. The air quality impact from vehicular NH₃ emissions, which could be particularly important in urban areas, has not been adequately evaluated. We developed high-resolution vehicular NH₃ emission inventories for Beijing and Shanghai based on detailed link-level traffic profiles and conducted atmospheric simulations of ambient PM_2.5 concentrations contributed by vehicular NH₃ emissions. We found that vehicular NH₃ emissions shared high proportions among total anthropogenic NH₃ emissions in the urban areas of Beijing (86%) and Shanghai (45%), where vehicular NH₃ was primarily emitted by gasoline vehicles. Local vehicular NH₃ emissions could be responsible for approximately 3% of urban PM_2.5 concentrations during wintertime, and the contributions could be much higher during polluted periods (∼3 μg m^-3). We also showed that controlling vehicular NH₃ emissions will be effective and feasible to alleviate urban PM_2.5 pollution for megacities in the near future.

Fabrication of PA-PEI-MOF303(Al) by Stepwise Impregnation Layer-by-Layer Growth for Highly Efficient Removal of Ammonia

NH₃ is a typical alkaline gaseous pollutant widely derived from industrial production and poses great risks to humans and other biota. Metal-organic frameworks (MOFs) have excellent adsorption capacities relative to materials traditionally used to adsorb NH₃. However, in practice, applications of MOFs as adsorbents are restricted because of its powder form. We prepared a polyamide (PA) macroporous polyester substrate using an emulsion template method and modified the surface with polyethylenimine (PEI) to improve the MOF growth efficiency on the substrate. The difficulty of loading the MOF because of the fast nucleation rate inside the PA macroporous polyester substrate was solved using a stepwise impregnation layer-by-layer (LBL) growth method, and a PA-PEI-MOF303(Al) hierarchical pore composite that very efficiently adsorbed NH₃ was successfully prepared. The PA-PEI-MOF303(Al) adsorption capacity for NH₃ was 16.07 mmol·g^-1 at 298 K and 100 kPa, and the PA-PEI-MOF303(Al) could be regenerated repeatedly under vacuum at 423 K. The NH₃ adsorption mechanism was investigated by in situ Fourier transform infrared spectroscopy and by performing two-dimensional correlation analysis. Unlike for the MOF303(Al) powder, the formation of multi-site hydrogen bonds between Al-O-Al/C-OH, N-H, -OH, C=O, and NH₃ in PA-PEI-MOF303(Al) was found to be an important reason for efficient NH₃ adsorption. This study will provide a reference for the preparation of other MOF-polymer composites.

Standoff sub-ppb level measurement of atmospheric ammonia with calibration-free wavelength modulation spectroscopy

Ammonia (NH₃) plays a significant role in the formation of atmospheric particulate matter, and influences on environmental and public health as well as climate change. Thus, it is important to sensitive measurement of atmospheric NH₃. In the present work, a sub-ppb level standoff open-path NH₃ sensor was developed for on line, sensitive measurement of atmospheric NH₃. A 9.06 μm distributed feedback quantum cascade laser was employed to probe the ammonia absorption lines located on fundamental rotational-vibrational absorption band and calibration-free wavelength modulation spectroscopy technique was employed to retrieve NH₃ concentration directly. The standoff open-path NH₃ sensor performance was investigated in laboratory corridor with 80 m open path length (Hefei, China) and a minimum detection limit of 0.46 ppb (3σ) was obtained. Finally, field campaign measurement was carried out in a winter wheat farmland (Changshu, China). Field measurement shown that the concentration of NH₃ varies from 7 ppb to 30 ppb with an average of 14 ppb. The developed standoff sensor has high potential to be a robust tool for monitoring atmospheric NH₃ or study of regional ammonia emissions in farmland or feedlot scale.

Potential Risk of NH3 Slip Arisen from Catalytic Inactive Site in Selective Catalytic Reduction of NOx with Metal-Free Carbon Catalysts

Ammonia emissions from industrial processes have rapidly increased in the past years. Recent advances have used carbon-based selective catalytic reduction (SCR) technology combined with a reaction-regeneration process to reduce NO_x from sintering flue gas, while NH₃ slip is seldom accounted for in this process. This study demonstrates that although the electrophilic carboxyl groups (-COOH) on metal-free carbon catalysts exhibit strong adsorption toward NH₃, they do not participate in the SCR reaction. As a result of the competitive adsorption of NH₃ in the reaction step, these catalytic inactive carboxyl groups not only prolong the time to the SCR steady state, but also result in the potential risk of NH₃ slip. A linear relationship with the equimolar ratio between carboxyl groups and slipped NH₃ was established in the regeneration steps. The slip of NH₃ could be alleviated by the decomposition of carboxyl groups, and special attention should be paid to the presence of inactive sites with strong NH₃ adsorption on industrial-employed carbon catalysts. In addition to advancing the understanding of the NH₃-SCR mechanism, this work also provides valuable opportunities for the control of ammonia emissions from industrial processes.

Isotopic imprints of aerosol ammonium over the north China plain

Atmospheric PM_2.5 poses a variety of health and environmental risks to urban environments. Ammonium is one of the main components of PM_2.5, and its role in PM_2.5 pollution will likely increase in the coming years as NH₃ emissions are still unregulated and rising in many cities worldwide. However, partitioning urban NH₄⁺ sources remains challenging. Although the ¹⁵N natural abundance (δ¹⁵N) analysis is a promising approach for this purpose, it has seldom been applied across multiple cities within a given region. This limits our understanding of the regional patterns and controls of NH₄⁺ sources in urban environments. Here, we collected PM_2.5 samples using an active sampling technique during winter at six cities in the North China Plain to characterize the concentrations, δ¹⁵N and sources of NH₄⁺ in PM_2.5. We found substantial variations in both the concentrations and δ¹⁵N of NH₄⁺ among the sites. The mean NH₄⁺ concentrations across the six cities ranged from 3.6 to 12.1 μg m^-3 on polluted days and from 0.9 to 10.6 μg m^-3 on non-polluted days. The δ¹⁵N ranged from 6.5‰ to 13.9‰ on polluted days and from 8.7‰ to 13.5‰ on non-polluted days. The δ¹⁵N decreased with increasing NH₄⁺ concentrations at all six sites. We found that non-agricultural sources (vehicle exhaust, ammonia slip and urban wastes) contributed 72%-94% and 56%-86% of the NH₄⁺ on polluted and non-polluted days, respectively, and that during polluted days, combustion-related emissions (vehicle exhaust and ammonia slip) were positively associated with the proportion of urban area, population density and number of vehicles, highlighting the importance of local sources of particulate pollution. This study suggests that the analysis of ¹⁵N in aerosol NH₄⁺ is a promising approach for apportioning atmospheric NH₃ sources over a large region, and this approach has potential for mapping rapidly and precisely the sources of NH₃ emissions.

Significant contributions of combustion-related sources to ammonia emissions

Atmospheric ammonia (NH3) and ammonium (NH4+) can substantially influence air quality, ecosystems, and climate. NH3 volatilization from fertilizers and wastes (v-NH3) has long been assumed to be the primary NH3 source, but the contribution of combustion-related NH3 (c-NH3, mainly fossil fuels and biomass burning) remains unconstrained. Here, we collated nitrogen isotopes of atmospheric NH3 and NH4+ and established a robust method to differentiate v-NH3 and c-NH3. We found that the relative contribution of the c-NH3 in the total NH3 emissions reached up to 40 ± 21% (6.6 ± 3.4 Tg N yr-1), 49 ± 16% (2.8 ± 0.9 Tg N yr-1), and 44 ± 19% (2.8 ± 1.3 Tg N yr-1) in East Asia, North America, and Europe, respectively, though its fractions and amounts in these regions generally decreased over the past decades. Given its importance, c-NH3 emission should be considered in making emission inventories, dispersion modeling, mitigation strategies, budgeting deposition fluxes, and evaluating the ecological effects of atmospheric NH3 loading.

Long-Term Source Apportionment of Ammonium in PM2.5 at a Suburban and a Rural Site Using Stable Nitrogen Isotopes

Ammonia gas (NH3) is an important alkaline air pollutant and a precursor to particulate matter, and its source has been thought to be agricultural, but in recent years, nonagricultural sources have been suspected. In this study, stable nitrogen isotope ratios of ammonium (δ15N-NH4+) in fine particulate matter (PM2.5) were measured at a suburban site and a rural site in Japan. Then, the long-term sources of NH4+ were identified using the δ15N-NH3 and an isotopic mixing model. The results showed that the averaged contribution from nonagricultural sources was 67% at the suburban site and 78% at the rural site. We also reanalyzed NH3 data collected at the same location. The result showed that the averaged contribution of nonagricultural sources to NH3 was 39%. This result is reasonable because bottom-up estimates are close to the contribution, and the NH3 emissions are affected by warm season activities in the rural site. It was first found that the sources vary greatly, depending on the gas and particles. Back-trajectory results suggested that PM2.5 measured at the rural site was derived from the Asian continent. We inferred that the NH4+ had been formed on the continent and that these particles thus represent transboundary pollution.

PM2.5 and water-soluble inorganic ion concentrations decreased faster in urban than rural areas in China

We investigated variations of PM_2.5 and water-soluble inorganic ions chemical characteristics at nine urban and rural sites in China using ground-based observations. From 2015 to 2019, mean PM_2.5 concentration across all sites decreased by 41.9 µg/m³ with a decline of 46% at urban sites and 28% at rural sites, where secondary inorganic aerosol (SIAs) contributed to 21% (urban sites) and 17% (rural sites) of the decreased PM_2.5. SIAs concentrations underwent a decline at urban locations, while sulfate (SO₄^2-), nitrate (NO₃^-), and ammonium (NH₄⁺) decreased by 49.5%, 31.3% and 31.6%, respectively. However, only SO₄^2- decreased at rural sites, NO₃^- increased by 21% and NH₄⁺ decreased slightly. Those changes contributed to an overall SIAs increase in 2019. Higher molar ratios of NO₃^- to SO₄^2- and NH₄⁺ to SO₄^2- were observed at urban sites than rural sites, being highest in the heavily polluted days. Mean molar ratios of NH₃/NH_x were higher in 2019 than 2015 at both urban and rural sites, implying increasing NH_x remained as free NH₃. Our observations indicated a slower transition from sulfate-driven to nitrate-driven aerosol pollution and less efficient control of NO_x than SO₂ related aerosol formation in rural regions than urban regions. Moreover, the common factor at urban and rural sites appears to be a combination of lower SO₄^2- levels and an increasing fraction of NO₃^- to PM_2.5 under NH₄⁺-rich conditions. Our findings imply that synchronous reduction in NO_x and NH₃ emissions especially rural areas would be effective to mitigate NO₃^--driven aerosol pollution.

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.

Development of a Near-Infrared Photoacoustic System for Selective, Fast, and Fully Automatized Detection of Isotopically Labeled Ammonia

Different environmental and industrial technologies seek for fast and automatic ammonia detection systems, capable of the selective measurement of the concentration of its isotopes at sub-ppm levels, without any interference with the common contaminants. In this work, we report the quasi-simultaneous measurement of ¹⁴NH₃ and ¹⁵NH₃ concentrations based on a near-infrared diode laser-based photoacoustic system. Using a widely tunable external cavity diode laser, four nearby wavelengths within the range of 1531.3–1531.8 nm were optimal circumstances for sensitive detection, while avoiding interference with water vapor. Subsequently, a more robust distributed feedback diode laser was employed to tune the laser wavelength on the sub-second timescale by varying its driving current rather than using much slower temperature tuning. The detection limit of our system is 0.15 and 0.73 ppm for ¹⁴NH₃ and ¹⁵NH₃ (with an accuracy below 0.1%), respectively, and the response time is 3.5 s.

Is fertilization the dominant source of ammonia in the urban atmosphere?

It was previously believed that ammonia (NH₃) has a short residence time in the atmosphere and cannot be transported far from its sources. In late March, however, this study observed a severe NH₃ episode in urban Beijing when fertilizer was intensively applied on the North China Plain, with the highest hourly concentrations of 66.9 μg m^-3 throughout the year. The stable nitrogen isotopic composition of NH₃ (δ¹⁵N-NH₃) during this episode (-37.0 to -20.0‰) fell in the range of endmembers of fertilizer and livestock, suggesting the long-range transport of NH₃ from agricultural to urban regions. Based on a Bayesian isotope mixing model, the contribution of agriculture (fertilization) to urban NH₃ concentrations was apportioned as 43.5% (26.0%) on polluted days. However, these contributions were reduced to 29.1% (12.8%) when nitrogen isotope fractionation between NH₃ and ammonium was considered. In contrast to the limited contribution of agricultural sources, we found that nonagricultural emissions, particularly vehicles, dominate the source of NH₃ in urban Beijing, even during the fertilization period. This finding indicated that nonagricultural sources should be considered when designing a control strategy for NH₃ to reduce haze pollution in the urban atmosphere.

Sources identification of ammonium in PM2.5 during monsoon season in Dhaka, Bangladesh

Ammonia (NH₃) is taken up by fine particulate matter (PM_2.5), and there are concerns about its impact on the environment and health. The source of NH₃, which was thought to be of agricultural sources, has recently been suspected to be non-agricultural sources in urban areas. Here, we collected PM_2.5 during the monsoon season in Dhaka, Bangladesh, the most polluted city in the world, and analyzed the δ¹⁵N-NH₄⁺ in PM_2.5. As the result, the δ¹⁵N-NH₄⁺ ranged from 9.2 ‰ to 34.4 ‰ (average: 20.7 ± 4.8 ‰), the highest of any of the averaged values annual reported in previous researches. In order to perform source analysis, the NH₃ concentrations were estimated using the thermodynamic model ISORROPIA-II. The estimated concentration of NH₃ gas averaged 40.8 μg/m³ (3.0-154.6 μg/m³). The contributions calculated with the mixing model to the δ¹⁵N-NH₄⁺ values in PM_2.5 in Dhaka, Bangladesh averaged 25.3 ± 14 %, 22.8 ± 10 %, 26.5 ± 15 %, and 25.4 ± 10 % for waste, fertilizer, NH₃ slip, and fossil fuel combustion, respectively. Non-agricultural sources (NH₃ slip, and fossil fuel combustion) accounted for almost half (51.9 %) of the contributions. In addition, the several validation tests of the isotope mixing model were also performed. For validating the uncorrected and corrected source data for δ¹⁵N-NH₃, the contribution of non-agricultural sources with uncorrected source data would have been very high (>80 %), much higher than the corrected source data.

Unexpected catalytic influence of atmospheric pollutants on the formation of environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) have been recognized as harmful and persistent environmental pollutants. In polluted regions, many acidic and basic atmospheric pollutants, which are present at high concentrations, may influence the extent of the formation of EPFRs. In the present paper, density functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations were performed to investigate the formation mechanisms of EPFRs with the influence of the acidic pollutants sulfuric acid (SA), nitric acid (NA), organic acid (OA), and the basic pollutants, ammonia (A), dimethylamine (DMA) on α-Al₂O₃ (0001) surface. Results indicate that both acidic and basic pollutants can enhance the formation of EPFRs by acting as "bridge" or "semi-bridge" roles by proceeding via a barrierless process. Acidic pollutants enhance the formation of EPFRs by first transferring its hydrogen atom to the α-Al₂O₃ surface and subsequently reacting with phenol to form an EPFR. In contrast, basic pollutants enhance the formation of EPFRs by first abstracting a hydrogen atom from phenol to form a phenoxy EPFR and eventually interacting with the α-Al₂O₃ surface. These new mechanistic insights will inform in understanding the abundant EPFRs in polluted regions with high mass concentrations of acidic and basic pollutants.

Spatiotemporal variations of nitrogen and phosphorus deposition across China

Atmospheric deposition is an important pathway for the input of anthropogenic and natural nutrients to terrestrial and aquatic ecosystems. However, previous measurements focused mainly on hotspot locations, ignoring the fact that the deposition magnitudes of various nutrient species (e.g., nitrogen (N), phosphorus (P)) at a national scale should be investigated jointly. To better characterize national scale bulk deposition, precipitation samples were collected at 41 sites across China from September 2015 to August 2016 and September 2017 to August 2018. The bulk deposition fluxes of total nitrogen (TN) and total phosphorus (TP) over the network were 27.5 kg N ha^-1 yr^-1 and 0.92 kg P ha^-1 yr^-1, respectively. Contributions of NH₄⁺, NO₃^-, and dissolved organic nitrogen (DON) to TN averaged 32%, 32%, and 36%, respectively. Significant spatial and seasonal variations in concentrations and deposition fluxes of all nutrient species were observed reflecting effects of local reactive nitrogen (Nr) and P emissions and rainfall amount. Major sources were energy resource consumption for NO₃^-, agricultural activities for NH₄⁺, and a mixed contribution of both anthropogenic and natural sources for DON and TP. Atmospheric N and P deposition represent important external nutrient inputs to ecosystems and a high ratio of TN to TP (29.9) may induce relative P-limitation and further increase the risk of eutrophication. This work reveals a new map of atmospheric N and P deposition and identifies regions where emissions should be controlled to mitigate long-term impacts of atmospheric deposition over China.

Robust Evidence of 14C, 13C, and 15N Analyses Indicating Fossil Fuel Sources for Total Carbon and Ammonium in Fine Aerosols in Seoul Megacity

Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the ¹⁴C fraction modern (f_M) and δ¹³C of total carbon (TC) and δ¹⁵N of NH₄⁺ in the PM_2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ¹³C values were similar to -25.1‰ ± 2.0‰ in warm and -24.2‰ ± 0.82‰ in cold seasons. Mean δ¹⁵N values were higher in warm (16.4‰ ± 2.8‰) than in cold seasons (4.0‰ ± 6.1‰), highlighting the temperature effects on atmospheric NH₃ levels and phase-equilibrium isotopic exchange during the conversion of NH₃ to NH₄⁺. While 37% ± 10% of TC was apportioned to fossil-fuel sources on the basis of f_M values, δ¹⁵N indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH₃ slip) to NH₃: 60% ± 26% in warm season and 66% ± 22% in cold season, based on a Bayesian isotope-mixing model. The collective evidence of multiple isotope analysis reasonably supports the major contribution of fossil-fuel-combustion sources to NH₄⁺, in conjunction with TC, and an increased contribution from vehicle emissions during the severe PM_2.5 pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM_2.5 in the urban atmosphere.

Seasonal variation of transport pathways and potential source areas at high inorganic nitrogen wet deposition sites in southern China

This study attempts to identify the dominant transport pathways, potential source areas, and their seasonal variation at sites with high inorganic nitrogen (IN) wet deposition flux in southern China. This is a long-term study (2010-2017) based on continuous deposition measurements at the Guangzhou urban site (GZ) and the Dinghushan Natural Reserve site (DHS) located in the Pearl River Delta (PRD) region. A dataset on monthly IN concentration in precipitation and wet deposition flux were provided. The average annual fluxes measured at both sites (GZ: 33.04±9.52, DHS: 20.52±10.22 kg N/(ha∙year)) were higher, while the ratios of reduced to oxidized N (GZ: 1.19±0.77, DHS: 1.25±0.84) were lower compared with the national mean level and the previous reported level throughout the PRD region. The dominant pathways were not always consistent with the highest proportional trajectory clusters. The transport pathways contributing most of deposition were identified in the north and north-northeast in the dry season and in the east-southeast, east, and south-southwest in the wet season. A weighted potential source contribution function (WPSCF) value >0.3 was determined reasonably to define the potential source area. Emission within the PRD region contributed the majority (≥95% at both sites) of the IN deposition in the wet season, while the contribution outside the region increased significantly in the dry season (GZ: 27.86%, DHS: 95.26%). Our results could help create more effective policy to control precursor emissions for IN fluxes, enabling reduction of the ecological risks due to excessive nitrogen.

Benefits of refined NH3 emission controls on PM2.5 mitigation in Central China

Atmospheric ammonia (NH₃) is one of the most crucial precursors of secondary inorganic aerosols. However, its emission control is still weakness over China. NH₃ emission inventories of 2015 with and without considering a set of refined emission reduction strategies covering seven major NH₃ emission sources were constructed in Central China. GEOS-Chem model simulations were conducted to quantify the benefits of NH₃ emission reduction on PM_2.5 mitigation in four typical months (January, April, July and October). The results showed that these control strategies could reduce approximately 47.0% (152 Gg) of the total NH₃ emissions in Hubei Province, with the agricultural (livestock and fertilizer application) source being reduced the most (133 Gg). NH₃ had a significant nonlinear relationship with sulfate, nitrate, ammonium and PM_2.5. NH₃ emission reduction exerted less effect on sulfate mitigations (the annual average sensitivity was 4.5%), but it obviously alleviated nitrate, ammonium and thus PM_2.5, with the annual average sensitivities of 81.9%, 34.8% and 22.0%, respectively. The average provincial concentrations of PM_2.5 were alleviated by 11.2% in January, 10.6% in October, 10.2% in April and 9.3% in July through NH₃ emission reduction by 47.0%. The reduction benefits were more pronounced in high NH₃ emission areas, such as Yichang, with the PM_2.5 reduction of 14.4% in January. This research could provide scientific support for formulating NH₃ emission reduction policies to further mitigate PM_2.5 pollution.

Rapid Increase in China's Industrial Ammonia Emissions: Evidence from Unit-Based Mapping

Ammonia (NH₃) is an important precursor of secondary inorganic aerosols and greatly impacts nitrogen deposition and acid rain. Previous studies have mainly focused on the agricultural NH₃ emissions, while recent research has noted that industrial sources could be significant in China. However, detailed estimates of NH₃ emitted from industrial sectors in China are lacking. Here, we established an unprecedented high-spatial-resolution data set of China's industrial NH₃ emissions using up-to-date measurements of NH₃ and point source-level information covering eight major industries and 27 subdivided process categories. We found that China emitted 798 (90% confidence interval: 668-933) gigagrams of industrial NH₃ into the atmosphere in 2019, equivalent to 44 ± 20% of the industrial emissions worldwide; this flux is 3-fold larger than that in 1998 and has fluctuated since 2014. Furthermore, although fertilizer production is responsible for approximately half of the emissions in China, the emissions from cement production and coal-fired power plants increased dramatically from near zero to 164 and 41 gigagrams, respectively, in the past two decades, primarily due to the NH₃ escape caused by the large-scale application of the denitration process. Our results reveal that, unlike other major air pollutants, China's industrial NH₃ emission control is still in a critical period, and stricter NH₃ emission standards and innovation in pollution control technologies are highly desirable.

Long-term effects of PM2.5 components on blood pressure and hypertension in Chinese children and adolescents

Growing evidence has linked fine particulate matter (PM_2.5) exposure to elevated blood pressure, but the effects of PM_2.5 components are unclear, particularly in children and adolescents. Based on a cross-sectional investigation in China, we analyzed the associations between long-term exposure to PM_2.5 and its major components with elevated blood pressure in children and adolescents. A representative sample (N = 37,610) of children and adolescents with age 7-18 years was collected in seven Chinese provinces. Exposures to PM_2.5 and five of its major components, including black carbon (BC), organic matter (OM), inorganic nitrate (NO₃^-), sulfate (SO₄^2-), and soil particles (SOIL), were estimated using satellite-based spatiotemporal models. The associations between long-term exposures to PM_2.5 and its components and diastolic blood pressure (DBP), systolic blood pressure (SBP), and hypertension were investigated using mixed-effects logistic and linear regression models. Within the populations, 11.5 % were classified as hypertension. After adjusting for a variety of covariates, per interquartile range (IQR) increment in PM_2.5 mass and BC levels were significantly associated with a higher hypertension prevalence with odds ratios (ORs) of 1.56 (95% confidence interval (CI): 1.08, 2.25) for PM_2.5 and 1.19 (95% CI: 1.04, 1.35) for BC. Long-term exposures to PM_2.5 and BC have also been associated with elevated SBP and DBP. Additionally, OM and NO₃^- were significantly associated with increased SBP, while SOIL was significantly associated with increased DBP. In the subgroup analysis, the associations between long-term exposures to BC and blood pressure vary significantly by urbanicity of residential area and diet habits. Our study suggests that long-term exposure to PM_2.5 mass and specific PM_2.5 components, especially for BC, are significantly associated with elevated blood pressure and a higher hypertension prevalence in Chinese children and adolescents.

Estimating urban air pollution contribution to South Platte River nitrogen loads with National Atmospheric Deposition Program data and SPARROW model

Air pollution is commonly disregarded as a source of nutrient loading to impaired surface waters managed under the Clean Water Act per states' 303(d) list programs. The contribution of air pollution to 2017-2018 South Platte River nitrogen (N) loads was estimated from the headwaters to the gage at Weldona, Colorado, USA (100 km downstream of Denver), using data from the National Atmospheric Deposition Program (NADP) and the SPAtially Referenced Regressions On Watershed attributes (SPARROW) model. The NADP offers wet-deposition raster created by spatial interpolation of data collected from regionally representative monitoring sites, excluding the influences from urban site data. For this study, NADP wet-deposition data obtained from sites within the Denver-Boulder, Colorado, urban corridor were included and excluded in new spatial interpolations of wet-deposition raster, which were used as input for SPARROW to model the influence of urban air pollution sources on South Platte River loads. Because urban air pollution is already incorporated into the NADP Total Deposition modeling methodology, dry N deposition was held constant for each SPARROW modeling scenario when dry deposition was included. By including the urban wet-deposition data in the model, estimated N loading to the South Platte River at Denver increased by 9-11 percent. Factoring in dry deposition at a 1:1.8 dry:wet ratio obtained from the results, urban air pollution was estimated to contribute as much as 20 percent of the nitrate Total Maximum Daily Load for Segment 14 of the South Platte River.

Spatial Distributions of Atmospheric Ammonia in a Rural Area in South Korea and the Associated Impact on a Nearby Urban Area

Ammonia (NH3) plays an important role in air quality and atmospheric chemistry, yet studies on the characteristics and impacts of NH3 are limited. Herein, we revealed the spatial distribution of atmospheric NH3, as measured by passive samplers, at three different sites (R1, R2, and R3) in the rural area (livestock environment) of Jeongeup, South Korea, from September 2019 to August 2020. At site R1, the boundary of a large-scale pig farm, dramatically high daily mean concentrations of NH3 were observed (118.7 ppb), whereas sites R2 and R3, located ~1 km from R1, exhibited lower concentrations of 18.2 and 30.4 ppb, respectively. In the rural environment, the monthly NH3 variations showed a peak in June (34.2 ppb), which was significantly higher than in the urban and remote areas. To examine the impact of NH3 from the rural area on a nearby urban area in June 2020, simultaneous measurements were performed using a real-time instrument in Jeonju. When high NH3 events occurred in the urban area in June, the results for the NH3 concentrations and observed meteorological conditions in the rural and urban areas showed that the rural area influenced the NH3 levels in the adjacent urban area.

Toxic effects of ammonia on intestinal health and microbiota in red-eared slider (Trachemys scripta elegans)

Ammonia is an important environmental pollutant and can induce serious damages to the organs of aquatic animals, especially the intestine which is mostly exposed to external environment. As important species of aquatic ecosystems, turtles may be potential risk targets of ammonia. However, it is not clear whether ammonia shows toxic effects on the intestines of turtles. Therefore, the worldwide species red-eared slider (Trachemys scripta elegans) was selected, to investigate the effects of ammonia on intestinal health and the composition of microbiota. Results showed that ammonia significantly changed the structure of intestines by decreasing the thickness of intestinal wall, shortening the length of intestinal villus, extending lamina proprias, and inducing inflammatory cells appearance when the turtles were exposed to ammonia (1.418 mg NH₃ L^-1) for 30 d. In addition, the downregulation of epithelial tight junction genes indicated that ammonia increased selective paracellular permeability. Simultaneously, the upregulation of cytokines suggested that ammonia induced intestinal immune and inflammatory responses. Furthermore, ammonia altered the dominant bacterial composition, and decreased the abundance of beneficial intestinal bacteria in the host. Our results demonstrated that ammonia impaired the intestinal health and changed the composition of residential microbiota in T. s. elegans. This study provides a new insight to evaluate the toxic effects of ammonia on aquatic turtles and helps to build a framework for the effective conservation of turtles.

Evolution of secondary inorganic aerosols amidst improving PM2.5 air quality in the North China plain

The Clean Air Action implemented by the Chinese government in 2013 has greatly improved air quality in the North China Plain (NCP). In this work, we report changes in the chemical components of atmospheric fine particulate matter (PM_2.5) at four NCP sampling sites from 2012/2013 to 2017 to investigate the impacts and drivers of the Clean Air Action on aerosol chemistry, especially for secondary inorganic aerosols (SIA). During the observation period, the concentrations of PM_2.5 and its chemical components (especially SIA, organic carbon (OC), and elemental carbon (EC)) and the frequency of polluted days (daily PM_2.5 concentration ≥ 75 μg m^-3) in the NCP, declined significantly at all four sites. Asynchronized reduction in SIA components (large decreases in SO₄^2- with stable or even increased NO₃^- and NH₄⁺) was observed in urban Beijing, revealing a shift of the primary form of SIA, which suggested the fractions of NO₃^- increased more rapidly than SO₄^2- during PM_2.5 pollution episodes, especially in 2016 and 2017. In addition, unexpected increases in the sulfur oxidation ratio (SOR) and the nitrogen oxidation ratio (NOR) were observed among sites and across years in the substantially decreased PM_2.5 levels. They were largely determined by secondary aerosol precursors (i.e. decreased SO₂ and NO₂), photochemical oxidants (e.g. increased O₃), temperature, and relative humidity via gas-phase and heterogeneous reactions. Our results not only highlight the effectiveness of the Action Plan for improving air quality in the NCP, but also suggest an increasing importance of SIA in determining PM_2.5 concentration and composition.

Assessing the emission sources and reduction potential of atmospheric ammonia at an urban site in Northeast China

Atmospheric ammonium and ammonia have brought negative environmental impacts and adverse health effects. However, ammonia emissions are generally less regulated worldwide. This study analyzed ammonium pollution character, quantified the dominant ammonia emission sources, and assessed ammonia reduction potential in urban Harbin (China). The results showed that ammonium recorded low concentration in the non-heating season (1.34 ± 1.57 μg/m³), and recorded sharply increased concentration (6.50 ± 7.02 μg/m³) and relative abundance in the heating season. It was closely correlated with vehicle-related pollutants (CO) in non-heating season, while with biomass burning-related pollutants (K⁺, Cl^-) in the heating season. Bayesian Mixing Model emphasized the increasing contribution of biomass burning and decreasing contribution of fertilizer as the pollution levels escalate. The results from the thermodynamic equilibrium model showed that a 50%-60% ammonium decrease could bring marketable decrements of the aerosol pH, aerosol water content, ammonium nitrate concentration, and inorganic ion mass. The results of this study would provide scientific bases for ammonia emission reduction and haze pollution control in urban Harbin.

Emission factors of ammonia for on-road vehicles in urban areas from a tunnel study in south China with laser-absorption based measurements

Vehicle emission is an important source of ammonia (NH₃) in urban areas. To better address the role of vehicle emission in urban NH₃ sources, the emission factor of NH₃ (NH₃-EF) from vehicles running on roads under real-world conditions (on-road vehicles) needs to update accordingly with the increasingly tightened vehicle emission standards. In this study, laser-absorption based measurements of NH₃ were conducted during a six-day campaign in 2019 at a busy urban tunnel with a daily traffic flow of nearly 40,000 vehicles in south China's Pearl River Delta (PRD) region. The NH₃-EF was measured to be 16.6 ± 6.3 mg km^-1 for the on-road vehicle fleets and 19.0 ± 7.2 mg km^-1 for non-electric vehicles, with an NH₃ to CO₂ ratio of 0.27 ± 0.09 ppbv ppmv^-1. Multiple linear regression revealed that the average NH₃-EFs for gasoline vehicles (GVs), liquefied petroleum gas vehicles, and heavy-duty diesel vehicles (HDVs) were 18.8, 15.6, and 44.2 mg km^-1, respectively. While NH₃ emissions from GVs were greatly reduced with enhanced performance of engines and catalytic devices to meet stricter emission standards, the application of urea selective catalytic reduction (SCR) in HDVs makes their NH₃ emission an emerging concern. Based on results from this study, HDVs may contribute over 11% of the vehicular NH₃ emissions, although they only share ∼4% by vehicle numbers in China. With the updated NH₃-EFs, NH₃ emission from on-road vehicles was estimated to be 9 Gg yr^-1 in the PRD region in 2019, contributing only 5% of total NH₃ emissions in the region, but still might be a dominant NH₃ source in the urban centers with little agricultural activity.

Inorganic nitrogen deposition in arid land ecosystems of Central Asia

Atmospheric reactive nitrogen (Nr) pollution leads to enhanced Nr deposition. There still big gaps in understanding atmospheric nitrogen deposition because of limited monitoring sites in arid land ecosystems of Central Asia. To determine Nr concentrations and deposition in the study area, we have set up 20 monitoring sites to collect gaseous, particulate, and precipitation samples and measure their Nr components since 2009. Nr concentrations in air showed large spatial variations. Based on the Nr concentrations, dry deposition was calculated using the monthly average Nr concentrations by the corresponding deposition velocities modeled, which was varied between 3.15 and 27.92 kg N ha^-1 yr^-1 across desert, grassland, desert-grassland, forest, farmland, and city/suburb ecosystems. Ammonia N deposition varied between 0.50 asnd 8.66 kg N ha^-1 yr^-1, and nitrate N deposition c varied between 0.67 and 4.22 kg N ha^-1 yr^-1, respectively, in precipitation. Annual N deposition is following the order of desert (4.0) < grassland (6.0) < desert-grassland (7.6) < forest (16.1) < farmland (18.4) < city/suburb (35.4) ecosystems. Dry deposition contributed 52.7, 53.8, 100, 68.2, 73.7, and 78.9% of total N deposition in grassland, desert-grassland, desert, forest, farmland and city/suburb ecosystems, respectively. Reduced nitrogen deposition accounted for 62% of total N deposition in the arid area. Dry NH₃ deposition made an important contribution (on average 40%) to total N deposition. Therefore, understanding the characteristics of Nr pollution especially NH₃ emission is indispensable to atmospheric pollution control in arid region.

Reapportioning the sources of secondary components of PM2.5: A combined application of positive matrix factorization and isotopic evidence

Secondary particles account for a considerable proportion of fine particles (PM_2.5) and reasonable reapportioning them to primary sources is critical for designing effective strategies for air quality improvement. This study developed a method which can reapportion secondary sources of PM_2.5 solved by positive matrix factorization (PMF) to primary sources based on the isotopic signals of nitrate, ammonium and sulfate. Actual PM_2.5 data in Beijing were used as a case study to assess the feasibility and capacity of this method. In the case, 20 chemical components were used to apportion PM_2.5 sources and source contributions of nitrate were applied to reapportion secondary source to primary sources. The model performance was also estimated by radiocarbon measurement (¹⁴C) of organic (OC) and elemental (EC) carbons of eight samples. The PMF apportioned seven sources: the secondary source (36.1%), vehicle exhausts (18.7%), industrial sources (13.6%), biomass burning (11.4%), coal combustion (8.10%), construction dust (7.93%) and fuel oil combustion (4.24%). After the reapportionment of the secondary source, vehicle exhausts (28.7%) contributed the most to PM_2.5, followed by biomass burning (25.1%) and industrial sources (18.9%). Fossil oil combustion and coal combustion increased to 8.00% and 11.4%, respectively, and construction dust contributed the least. The average gap between contributions of identified sources to OC and EC and the ¹⁴C measurements decreased 2.5 ± 1.2% after the reapportionment than 13.2 ± 10.8%, indicating the good performance of the developed method.

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 PM2.5 (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).

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.