Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from (15)N-Stable Isotope in Size-Resolved Aerosol Ammonium

Decreased nitrogen deposition in Beijing over the recent decade and its implications

Atmospheric reactive nitrogen (Nr) deposition has been modified significantly by human activities such as agriculture and fossil fuel combustion. Understanding the changes in Nr deposition is essential for maintaining the functionality and sustainability of ecosystems. Taking Beijing as a case study, we report long-term measurements of wet Nr deposition from 1999 to 2022 and dry Nr deposition from 2010 to 2022 and their relationship with China's air pollution control. Total Nr deposition to Beijing decreased by 34 % during 2010-2022, mainly caused by a decrease in dry N deposition by 54.27 %, from 47.86 kg N ha-1 yr-1 in 2010-2014 to 21.89 kg N ha-1 yr-1 in 2018-2022; reduced and oxidized N in dry deposition decreased by 29.93 % and 72.05 %, respectively. This was a result of the "Action Plan for Prevention and Control of Air Pollution (APCP)" and the implementation of the "Zero Growth in Fertilizer Use by 2020" in 2015. Our ground-based measurements provide evidence to support recent achievements in air pollution control and a reference and guidance for other regions of China and other countries for abating Nr pollution.

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+.

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

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

Strategic control of combustion-induced ammonia emissions: A key initiative for substantial PM2.5 reduction in Tianjin, North China Plain

Information on the temporal and spatial variations in the sources of ammonium salts (NH4+), a crucial alkaline component in PM2.5, is limited. Here, we simultaneously collected PM2.5 and gaseous ammonia (NH3) samples in both summer and winter from two sites in Tianjin: an urban site (Tianjin University, TJU) and a suburban site (Binhai New-region, BH). NH3 concentrations, the contents of major water-soluble inorganic ions in PM2.5, and the compositions of ammonium‑nitrogen isotopes (δ15N-NH4+) were measured. As a result, (NH4)2SO4 and NH4NO3 were the predominant forms of NH4+ in PM2.5 during summer and winter, respectively. However, the NH4NO3 concentrations were notably greater at TJU (6.2 ± 7.3 μg m-3) than at BH (3.8 ± 4.7 μg m-3) in summer, with no regional differences observed in winter. Both sites displayed almost half the contribution of c-NH3 (combustion-related NH3) to NH4+, differing from the finding of previous isotope-based studies. This discrepancy could be attributed to the combined effects of NHx isotope fractionation and seasonal δ15N value variations in NH3 sources. The contribution fractions of v-NH3 (volatile NH3) and c-NH3 exhibited similar patterns at both sites seasonally, probably caused by coal combustion for heating in winter and temperature fluctuations. However, the contribution fraction of c-NH3 was lower at BH than at TJU in summer but greater in winter than at TJU. In summer, NH4NO3 was unstable and limited its delivery to TJU from BH, and the high contribution of c-NH3 to NH4+ at TJU could be attributed to local vehicle emissions. In winter, the stable particulate NH4NO3 that formed from the c-NH3 in the upwind area could be transported to the downwind area, increasing the NH4+ concentration at BH. Our study provides valuable insights for devising emission mitigation strategies to alleviate the increasing burden of NH3 in the local atmosphere.

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.

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

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

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.

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.

Isotopic characteristics and source analysis of atmospheric ammonia during agricultural periods in the Xichuan area of the Danjiangkou Reservoir

Nitrogen deposition is an important means of exogenous nitrogen input in reservoir water. Agricultural activities around the reservoir lead to a sharp increase in the concentration of ammonia in the atmosphere, which poses a threat to the reservoir water body. Clarifying the contribution of agricultural ammonia release to atmospheric NHx (gaseous NH3 and particulate NH4+), in the reservoir area can provide a theoretical foundation for local reactive nitrogen control. We collected atmospheric NH3 and NH4+ samples during the agricultural periods and analyzed the isotopic characteristics of atmospheric NHx and the contribution rates of different ammonia sources in the Xichuan area of the Danjiangkou Reservoir. The results showed that the initial δ15N values of NH3 (-30.0‰ to -7.2‰) and particulate NH4+(-33‰ to +4.9‰ for finer and coarser particles, respectively) are different, and their contribution ratios from dissimilar ammonia sources are also different, among which NH4+ is more susceptible to meteorological factors. However, since the atmospheric NHx in the Xichuan area is mainly gaseous NH3, the final sources of atmospheric ammonia nitrogen source depend on gaseous NH3. Agricultural sources (59%-74%) were the main NH3 sources in this area. Among them, the fertilizer use emission was dominant; it had the highest contribution rate in summer during the agricultural period and a more prominent impact in areas with less human interference. Reasonable regulation of the application of high-ammonia releasing fertilizer, especially during the agricultural period in summer, is an effective way to reduce the threat of atmospheric ammonia to water health.

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.

Influence of Oxygen Vacancy-Induced Coordination Change on Pd/CeO2 for NO Reduction

The byproduct formation in environmental catalysis is strongly influenced by the chemical state and coordination of catalysts. Herein, two Pd/CeO2 catalysts (PdCe-350 and PdCe-800) with varying oxygen vacancies (Ov) and coordination numbers (CN) of Pd were prepared to investigate the mechanism of N2O and NH3 formation during NO reduction by CO. PdCe-350 exhibits a higher density of Ov and Pd sites with higher CN, leading to an enhanced metal-support interaction by electron transformation from the support to Pd. Consequently, PdCe-350 displayed increased levels of byproduct formation. In situ spectroscopies under dry and wet conditions revealed that at low temperatures, the N2O formation strongly correlated with the Ov density through the decomposition of chelating nitro species on PdCe-350. Conversely, at high temperatures, it was linked to the reactivity of Pd species, primarily facilitated by monodentate nitrates on PdCe-800. In terms of NH3 formation, its occurrence was closely associated with the activation of H2O and C3H6, since a water-gas shift or hydrocarbon reforming could provide hydrogen. Both bridging and monodentate nitrates showed activity in NH3 formation, while hyponitrites were identified as key intermediates for both catalysts. The insights provide a fundamental understanding of the intricate relationship among the local coordination of Pd, surface Ov, and byproduct distribution.

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.

Achievements and challenges in improving air quality in China: Analysis of the long-term trends from 2014 to 2022

Due to the implementation of air pollution control measures in China, air quality has significantly improved, although there are still additional issues to be addressed. This study used the long-term trends of air pollutants to discuss the achievements and challenges in further improving air quality in China. The Kolmogorov-Zurbenko (KZ) filter and multiple-linear regression (MLR) were used to quantify the meteorology-related and emission-related trends of air pollutants from 2014 to 2022 in China. The KZ filter analysis showed that PM2.5 decreased by 7.36 ± 2.92% yr-1, while daily maximum 8-h ozone (MDA8 O3) showed an increasing trend with 3.71 ± 2.89% yr-1 in China. The decrease in PM2.5 and increase in MDA8 O3 were primarily attributed to changes in emission, with the relative contribution of 85.8% and 86.0%, respectively. Meteorology variations, including increased ambient temperature, boundary layer height, and reduced relative humidity, also contributed to the reduction of PM2.5 and the enhancement of MDA8 O3. The emission-related trends of PM2.5 and MDA8 O3 exhibited continuous decrease and increase, respectively, from 2014 to 2022, while the variation rates slowed during 2018-2020 compared to that during 2014-2017, highlighting the challenges in further improving air quality, particularly in simultaneously reducing PM2.5 and O3. This study recommends reducing NH3 emissions from the agriculture sector in rural areas and transport emissions in urban areas to further decrease PM2.5 levels. Addressing O3 pollution requires the reduction of O3 precursor gases based on site-specific atmospheric chemistry considerations.

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.

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.

Nitrogen isotopes suggest agricultural and non-agricultural sources contribute equally to NH3 and NH4+ in urban Beijing during December 2018

Agricultural and non-agricultural sources emission contribute to atmospheric ammonia (NH₃) and particulate ammonium (NH₄⁺). However, our understanding on the sources of NH₃ and NH₄⁺ in PM_2.5 (particles smaller than 2.5 μm) during the winter period in the urban atmosphere is limited. Here, we measured the concentrations and stable nitrogen isotopic composition (δ¹⁵N) of NH₃ and NH₄⁺ in parallel during December 2018 in urban Beijing to assess the non-agricultural and agricultural sources contributions to NH₃ and NH₄⁺ in ambient air based on the Chemical Transport Model (CTM), a Bayesian isotope mixing model (SIMMR), and the δ¹⁵N signatures that we developed. Our study found weekly NH₄⁺ and NH₃ concentrations were on average 2.5 ± 1.4 μg m^-3 and 3.8 ± 1.7 μg m^-3, respectively during December 2018. Weekly concentration weighted δ¹⁵N(NH₄⁺) values ranged from 4.5‰ to 13.7‰ with an average value of 8.2 ± 3.9‰ during December 2018. After accounting for nitrogen isotopic fractionation from NH₃ gas to NH₄⁺ conversion, initial δ¹⁵N(NH₃) values ranged from -22.5‰ to -12.8‰ with an average value of -17.4 ± 3.5‰. Further, weekly measured δ¹⁵N(NH₃) values ranged from -22.2‰ to -10.2‰ (after correction) with an average value of -15.6 ± 5.3‰ during December 2018. Results from two different isotope-based method showed non-agricultural sources contributed 31.2%-63.1%, with an average value of 47.5 ± 14.6%, to NH₄⁺ and 32.3%-71.2%, with an average of 53.4 ± 16.1%, to ambient NH₃ during December 2018 in Beijing. Results from CMAQ-ISAM suggest non-agricultural sources contributed on average 66.2 ± 1.9% to ambient NH₄⁺ and 66.4 ± 1.9% to ambient NH₃ during December 2018. Results from this study suggest that agricultural and non-agricultural sources nearly equally contributed to NH₃ and NH₄⁺ in urban Beijing during December 2018 with an uncertainty range of 13%-19% between isotope-based methods and CTM method.

Atmospheric ammonia in China: Long-term spatiotemporal variation, urban-rural gradient, and influencing factors

In recent years, atmospheric ammonia (NH₃) concentrations have increased in China. Ammonia control has become one of the next hot topics in air pollution mitigation with the increasing cost of acid gas emission reduction. In this study, using Infrared Atmospheric Sounding Interferometer (IASI) satellite observations, we analyzed the spatiotemporal distribution, the urban-rural gradient of the vertical column densities (VCDs) of NH₃ and the contribution of influencing factors (meteorology, social, atmospheric acid gases, and NH₃ emissions) in China from 2008 to 2019 using hotspot analysis, circular gradient analysis, geographical and temporal weighted regression, and some other methods. Our results showed that NH₃ VCDs in China have significantly increased (31.88 %) from 2008 to 2019, with the highest occurring in North China Plain. The average NH₃ VCDs in urban areas were significantly higher than those in rural areas, and the urban-rural gap in NH₃ VCDs was widening. The results of circular gradient analysis showed an overall decreasing trend in NH₃ VCDs along the urban-rural gradient. We used a geographically and temporally weighted regression model to analyze the contribution of various influencing factors to NH₃ VCDs: meteorology (30.13 %), social (27.40 %), atmospheric acid gases (23.20 %), and NH₃ emissions (19.28 %) factors. The results showed substantial spatiotemporal differences in the influencing factors. Atmospheric acid gas was the main reason for the increase in NH₃ VCDs from 2008 to 2019. A more thorough understanding of the spatiotemporal distribution, urban-rural variations, and factors influencing NH₃ in China will aid in developing control strategies to reduce PM_2.5.

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.

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.

Contributions of various driving factors to air pollution events: Interpretability analysis from Machine learning perspective

The air quality in China has been improved substantially, however fine particulate matter (PM_2.5) still remain at a high level in many areas. PM_2.5 pollution is a complex process that is attributed to gaseous precursors, chemical, and meteorological factors. Quantifying the contribution of each variable to air pollution can facilitate the formulation of effective policies to precisely eliminate air pollution. In this study, we first used decision plot to map out the decision process of the Random Forest (RF) model for a single hourly data set and constructed a framework for analyzing the causes of air pollution using multiple interpretable methods. Permutation importance was used to qualitatively analyze the effect of each variable on PM_2.5 concentrations. The sensitivity of secondary inorganic aerosols (SIA): SO₄^2-, NO₃^- and NH₄⁺ to PM_2.5 was verified by Partial dependence plot (PDP). Shapley Additive Explanation (Shapley) was used to quantify the contribution of drivers behind the ten air pollution events. The RF model can accurately predict PM_2.5 concentrations, with determination coefficient (R²) of 0.94, root mean square error (RMSE) and mean absolute error (MAE) of 9.4 μg/m³ and 5.7 μg/m³, respectively. This study revealed that the order of sensitivity of SIA to PM_2.5 was NH₄⁺＞NO₃^-＞SO₄^2-. Fossil fuel and biomass combustion may be contributing factors to air pollution events in Zibo in 2021 autumn-winter. NH₄⁺ contributed 19.9-65.4 μg/m³ among ten air pollution events (APs). K, NO₃^-, EC and OC were the other main drivers, contributing 8.7 ± 2.7 μg/m³, 6.8 ± 7.5 μg/m³, 3.6 ± 5.8 μg/m³ and 2.5 ± 2.0 μg/m³, respectively. Lower temperature and higher humidity were vital factors that promoted the formation of NO₃^-. Our study may provide a methodological framework for precise air pollution management.

Distribution, source identification and ecological effects of aerosol dissolved nutrients in the Bohai Bay

The atmospheric aerosols around the Bohai Bay are affected intensively by the surrounding industrial, shipping and other human activities. Although atmospheric dry deposition is an important way for nutrients to enter the Bohai Bay, few studies explore the distribution patterns, source and deposition fluxes of typical nutrients in aerosols and their impacts on the marine ecosystem. This paper explored the spatial-temporal distribution of typical aerosol nutrients in summer and autumn, and their source and ecological effects were illustrated further. The mean concentration of dissolved total phosphorus (DTP), dissolved total nitrogen (DTN), dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN), ammonium (NH₄-N), nitrate (NO₃-N), nitrite (NO₂-N), silicate (SiO₃-Si), phosphate (PO₄-P), and dissolved organic phosphorus (DOP) were 31.22, 847.22, 288.19, 559.77, 288.19, 304.00, 253.65, 2.12, 15.74 and 15.48 nmol/m³, respectively, while their fluxes were corresponding to 0.61, 8.36, 2.52, 4.90, 1.41, 2.49, 0.02, 0.04, 0.19 and 0.26 mmol/(m² month). Typical aerosol nutrient concentrations in autumn were mostly higher than those in summer, with high values occurring mainly in the central region. The potential sources of pollution were mainly concentrated in Shandong and Mongolia, and the sources of pollution were mainly agriculture, dust and industry. The large N:P and N:Si ratios in the dry deposition likely exacerbated Si and P limitation in the water column. These results provided the data basis for evaluating the pollution status and revealed that the dry deposition of aerosol nutrients should not be neglected by the ecological environment in the Bohai Bay.

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.

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.

Urgency of controlling agricultural nitrogen sources to alleviate summertime air pollution in the North China Plain

Agricultural nitrogen sources (ANS) have played an increasingly important role in the air quality since ANS emission controls are much weaker than those for fossil fuel combustion sources due to the increasing food demand. However, ANS emissions are highly uncertain due to stochastic agricultural management activities and limited field measurements, and impacts of ANS on the air quality remain elusive. In the study, the WRF-Chem model has been used to investigate ANS shares in near surface air pollutant concentrations during a growing season in the North China Plain (NCP), with ANS emissions constrained by satellite retrievals. Soil NOX and agricultural NH3 emissions are about 36% and 92% of their total emissions during the growing season. Sensitivity studies demonstrate that ANS count 16.9 μg m-3 (9.9%) of the mean maximum daily average 8-h ozone concentrations (MDA8 [O3]) and 8.9 μg m-3 (31.7%) of fine particulate matter concentrations ([PM2.5]) on average in the NCP. Additionally, the contributions of ANS to MDA8 [O3] and [PM2.5] increase with the deterioration of air pollution in cities. A 50% emission reduction in ANS decreases MDA8 [O3] ([PM2.5]) from 4.2% to 8.4% (from 19.7% to 31.9%) when the air quality changes from being lightly to heavily polluted in terms of MDA8 [O3] (hourly [PM2.5]). Without fossil fuel combustion emissions, the simulated average MDA8 [O3] and [PM2.5] are 111.7 and 8.2 μg m-3 in cities of the NCP, respectively, exceeding the new standards from the World Health Organization. Our study highlights important contributions of ANS to air quality and the urgency of ANS emission abatement for air pollution alleviation during summertime in the NCP.

Observation and Source Apportionment of Atmospheric Alkaline Gases in Urban Beijing

Alkaline gases, including NH₃, C_1-3-amines, C_1-3-amides, and C_1-3-imines, were measured in situ using a water cluster-CIMS in urban Beijing during the wintertime of 2018, with a campaign average of 2.8 ± 2.0 ppbv, 5.2 ± 4.3, 101.1 ± 94.5, and 5.2 ± 5.4 pptv, respectively. Source apportionment analysis constrained by emission profiles of in-use motor vehicles was performed using a SoFi-PMF software package, and five emission sources were identified as gasoline-powered vehicles (GV), diesel-powered vehicles (DV), septic system emission (SS), soil emission (SE), and combustion-related sources (CS). SS was the dominant NH₃ source (60.0%), followed by DV (18.6%), SE (13.1%), CS (4.3%), and GV (4.0%). GV and DV were responsible for 69.9 and 85.2% of C₁- and C₂-amines emissions, respectively. Most of the C₃-amines were emitted from nonmotor vehicular sources (SS = 61.3%; SE = 17.8%; CS = 9.1%). DV accounted for 71.9 and 34.1% of C₁- and C₂-amides emissions, respectively. CS was mainly comprised of amides and imines, likely originating from the pyrolysis of nitrogen-containing compounds. Our results suggested that motor vehicle exhausts can not only contribute to criteria air pollutants emission but also promote new particle formation, which has not been well recognized and considered in current regulations. Urban residential septic system was the predominant contributor to background NH₃. Enhanced NH₃ emissions from soil and combustion-related sources were the major cause of PM_2.5 buildup during the haze events. Combustion-related sources, together with motor vehicles, were responsible for most of the observed amides and imines and may be of public health concern within the vicinity of these sources.

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.

Inter-comparison of chemical fingerprint and source apportionment of marine fine particles at two islands through the west and east passages of the Taiwan Island

In this study, the concentrations of marine fine particles (PM_2.5) and their chemical fingerprints were inter-compared at two islands located aside from the west and east waters of Taiwan Island and the variability of west and east passages (i.e., Routes A1 and A2) were explored. Marine PM_2.5 was simultaneously sampled at the Green and Dongsha Islands and five chemical components (i.e., water-soluble ions, metallic elements, carbonaceous content, anhydrosugars, and organic acids) were further analyzed in PM_2.5 to characterize their chemical fingerprints. The highest concentrations of chemical composition and PM_2.5 were commonly observed during the Asian Northeastern Monsoons (ANMs) via long-range transport (LRT). Water-soluble ions (WSIs) were dominated by secondary inorganic aerosols (SIAs), and followed by oceanic spray. The major metallic content of PM_2.5 was crustal elements, while trace metals originated from anthropogenic sources with an enrichment factor (EF) > 10. In terms of carbonaceous content in PM_2.5, organic carbon (OC) was superior to elemental carbon (EC). High levoglucosan concentrations were also observed during the periods of ANMs. Secondary organic aerosols (SOAs) were formed by atmospheric chemical reactions during the LRT procedure. The PM_2.5 concentration of Route A1 was 37.51 % higher than that of Route A2, and trace metals (V, Mn, Ni, Pb, Cr, and Cu) increased significantly by 96.16-325.83 %. Positive matrix factorization (PMF) results revealed that the dominant factor of PM_2.5 for Route A1 was shipping emissions and vehicular exhausts (41.2 %), while that for Route A2 was oceanic spray (30.2 %). Route A1 was mainly attributed to highly industrialized regions, densely populated urbanized areas, and ship-intensive traffics in East Asia.

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.

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.

Ammonia Abatement via Selective Oxidation over Electron-Deficient Copper Catalysts

Selective catalytic ammonia-to-dinitrogen oxidation (NH₃-SCO) is highly promising for the abatement of NH₃ emissions from flue gas purification devices. However, there is still a lack of high-performance and cost-effective NH₃-SCO catalysts for real applications. Here, highly dispersed, electron-deficient Cu-based catalysts were fabricated using nitrogen-doped carbon nanotubes (NCNT) as support. In NH₃-SCO catalysis, the Cu/NCNT outperformed Cu supported on N-free CNTs (Cu/OCNT) and on other types of supports (i.e., activated carbon, Al₂O₃, and zeolite) in terms of activity, selectivity to the desired product N₂, and H₂O resistance. Besides, Cu/NCNT demonstrated a better structural stability against oxidation and a higher NH₃ storage capacity (in the presence of H₂O vapor) than Cu/OCNT. Quasi in situ X-ray photoelectron spectroscopy revealed that the surface N species facilitated electron transfer from Cu to the NCNT support, resulting in electron-deficient Cu catalysts with superior redox properties, which are essential for NH₃-SCO catalysis. By temperature-programmed surface reaction studies and systematic kinetic measurements, we unveiled that the NH₃-SCO reaction over Cu/NCNT proceeded via the internal selective catalytic reaction (i-SCR) route; i.e., NH₃ was oxidized first to NO, which then reacted with NH₃ and O₂ to form N₂ and H₂O. This study paves a new route for the design of highly active, H₂O-tolerant, and low-cost Cu catalysts for the abatement of slip NH₃ from stationary emissions via selective oxidation to N₂.

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.

Ammonium Chloride Associated Aerosol Liquid Water Enhances Haze in Delhi, India

The interaction between water vapor and atmospheric aerosol leads to enhancement in aerosol water content, which facilitates haze development, but its concentrations, sources, and impacts remain largely unknown in polluted urban environments. Here, we show that the Indian capital, Delhi, which tops the list of polluted capital cities, also experiences the highest aerosol water yet reported worldwide. This high aerosol water promotes secondary formation of aerosols and worsens air pollution. We report that severe pollution events are commonly associated with high aerosol water which enhances light scattering and reduces visibility by 70%. Strong light scattering also suppresses the boundary layer height on winter mornings in Delhi, inhibiting dispersal of pollutants and further exacerbating morning pollution peaks. We provide evidence that ammonium chloride is the largest contributor to aerosol water in Delhi, making up 40% on average, and we highlight that regulation of chlorine-containing precursors should be considered in mitigation strategies.

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.

Unprecedented levels of ultrafine particles, major sources, and the hydrological cycle

Ultrafine particles (UFP) acting as cloud condensation nuclei (CCN) are the driving force behind changing rainfall patterns. Recently observed weather extremes like floods and drought might be due to changing anthropogenic UFP emissions. However, the sources and budgets of anthropogenic primary and secondary particles are not well known. Based on airborne measurements we identified as a major contribution modern fossil fuel flue gas cleaning techniques to cause a doubling of global primary UFP number emissions. The subsequent enhancement of CCN numbers has several side effects. It’s changing the size of the cloud droplets and delays raindrop formation, suppressing certain types of rainfall and increasing the residence time of water vapour in the atmosphere. This additional latent energy reservoir is directly available for invigoration of rainfall extremes. Additionally it’s a further contribution to the column density of water vapour as a greenhouse gas and important for the infrared radiation budget. The localized but ubiquitous fossil fuel related UFP emissions and their role in the hydrological cycle, may thus contribute to regional or continental climate trends, such as increasing drought and flooding, observed within recent decades.

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.

Vehicular Emissions Enhanced Ammonia Concentrations in Winter Mornings: Insights from Diurnal Nitrogen Isotopic Signatures

A general feature in the diurnal cycle of atmospheric ammonia (NH₃) concentrations is a morning spike that typically occurs around 07:00 to 10:00 (LST). Current hypotheses to explain this morning's NH₃ increase remain elusive, and there is still no consensus whether traffic emissions are among the major sources of urban NH₃. Here, we confirmed that the NH₃ morning pulse in urban Beijing is a universal feature, with an annual occurrence frequency of 73.0% and a rapid growth rate (>20%) in winter. The stable nitrogen isotopic composition of NH₃ (δ¹⁵N-NH₃) in winter also exhibited a significant diurnal variation with an obvious morning peak at 07:00 to 10:00 (-18.6‰, mass-weighted mean), higher than other times of the day (-26.3‰). This diurnal pattern suggests that a large fraction of NH₃ in the morning originated from nonagricultural sources, for example, power plants, vehicles, and coal combustion that tend to have higher δ¹⁵N-NH₃ emission signatures relative to agricultural emissions. In particular, the contribution from vehicular emissions increased from 18% (00:00 to 07:00) to 40% (07:00 to 10:00), while the contribution of fertilizer sources to NH₃ was reduced from 15.8% at 00:00 to 07:00 to 5.2% at 07:00 to 10:00. We concluded that NH₃ concentrations in winter mornings in urban Beijing were indeed enhanced by vehicle emissions, which should be considered in air pollution regulations.

Physicochemical and isotopic properties of ambient aerosols and precipitation particles during winter in Seoul, South Korea

The aim of this study was to characterize the physicochemical properties and microbial communities of particulate matter (PM) in Seoul, Korea. We collected long-term (2017-2019) precipitation samples and PM₁₀ and PM_2.5 monitoring data to determine the impact of soluble and insoluble chemical species on the soil surface. Ambient PM₁₀ concentrations were higher than PM_2.5 concentrations during the monitoring period, but both decreased during rainfall due to the washing effect of precipitation. PM_2.5 particles had a "fluffy" shape and contained sulfur (0.2%), but suspended particles (SPs) contained many carbon particles (approximately 60%). Spherical particles containing metal oxides, Fe and Al, might be originated from coal combustion, wild fires, and metal-refining processes under high-temperature conditions. Dissolved ions in precipitation included those eluted from salts and coal combustion based on the correlation coefficients of Na and Cl (R = 0.953) and F and NO₃ (R = 0.706). The δ¹⁵N-NO₃ and δ³⁴S-SO₄ of precipitation were enriched as the atmospheric temperature decreased from 9.8 to -1.6°C, implying the influence of domestic coal combustion. Backward trajectories showed that, in winter, air parcels passed through industrialized cities from China to South Korea. The microbial communities associated with PM were strongly influenced by atmospheric conditions. Proteobacteria (range from 4.6 to 76.7%) and Firmicutes (range from 6.0 to 91.4%) were the most dominant phyla and were significantly affected by changes in the PM_2.5 environment. The results indicate that the acidity of precipitation and the composition of aerosols were affected by fossil fuel combustion and mineral dust, and that atmospheric conditions may change as PM_2.5 concentrations increase.

Quantifying the Influence of a Burn Event on Ammonia Concentrations Using a Machine-Learning Technique

Although combustion is considered a common source of ammonia (NH3) in the atmosphere, field measurements quantifying such emissions of NH3 are still lacking. In this study, online measurements of NH3 were performed by a cavity ring-down spectrometer, in the cold season at a rural site in Xianghe on the North China Plain. We found that the NH3 concentrations were mostly below 65 ppb during the study period. However, from 18 to 21 November 2017, a close burn event (~100 m) increased the NH3 concentrations to 145.6 ± 139.9 ppb. Using a machine-learning technique, we quantified that this burn event caused a significant increase in NH3 concentrations by 411%, compared with the scenario without the burn event. In addition, the ratio of ∆NH3/∆CO during the burn period was 0.016, which fell in the range of biomass burning. Future investigations are needed to evaluate the impacts of the NH3 combustion sources on air quality, ecosystems, and climate in the context of increasing burn events worldwide.

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

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

Atmospheric ammonia and its effect on PM2.5 pollution in urban Chengdu, Sichuan Basin, China

Controlling ammonia (NH₃) emissions has been proposed as a strategy to mitigate haze pollution. To explore the role of NH₃ in haze pollution in Sichuan Basin, where agricultural activities are intense, hourly in situ data of NH₃, as well as nitric acid and secondary inorganic aerosols (SIAs) were gathered in Chengdu from April 2017 to March 2018. We found that NH₃ had an annual mean concentration of 9.7 ± 3.5 (mean ± standard deviation) μg m^-3, and exhibited seasonal variations (spring > summer > autumn and winter) due to changes in emission sources and meteorological conditions (particularly temperature). Chengdu's atmosphere is generally NH₃-sufficient, especially in the warm seasons, implying that the formation of SIAs is more sensitive to the availability of nitric acid. However, an NH₃ "sufficient-to-deficient" transition was found to occur during winter pollution periods, and the frequency of NH₃ deficiency increased with the aggravation of pollution. Under NH₃-deficient conditions, the nitrogen oxidation ratio increased linearly with the increase in free NH₃, implying that NH₃ contributes appreciably to the formation of nitrate and thus to high PM_2.5 loadings. No relationships of NH₃ with fossil fuel combustion-related pollutants were found. The NH₃ emissions from farmland and livestock waste in the suburbs of Chengdu and regional transport from west of Chengdu probably contribute to the occurrence of high PM_2.5 loading in winter and spring, respectively. These results suggest that to achieve effective mitigation of PM_2.5 in Chengdu, local and regional emission control of NH₃ and NOx synergistically would be effective.