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Stringent emission controls during the Asia Pacific Economic Cooperation Summit (APEC; November 5–11, 2014) provide a valuable opportunity to examine the impact of such measures on the chemical properties of PM 2.5 and other air pollutants. Here, we measured the water-soluble inorganic ions (WSII) and carbonaceous species in PM 2.5 , NH 3 and NO 2 at multiple sites in Beijing between September and November 2014. Relative to the pre-APEC period (September and October 2014), significant reductions in the average concentrations of WSII (69% for NO 3 −, 68% for SO 4 2−, 78% for NH 4 +, and 29–71% for other species), elemental carbon (EC, 43%) and organic carbon (OC, 45%) in PM 2.5 were found during the APEC period. The contributions of secondary inorganic ions (SIA, including SO 4 2−, NO 3 −, and NH 4 +) to PM 2.5 were significantly lower during the APEC period (9–44%), indicating a combination of lower gaseous precursor emissions and a relative weak secondary aerosol formation. Ion-balance calculations indicated that the PM 2.5 sample in the pre-APEC period was alkaline but was acidic during the APEC period. Relatively lower mean concentrations of EC (1.5 μg m−3), OC (10.5 μg m−3), secondary organic carbon (SOC, 3.3 μg m−3), secondary organic aerosol (SOA, 5.9 μg m−3) and primary organic aerosol (POA, 10.0 μg m−3) appeared during the APEC period. The average concentrations of NH 3 and NO 2 at all road sites were significantly reduced by 48 and 60% during the APEC period, which is consistent with clear reductions in satellite NH 3 columns over Beijing city in the same period. This finding suggests that reducing traffic emissions could be a feasible method to control urban NH 3 pollution. During the APEC period, concentrations of PM 2. 5 , PM 10 , NO 2 , SO 2 and CO from the Beijing city monitoring network showed significant reductions at urban (20–60%) and rural (18–57%) sites, whereas O 3 concentrations increased significantly (by 93% and 53%, respectively). The control measures taken in the APEC period substantially decreased PM 2.5 pollution but can increase ground O 3 , which also merits attention. • Concentrations of PM 2.5 and components reduced significantly during the APEC period. • Major components in PM 2.5 and other pollutants were comparable with other cities. • Vehicle restrictions could lower atmospheric NH 3 concentration over Beijing. • Emission control measures significantly increased ground-level O 3 concentrations. [ABSTRACT FROM AUTHOR]

Paddy fields in China contributed to one third of the global cropland ammonia (NH 3) emission inventory, while rice accounted for half of cereal consumption, necessitating exhaustive considerations of the balance between NH 3 emissions abatement and food demand. The concept of yield-scaled emission intensity (emissions per unit crop production) has the potential to guide sustainable intensification strategies, yet its application to NH 3 emissions remains poorly understood. Here, by constructing novel crop-specific models for single rice production and NH 3 emissions in the Middle and Lower Yangtze River Basin (LYRB) as a case study, the relationships between fertilizer N application and yield-scaled NH 3 were estimated. Contrary to our hypothesis of a tipping point, our results showed that yield-scaled NH 3 curves could not directly identify optimal nitrogen (N) rates. However, the benefit of lower N fertilizer rate on NH 3 abatement consistently outweighed the risk of yield loss. The exponential relationships between yield-scaled NH 3 and N surplus allowed us to estimate the N surplus criterion as 15.6 kg N ha−1 (or 190 kg N ha−1 fertilizer N rate) for the LYRB. Under the N surplus criterion, NH 3 emissions can be reduced by 23–27% without severely impacting rice yield, compared to the N rate required for the highest yield. Moreover, five major controlling factors for yield-scaled NH 3 were estimated by random forest models, ranked in order of importance as N rate, total N, K rate, mean annual precipitation, and soil organic carbon. Among the agricultural practices (irrigation, tillage, and fertilizer management), deep placement was the most effective measure to reduce yield-scaled NH 3, showing 48% reduction potential, followed by proper N splitting frequency (43%). Overall, this study highlights the efficacy of N application optimization and targeted farm management in mitigating NH 3 emission while maintaining crop productivity. [Display omitted] • Yield-scaled NH 3 cannot directly guide N rate due to high abatement efficacy. • The Yield-scaled NH 3 –N surplus criterion reduces NH 3 emissions by 23–27%. • Five key factors contributing to spatial variations were unraveled. • Deep placement and optimal splitting frequency perform best in yield-scaled NH 3 mitigation. [ABSTRACT FROM AUTHOR]

The losses of excessive reactive nitrogen (N) from agricultural production pose detrimental impacts on water, air and land. However, N budgets of agroecosystems are still poorly quantified, presenting a barrier to understand the N turnover in agriculture. Agricultural ammonia (NH 3) volatilization has been recognized as a crucial contribution to the pollution of fine particulate matters over China through reacting with acid gases. Building on these challenges, the first national-scale model analysis was constructed on the N budgets to gain an overall insight into the current status of N flows in Chinese dryland systems towards sustainable N management. Total inputs of soil N in Chinese dryland soils were estimated at 121 kg N ha−1 in 2010, considering all pathways including N manure, fertilizer, atmospheric deposition and litter from crop residues. Atmospheric N deposition accounted for 25% of N fertilizer plus N manure in Chinese dryland soils, suggesting that N deposition could not be ignored when estimating total N inputs to Chinese dryland soils. The highest ratio of NH 3 volatilization to total N outputs was found at 43 kg N ha−1 (∼21%) in Northern China, followed by 41 kg N ha−1 (∼20%) in Sichuan Basin and 25 kg N ha−1 (∼26%) in Northeastern China. The modeling results indicated that, if a 20% decrease in N fertilizer plus N manure was achieved, it would lead to a 24% (7–49%) reduction in NH 3 volatilization. Substantial reductions of NH 3 volatilization would also be achieved by making an improvement in changing management practices (controlled release fertilizer and full irrigation). The results would give an overall insight into N budgets in Chinese dryland soils. The constructed N budgets assisted with understanding agricultural N flows and NH 3 pollution, and evaluated the impacts of human activities on N cycle towards a precise way to regulate agricultural management. Image 1 • N deposition accounted for 25% of N fertilizer in dryland soils. • NH 3 volatilizations were identified as the major pathways of soil N outputs. • NH 3 volatilizations can be reduced by appropriate fertilizer and irrigation. [ABSTRACT FROM AUTHOR]

Atmospheric reactive nitrogen (Nr) emissions and deposition have caused much damage to the global environment and also impacted human health. Determining long-term trends in atmospheric Nr concentrations and deposition is important for evaluating air quality and ecosystem effects and formulating control measures to reduce negative impacts. We measured monthly concentrations of NH 3 , pNH 4 +, HNO 3 , pNO 3 -, and NO 2 and calculated their dry deposition rates using an inferential method from 2011 to 2018 at an urban site in Zhengzhou in the North China Plain. Inorganic N (NH 4 + and NO 3 −) concentrations in precipitation and wet/bulk N deposition were also quantified in 2011, 2016, 2017 and 2018. The results showed high concentrations of atmospheric Nr species, with considerable variation in some species over the monitoring period. Annual mean NH 3 concentrations significantly increased from 2011 to 2018 (p < 0.001) with that in 2018 being twice that in 2011. However, pNO 3 - and pNH 4 + concentrations decreased after 2013. The annual mean NO 2 concentration in 2018 had decreased by 33.4% compared to the peak in 2011. Highest NH 3 concentrations were measured in spring or summer, and lowest concentrations in winter. pNH 4 + and pNO 3 - concentrations peaked mainly in winter. Total annual dry N deposition ranged from 21.9 to 41.2 kg N ha−1. Bulk NH 4 + and NO 3 − deposition was lower in 2016–2018 than in 2011. However, atmospheric Nr concentrations and N deposition remained high in this urban ecosystem. NH 3 emissions urgently need control measures, and Nr impacts on the surrounding ecosystems and human health should be considered. Continuous long-term measurements of atmospheric Nr concentrations and deposition at a regional scale are necessary to help formulate emission control measures and protect human and ecosystem health. • Significant changes in atmospheric reactive nitrogen were observed at an urban site in north China. • NH 3 concentrations increased by 110% from 2011 to 2018. • pNO 3 - and pNH 4 + concentrations in 2018 decreased by 65.1% and 62.8% compared to the maxima in 2012. • As a consequence, N deposition slightly decreased, with some improvement in air quality. [ABSTRACT FROM AUTHOR]

The North China Plain (NCP) is an intensive agricultural area and ammonia (NH 3) emission hotspot in China. Urgent mitigation actions are needed to reduce its environmental impact. A field trial was conducted, and results combined with a synthesis of existing data to assess the NH 3 loss and mitigation potentials in a wheat-maize double-crop rotation, a dominant crop production system in the NCP. Ammonia volatilization losses from N fertilizer applied to summer maize and winter wheat were 9.1% and 5.0%, respectively, based on the data synthesis, and were significantly affected by fertilizer application method and N fertilizer type. From the field trial, optimized reduction in N application rate, compared with conventional practice, had great efficiency for NH 3 mitigation. Despite their high NH 3 reduction potential, the higher input costs associated with the use of controlled-release fertilizers, replacement of urea with calcium ammonium nitrate or inclusion of double inhibitors (urease and nitrification inhibitors) with urea limited their applicability from an economic perspective. The inclusion of a urease inhibitor with urea achieved over 70% reduction in NH 3 emission and also gave a significant improvement in grain yield, net income and N use efficiency. Future adoption of NH 3 mitigation options in the NCP should focus on establishing a reduction target and consider the cost-benefits of simultaneous implementation of multiple mitigation techniques. [Display omitted] • Placement of fertilizer and fertilizer types significantly influence NH 3 emissions. • High cost of fertilizer amendment compromised economic benefits. • Using urease inhibitor showed good economic and environmental performance. [ABSTRACT FROM AUTHOR]

We present a national-scale model analysis on the sources and processes of inorganic nitrogen deposition over China using the GEOS-Chem model at 1/2° × 1/3° horizontal resolution. Model results for 2008–2012 are evaluated with an ensemble of surface measurements of wet deposition flux and gaseous ammonia (NH 3 ) concentration, and satellite measurements of tropospheric NO 2 columns. Annual total inorganic nitrogen deposition fluxes are simulated to be generally less than 10 kg N ha −1 a −1 in western China (less than 2 kg N ha −1 a −1 over Tibet), 15–50 kg N ha −1 a −1 in eastern China, and 16.4 kg N ha −1 a −1 averaged over China. Annual total deposition to China is 16.4 Tg N, with 10.2 Tg N (62%) from reduced nitrogen (NH x ) and 6.2 Tg N from oxidized nitrogen (NO y ). Domestic anthropogenic sources contribute 86% of the total deposition; foreign anthropogenic sources 7% and natural sources 7%. Annually 23% of domestically emitted NH 3 and 36% for NO x are exported outside the terrestrial land of China. We find that atmospheric nitrogen deposition is about half of the nitrogen input from fertilizer application (29.6 Tg N a −1 ), and is much higher than that from natural biological fixation (7.3 Tg N a −1 ) over China. A comparison of nitrogen deposition with critical load estimates for eutrophication indicates that about 15% of the land over China experiences critical load exceedances, demonstrating the necessity of nitrogen emission controls to avoid potential negative ecological effects. [ABSTRACT FROM AUTHOR]

Large uncertainties and challenges remain in the quantification of land-atmosphere exchanges of ammonia (NH 3), mainly due to limitations in measurement techniques. In this study, the eddy covariance (EC) technique based on an open-path quantum cascade laser (QCL) spectrometer was used to measure the NH 3 fluxes at a typical cropland in the North China Plain, one of the global NH 3 hotspots. It is the first attempt of measuring both NH 3 volatilization and deposition fluxes at an agricultural ecosystem using the open-path instrument-based EC technique. During the one-month experimental period, the NH 3 fluxes were characterized by enhanced emissions after fertilizer application and persistent dry depositions after ploughing practice, with the flux magnitude varying from −0.293 to 7.29 (median: −0.042) mg N m−2 h−1. Nearly 93% of the half-hourly fluxes (absolute values) exceeded the mean instrumental flux detection limit of 9.6 μg N m−2 h−1. Nitrogen loss rate from the wheat field through NH 3 volatilization was 0.57–0.71% relative to the applied nitrogen fertilizers. Ammonia deposition dominated the period after the ploughing practice, showing an average deposition rate of 0.014 kg N ha−1 d−1 with a significant diurnal pattern. Both the local emission sources and meteorological conditions influenced the variation of NH 3 dry deposition. The deposition fluxes measured by the EC technique were nearly one third the results modelled by the widely used inferential model method, but the difference needs to be validated by more observations that includes different surface and meteorological conditions. This study suggests that this open-path QCL-based EC instrument is an effective tool for long-term and high-frequency measurements of ecosystem NH 3 exchanges. It also enables cross-validation between different methods to improve the understanding in the atmospheric reactive nitrogen cycling. • The open-path QCL-based eddy covariance instrument can detect small NH 3 fluxes. • Nitrogen loss rate from the wheat field through NH 3 volatilization was 0.57–0.71%. • NH 3 dry deposition averaged to 0.014 kg N ha−1 d−1 with a significant diurnal pattern. • Depositions estimated by inferential model were two times larger than the observed. [ABSTRACT FROM AUTHOR]

Few related studies have been carried out for northwest China's the inland, although the increasing inorganic reactive nitrogen (Nr) deposition and severe air pollution in China have attracted considerable attention. Based on eight-year measurements of the concentrations of NH 3 , HNO 3 , particulate (p) NH 4 +, pNO 3 −, and NO 2 in the air and inorganic nitrogen (NH 4 +, NO 3 −) in the precipitation at a rural site in Wuwei, Gansu Province, we quantified the dry and bulk/wet Nr deposition fluxes and analyzed their temporal trends in combination with air pollution data (2015–2018). The concentrations of the single Nr species in the air did not exhibit significant seasonal variations, except for that of pNO 3 − which were the highest in summer and the lowest in winter. The average annual total Nr (dry + bulk) deposition was 27.2 ± 6.7 kg N ha−1 with a significant (p <.05) annual increase rate of 10%, which was dominated by the dry deposition flux (20.8 ± 7.1 kg N ha−1 yr−1) during the entire period. The NH 4 +-N and NO 3 −-N deposition fluxes in the precipitation were significantly (p <.05) higher in the summer than in the other three seasons owing to the heavy rainfall. The reduced N (NH 3 , pNH 4 +, and NH 4 +) deposition flux was 4.7 times that of the oxidized N (HNO 3 , pNO 3 −, NO 2 , and NO 3 −), which increased with a high annual rate (25%). The annual mean concentrations of the main air pollutants (PM 2.5 , PM 10 , SO 2 , NO 2 , and CO) significantly decreased by 15%–76%; in contrast, increasing trends of the O 3 and NH 3 concentrations were observed. Our results highlight the importance of the control of NH 3 emissions and O 3 pollution to comprehensively mitigate N deposition and improve air quality in the considered region. • The total nitrogen deposition increased at an annual rate of 10% from 2010 to 2018. • Nitrogen deposition has become an important nitrogen source to the agroecosystem. • Air pollutants significantly decreased by 15%–76%, except for O 3 and NH 3. • NH 3 concentrations increased at an annual rate of 25% from 2010 to 2018. • Control of NH 3 emission and O 3 pollution is imperative to improve air quality. [ABSTRACT FROM AUTHOR]

• Atmospheric N deposition in China was evaluated by two time-scale deposition databases. • Bulk and dry N deposition reached a peak between 2000 and 2010 and then began a steady decline. • Significant decrease of oxidized N deposition proved the effectiveness of NO x emissions control. • Increase of NH 3 dry deposition highlighted the importance of regional NH 3 mitigation. China has experienced a dramatic change in atmospheric reactive nitrogen (Nr) emissions over the past four decades. However, it remains unclear how nitrogen (N) deposition has responded to increases and/or decreases in Nr emissions. This study quantitatively assesses temporal and spatial variations in measurements of bulk and calculated dry N deposition in China from 1980 to 2018. A long-term database (1980–2018) shows that bulk N deposition peaked in around 2000, and had declined by 45% by 2016–2018. Recent bulk and dry N deposition (based on monitoring from 2011 to 2018) decreased from 2011 to 2018, with current average values of 19.4 ± 0.8 and 20.6 ± 0.4 kg N ha−1 yr−1, respectively. Oxidized N deposition, especially dry deposition, decreased after 2010 due to NO x emission controls. In contrast, reduced N deposition was approximately constant, with reductions in bulk NH 4 +-N deposition offset by a continuous increase in dry NH 3 deposition. Elevated NH 3 concentrations were found at nationwide monitoring sites even at urban sites, suggesting a strong influence of both agricultural and non-agricultural sources. Current emission controls are reducing Nr emissions and deposition but further mitigation measures are needed, especially of NH 3 , built on broader regional emission control strategies. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3), as an alkaline gas, contributes substantially to atmospheric nitrogen deposition, which can cause biodiversity loss, water eutrophication and soil acidification. Advances in the application of satellite observations allow us to gain deeper insights into atmospheric NH 3 concentrations at large spatial scales. A new satellite-based methodology is proposed for estimating dry NH 3 deposition with consideration of bi-directional NH 3 exchange. We estimate the global dry NH 3 deposition for nine years (2008–2016) by using the Infrared Atmospheric Sounding Interferometer Instrument (IASI) NH 3 retrievals. Satellite-based dry NH 3 deposition is in general consistent with measured dry NH 3 deposition over the monitoring sites (R2 = 0.65). Global dry NH 3 deposition over 8 kg N ha−1 is mainly distributed in the Eastern China, Northern and Central Pakistan, and Northern India. An annual increase rate of 0.27 and 0.13 kg N ha−1 y−1 in dry NH 3 deposition during 2008–2016 occurs in Eastern China and Sichuan Basin, which are the major Chinese agricultural regions. The NH 3 compensation point is high during warm months, and can be above 1 μg m−3 such as in Eastern China, implying the importance of considering the NH 3 compensation points for estimating dry NH 3 deposition. We find, if the upward NH 3 flux was ignored, it will cause 11%, 17%, 5% and 3% overestimation in dry NH 3 deposition in Eastern China, Northern India, Eastern United States and Western Europe, respectively. This study presents the potential of using the satellite retrievals to estimate the large-scale dry NH 3 deposition, and the methodology is able to provide temporally continuous and spatially complete fine-resolution datasets. Unlabelled Image • A new satellite-based method is proposed to estimate dry ammonia deposition. • An overestimation in dry ammonia deposition will be caused using unidirectional methods. • Satellite-based estimates capture the general variability of dry ammonia deposition. [ABSTRACT FROM AUTHOR]

Precipitation chemistry and atmospheric nitrogen (N) deposition are of great concern worldwide due to their close relationships with air quality and impacts on ecosystems. However, evaluation of the chemical composition of precipitation and N deposition flux in rural areas of Beijing has received little attention to date. This paper presents the chemical constituents, possible sources and wet deposition fluxes of water-soluble ions (NH 4 +, NO 3 −, SO 4 2−, Cl−, Na+, K+, Ca2+, Mg2+) in precipitation samples collected during 2017–2018 at a rural site located at northwest of Beijing city; meanwhile, dry deposition of reactive N species (gaseous ammonia, nitrogen dioxide, nitric acid, and particulate (p) NH 4 + and NO 3 −) were also quantified. During this 2-year period, the volume-weighted mean (VWM) pH of precipitation was 6.73, and all samples had pH values above 6.0. The VWM electric conductivity of precipitation and the mean sum of all measured ions was 43.8 μS cm−1 and 591.9 μeq L−1, respectively, indicating a significant impact of atmospheric pollution. Ca2+ and NH 4 + were the dominant neutralizing species for precipitation acidity. Positive matrix factorization analysis further confirmed five sources for water-soluble ions, including sea salt aging, secondary formation, agriculture, crust, and biomass burning. The annual mean wet N deposition was 4.6, 3.4 and 8.0 kg N ha−1 yr−1 for NH 4 +, NO 3 − and total inorganic N, respectively. The total dry N deposition was dominated by gaseous ammonia (11.5 kg N ha−1 yr−1). The total N deposition (wet plus dry) was 27.7 kg N ha−1 yr−1, where dry deposition contributed to 75% and wet deposition 25% of the total. The simulations from the GEOS-Chem model indicate that agricultural (fertilizer use and livestock) and nonagricultural sources (industry, power plant, and transportation) are both important contributors to total N deposition. These results could be useful in evaluating/developing emission control policies to protect the eco-environment in Beijing. • Precipitation chemistry and nitrogen (N) deposition were investigated in Beijing. • The pH values of precipitation samples were all above 6.0 and averaged 6.7. • Ion source was sea salt, secondary source, agriculture, crust, and biomass burning. • Total N deposition was 27.7 kg N ha−1 yr−1, of which 75% was from dry deposition. • Agricultural and nonagricultural sources contributed equally to total N deposition. [ABSTRACT FROM AUTHOR]