A database of atmospheric nitrogen concentration and deposition from the nationwide monitoring network in China

Substantial nitrogen abatement accompanying decarbonization suppresses terrestrial carbon sinks in China

China faces challenges in reaching its carbon neutrality goal by the year 2060 to meet the Paris Agreement and improving air quality simultaneously. Dramatic nitrogen emission reductions will be brought by this ambitious target, yet their impact on the natural ecosystem is not clear. Here, by combining two atmospheric chemistry models and two process-based terrestrial ecosystem models constrained using nationwide measurements, we show that atmospheric nitrogen deposition in China's terrestrial land will decrease by 44-57% following two emission control scenarios including one aiming at carbon neutrality. They consequently result in a pronounced shrinkage in terrestrial net ecosystem production, by 11-20% depending on models and emission scenarios. Our results indicate that the nitrogen emission reductions accompanying decarbonization would undermine natural carbon sinks and in turn set back progress toward carbon neutrality. This unintended impact calls for great concern about the trade-offs between nitrogen management and carbon neutrality.

Does Bidens pilosa L. Affect Carbon and Nitrogen Contents, Enzymatic Activities, and Bacterial Communities in Soil Treated with Different Forms of Nitrogen Deposition?

The deposition of nitrogen in soil may be influenced by the presence of different nitrogen components, which may affect the accessibility of soil nitrogen and invasive plant-soil microbe interactions. This, in turn, may alter the success of invasive plants. This study aimed to clarify the influences of the invasive plant Bidens pilosa L. on the physicochemical properties, carbon and nitrogen contents, enzymatic activities, and bacterial communities in soil in comparison to the native plant Pterocypsela laciniata (Houtt.) Shih treated with simulated nitrogen deposition at 5 g nitrogen m-2 yr-1 in four forms (nitrate, ammonium, urea, and mixed nitrogen). Monocultural B. pilosa resulted in a notable increase in soil pH but a substantial decrease in the moisture, electrical conductivity, ammonium content, and the activities of polyphenol oxidase, β-xylosidase, FDA hydrolase, and sucrase in soil in comparison to the control. Co-cultivating B. pilosa and P. laciniata resulted in a notable increase in total soil organic carbon content in comparison to the control. Monocultural B. pilosa resulted in a notable decrease in soil bacterial alpha diversity in comparison to monocultural P. laciniata. Soil FDA hydrolase activity and soil bacterial alpha diversity, especially the indices of Shannon's diversity, Simpson's dominance, and Pielou's evenness, exhibited a notable decline under co-cultivated B. pilosa and P. laciniata treated with nitrate in comparison to those treated with ammonium, urea, and mixed nitrogen.

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.

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.

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

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

Spatial variation in actual and required nitrogen use efficiency and the potential to close the gap by management practices

To boost crop production, China uses almost a third of the world's nitrogen (N) fertilizer. However, N losses due to enhanced application of N fertilizers has led to surface water and groundwater pollution. A reduction in N losses without reducing crop yields is possible by increasing nitrogen use efficiency (NUE), which is important for the effective management of local crop production and water quality. This study used two representative agricultural counties in China (Quzhou and Qiyang) to assess if it is possible to achieve N loss thresholds in surface and groundwater by optimizing N management measures while maintaining actual crop production. We used a spatially explicit N balance model to assess the spatial variation in actual N inputs to soil and N losses to water, and in critical N losses and associated agricultural N inputs. We also used this model to calculate the spatial variation in actual NUEs and the required NUE to align actual crop production with N thresholds. We then assessed the feasibility of achieving the necessary NUE changes through optimizing agricultural N management strategies. It was found that actual N input exceeded critical N input in 95 and 83 % of the agricultural area in Quzhou and Qiyang, respectively. To meet actual crop production without exceeding N loss thresholds, the NUE needs to increase with 11 to 15 % whereas the total N input needs to be reduced by 37 %. NUE gaps can be closed by reducing N rates, enhancing organic manure recycling, and using efficiency-enhancing fertilizers, with optimal combinations being dependent on site conditions.

A database of atmospheric inorganic nitrogen deposition fluxes in China from satellite monitoring

Over the past century, atmospheric inorganic nitrogen (IN) deposition to terrestrial ecosystems has significantly increased and caused various environmental issues. China has been one of the hotspot regions for IN deposition, yet limited data exist regarding IN deposition fluxes in China at the regional scale. In this study, based on NO2 and NH3 columns acquired by satellite sensors, coupled with atmospheric chemical transport model (CTM), mixed-effects model and site observations, we constructed regional-scale IN dry and wet deposition models respectively, and finally proposed a spatially explicit database of IN deposition fluxes in China. The database includes the dry, wet and total deposition fluxes in China during 2011-2020, and the data are presented in raster form with a resolution of 0.25° × 0.25°. Overall, the database is of great importance for monitoring and simulating the trends of IN deposition over a long time series in China.

Global food loss and waste embodies unrecognized harms to air quality and biodiversity hotspots

Global food loss and waste (FLW) undermines the resilience and sustainability of food systems and is closely tied to the United Nation's Sustainable Development Goals on climate, resource use and food security. Here we reveal strong yet under-discussed interconnections between FLW and two other Sustainable Development Goals of Human Health and Life on Land via the nitrogen cycle. We find that eliminating global FLW in 2015 would have reduced anthropogenic NH3 emissions associated with food production by 11.4 Tg (16%), decreased local PM2.5 concentrations by up to 5 μg m-3 and PM2.5-related years of life lost by 1.5 million years, and mitigated nitrogen critical load exceedances in global biodiversity hotspots by up to 19%. Halving FLW in 2030 will reduce years of life lost by 0.5-0.8 million years and nitrogen deposition by 4.7-6.0 Tg N per year (4%) (range for socioeconomic pathways). Complementary to near-term NH3 mitigation potential via technological measures, our study emphasizes incentivizing FLW reduction efforts from air quality and ecosystem health perspectives.

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.

Evaluating the anthropogenic nitrogen emissions to water using a hybrid approach in a city cluster: Insights into historical evolution, attribution, and mitigation potential

Anthropogenic reactive nitrogen (Nr) emissions from agricultural production and food consumption in city clusters have caused water quality degradation and scarcity. In this study, anthropogenic Nr emissions to the water environment were quantitatively evaluated in the Yangtze River Delta city cluster from 2011 to 2020 using coupling nitrogen (N) flow analysis and the grey water footprint (GWF) method. The spatiotemporal characteristics of the GWF and the relative contributions of natural and human factors to the water pollution level (WPL) were analyzed. The results showed that from 2011 to 2020, the total N-related GWF decreased by 12.1 %, mainly driven by reduced fertilizer application and livestock numbers. In 2020, the primary pollution source changed from livestock to humans; however, non-point sources still dominated the GWF. The spatial clustering trend of the GWF was significant: high and low GWF were mainly concentrated in the northeast and southwest regions, respectively. From 2011 to 2020, the mean center of the GWF moved west due to the decrease and increase in the eastern and western regions, respectively, supporting the pollution haven hypothesis. The WPL ranged from 2.67 to 5.03 and fluctuated due to variations in precipitation. The relative contributions of natural and human factors to the WPL evolution were 72.9 % and 27.1 %, respectively. According to the scenario analysis, increasing the N use efficiency to 50 %, manure recycling rate to 80 %, and sewage treatment rate in urban and rural regions to 98 % and 40 %, respectively, could decrease GWF by 39.6 %. The present study establishes an open framework to evaluate anthropogenic N emissions to water, and the outcomes provide valuable references for sustainable N management in city clusters.

Quantifying the wet deposition of reactive nitrogen over China: Synthesis of observations and models

Accurate estimation on reaction nitrogen (N_r) deposition is highly demanded for assessing the impacts on the environment and human beings. This study investigated the wet deposition of inorganic nitrogen (IN) in mainland China by measurements from over 500 sites from five observational networks/databases and ensemble results of eleven chemical transport models (CTMs). Each data source has its focus and limitations and together formed a comprehensive view over China. But the inconsistency among different sources may hinder the appropriate usage of data. Model evaluation results demonstrated the models' deficiency in simulating the wet NO₃^- deposition over Southeast China (40 % underestimation) and showed an overall underestimation of wet NH₄⁺ deposition over the hotspot regions (5-60 % underestimation). A synthesis of this study and twelve reference studies was conducted to quantify the national amount of wet IN deposition. The estimations by CTMs ranged 2.4-3.9 Tg(N) yr^-1 for wet NO_y deposition and 4-6.7 Tg(N) yr^-1 for wet NH_x deposition, after adjusting the results with 10-19 % underestimations in wet NO_y deposition and 1-40 % underestimations in wet NH_x deposition. The estimations by ground observations ranged 7.1-9 Tg(N) yr^-1 for wet NO_y deposition and 8-13.1 Tg(N) yr^-1 for wet NH_x deposition, which were 20-275 % higher than the estimation by CTMs, but the results were strongly influenced by the abundances and representative of measurements. Studies using statistical techniques to interpolate site observations predicted 3-5.5 Tg(N) yr^-1 for wet NO_y deposition and 3.9-7.2 Tg(N) yr^-1 for wet NH_x deposition. This approach benefited from high accuracy and good robustness of the statistical models, but the uncertainty in the interpolation methods could be a potential drawback.

Environmental significance of atmospheric nitrogen deposition in the transition zone between the Tibetan Plateau and arid region

The ecological effect of atmospheric N deposition has become a hot research point along with intensive human activities and global climatic change. As the transition zone between the Tibetan Plateau and the arid region, the Qilian Mountains are important ecological barriers and source regions of inland rivers in northwest China. However, the quantification of N deposition in the transition zone between the Tibetan Plateau (TP) and the arid region remains unclear, primarily due to the lack of in situ measurements. Hence, an observational study was conducted on the Qilian Mountains, and precipitation data were collected. Approximately 1382 samples were collected and analyzed regarding their characteristics and environmental effects of the atmospheric N wet deposition. The annual wet deposition of atmospheric N was 10.05 kg/hm², and showed a decreasing trend from the south to the north of the Qilian Mountains. NO₃^--N deposition was the main form of wet deposition of atmospheric N on the Qilian Mountains, accounting for 73.83% of the DIN deposition. Additionally, altitude, meteorological factors, and ecosystem types were found to influence the wet deposition of atmospheric N. The contribution of NO₃^--N to the wet deposition of atmospheric N in meadows, forests, grasslands, farmlands, and towns was 48.38%, 71.55%, 77.54%, 69.61%, and 82.84%, respectively. This study provides a scientific basis for the effective management and sustainable development of environmental protection in the transition zone between the TP and the arid region. The results showed that relevant policies, as well as scientific and governmental measures, could contribute to reducing N deposition effectively. However, the further mitigation measures should be proposed and strictly enforced.

Unexpected response of nitrogen deposition to nitrogen oxide controls and implications for land carbon sink

Terrestrial ecosystems in China receive the world’s largest amount of reactive nitrogen (N) deposition. Recent controls on nitrogen oxides (NO_x = NO + NO₂) emissions in China to tackle air pollution are expected to decrease N deposition, yet the observed N deposition fluxes remain almost stagnant. Here we show that the effectiveness of NO_x emission controls for reducing oxidized N (NO_y = NO_x + its oxidation products) deposition is unforeseen in Eastern China, with one-unit reduction in NO_x emission leading to only 55‒76% reductions in NO_y-N deposition, as opposed to the high effectiveness (around 100%) in both Southern China and the United States. Using an atmospheric chemical transport model, we demonstrate that this unexpected weakened response of N deposition is attributable to the enhanced atmospheric oxidizing capacity by NO_x emissions reductions. The decline in N deposition could bear a penalty on terrestrial carbon sinks and should be taken into account when developing pathways for China’s carbon neutrality.

Recent vigorous controls in anthropogenic nitrogen oxide emissions in China cannot result in proportionate decreases in regional atmospheric nitrogen deposition. Enhanced atmospheric oxidizing capacity offsets those reductions of precursor emissions.

Effects of Strigolactone on Torreya grandis Gene Expression and Soil Microbial Community Structure Under Simulated Nitrogen Deposition

Nitrogen enters the terrestrial ecosystem through deposition. High nitrogen levels can affect physical and chemical properties of soil and inhibit normal growth and reproduction of forest plants. Nitrogen modulates the composition of soil microorganisms. Strigolactones inhibits plant branching, promotes root growth, nutrient absorption, and promotes arbuscular fungal mycelia branching. Plants are subjected to increasing atmospheric nitrogen deposition. Therefore, it is imperative to explore the relationship between strigolactone and nitrogen deposition of plants and abundance of soil microorganisms. In the present study, the effects of strigolactone on genetic responses and soil microorganisms of Torreya grandis, under simulated nitrogen deposition were explored using high-throughput sequencing techniques. T. grandis is a subtropical economic tree species in China. A total of 4,008 differentially expressed genes were identified in additional N deposition and GR24 treatment. These genes were associated with multiple GO terms and metabolic pathways. GO enrichment analysis showed that several DEGs were associated with enrichment of the transporter activity term. Both additional nitrogen deposition and GR24 treatment modulated the content of nutrient elements. The content of K reduced in leaves after additional N deposition treatment. The content of P increased in leaves after GR24 treatment. A total of 20 families and 29 DEGs associated with transporters were identified. These transporters may be regulated by transcription factors. A total of 1,402,819 clean reads and 1,778 amplicon sequence variants (ASVs) were generated through Bacterial 16S rRNA sequencing. Random forest classification revealed that Legionella, Lacunisphaera, Klebsiella, Bryobacter, and Janthinobacterium were significantly enriched in the soil in the additional N deposition group and the GR24 treatment group. Co-occurrence network analysis showed significant differences in composition of soil microbial community under different treatments. These results indicate a relationship between N deposition and strigolactones effect. The results provide new insights on the role of strigolactones in plants and composition of soil microorganisms under nitrogen deposition.

Nitrogen losses from food production in the North China Plain: A case study for Quzhou

Nitrogen (N) management is essential for food security. The North China Plain is an important food producing region, but also a hotspot of N losses to the environment. This results in water, soil, and air pollution. In this study, we aim to quantify the relative contribution of different crops and animals to N losses, by taking the Quzhou county as a typical example in the North China Plain. We developed and applied a new version of the NUtrient flows in Food chains, Environment, and Resource use (NUFER) model. Our model is based on updated information for N losses in Quzhou. Our results show that N losses to the environment from crop and animal production in Quzhou were approximately 9 kton in 2017. These high N losses can be explained by the low N use efficiency in food production because of poor N management. For crop production, wheat, maize, and vegetables contributed 80% to N losses. Ammonia emissions and N leaching have dominant shares in these N losses. Pigs and laying hens were responsible for 74% of N losses from animal production. Ammonia emissions to air and direct discharges of manure to water were the main contributors to these N losses. Effective reduction of N losses requires improving the nutrient management in crop (wheat, maize, vegetables) and animal (pigs, laying hens) production. Our work could support the Agricultural Green Development in the North China Plain.

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.

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

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

Effects of exogenous N and endogenous nutrients on alpine tundra litter decomposition in an area of high nitrogen deposition

The effects of N deposition on the C and N cycles via altered litter decomposition rates are an important aspect of global environmental change. The Changbai Mountain region experienced a high N deposition (2.7 g·m^-2·year^-1 in 2015) and corresponding expansion of Deyeuxia purpurea into the alpine tundra, resulting in changes in endogenous nutrients. However, the relative contributions of the N deposition and endogenous litter nutrients to litter decompositions remain unclear. Therefore, a 5-year N addition and 2-year litter decomposition experiments were conducted. Exogenous N reduced the remaining litter mass of Rhododendron aureum at the early stage (30-240 d) by promoting soluble sugar release, and increased it at the late stage (360-720 d) by suppressing lignin release and decreasing soil microbial community and enzyme activity. A higher proportion of D. purpurea litter (representing higher N, lower lignin, and C:N ratio) decreased remaining litter mass and increased net N release. Exogenous N decreased decomposition rate from 0.32 to 0.21 and net N release from 34% to 24%, whereas litter compositions increased decomposition rates from 0.32 to 0.69 and net litter N release from 34% to 69%. Endogenous litter nutrients directly explained 62% and 40% of the litter decomposition and net N release variables, respectively, whereas exogenous N indirectly explained 12% and 9%, respectively. Thus, we infer that the reductions in C and N storage following D. purpurea expansion may offset the increases of C and N storage under N deposition and the expansion of D. purpurea has a potential long-term negative impact on the ability of tundra plants to sequester C and N in the alpine tundra of the Changbai Mountains. These findings highlight how shifting plant expansion, through changes endogenous nutrients, can influence tundra litter decomposition and C and N storage responses to N deposition.

Nitrogen emission and deposition budget in an agricultural catchment in subtropical central China

The study of emissions and depositions of atmospheric reactive nitrogen species (N_rs) in a region is important to uncover the sources and sinks of atmospheric N_rs in the region. In this study, atmospheric total N_rs depositions including both wet-only and dry deposition were monitored simultaneously across major land-use types in a 105 km² catchment called Jinjing River Catchment (JRC) in subtropical central China from 2015 to 2016. Based on activity data and emission factors for the main N_rs emission sources, ammonia (NH₃) and nitrogen oxides (NO_x) emission inventories for the catchment were also compiled. The estimated total N_rs deposition in JRC was 35.9 kg N ha^-1 yr^-1, with approximately 49.7 % attributed to reduced compounds (NH_x), and 40.5 % attributed to oxidized (NO_y). The total N_rs emission rate in JRC was 80.4 kg N ha^-1 yr^-1, with 61.5 and 18.9 kg N ha^-1 yr^-1 from NH₃ and NO_x emissions, respectively. Livestock excretion and fertilization were the two main contributing emission sources for NH₃, while vehicle sources contributed the bulk of NO_x emissions. The net atmospheric budgets of N_rs in paddy field, forest, and tea field were +3.7, -36.1, and +23.8 kg N ha^-1 yr^-1, respectively. At the catchment scale, the net atmospheric budget of N_rs was +47.7 kg N ha^-1 yr^-1, with +43.7 kg N ha^-1 yr^-1 of NH_x and +4.0 kg N ha^-1 yr^-1 of NO_y, indicating that the subtropical catchment was net sources of atmospheric N_rs. Considering that excessive atmospheric N_r emissions and deposition may cause adverse effects on the environment, effects should be conducted to mitigate the N_rs emissions from agriculture and transportation, and increasing the area of forest is good for reducing the net positive budget of atmospheric N_rs in the subtropical catchments in China.

Aerosol water content enhancement leads to changes in the major formation mechanisms of nitrate and secondary organic aerosols in winter over the North China Plain

In recent years, severe air pollution still frequently occurs in winter despite the effective implementation of clean air actions in China. Therefore, field measurements of particle composition and gas precursors were collected from December 1, 2018 to January 15, 2019 at an urban site in a central Chinese city to investigate the existing mechanisms of pollution. The hourly averaged PM_2.5 concentration during the campaign was 92.7 μg m^-3, with nitrate and organic aerosol (OA) demonstrated as the principal components. Generally, NO₂ oxidation in the daytime was observed as the major mechanism for nitrate generation, and aerosol water content (AWC) showed its influential role with the associated increases in the nitrogen oxidation and nitrate partitioning ratios. When AWC increased from dozens to hundreds of μg m^-3 after the afternoon, nocturnal N₂O₅ hydrolysis was demonstrated as the overriding mechanism and provoked extreme contamination of nitrates. Five sources of organic aerosols (OAs) were identified: hydrocarbon-like OAs (HOAs, 16.5%), coal combustion OAs (CCOAs, 19.2%), biomass burning OAs (BBOAs, 9.9%), semi-volatile oxygenated OAs (SV-OOAs, 29.4%), and low-volatile oxygenated OAs (LV-OOAs, 25.0%). SV-OOAs and LV-OOAs were identified as gasSOAs and aqSOAs according to their sensitivities to the atmospheric oxidation capacity and AWC. In addition, aqueous-phase processing was found to be the dominant pathway for SOA formation when the AWC concentration was higher than 80 μg m^-3. As an influential factor for nitrate and SOA formation, AWC could be greatly affected by RH and the concentrations of inorganic species. Sulfate, which was mainly contributed by anthropogenic emissions, was demonstrated to be a significant factor for active aqueous phase reactions, although SO₂ has been dramatically reduced in recent years. Above all, this study revealed the significant role of AWC in current pollution episode in winter, and will assist in establishing future measures for pollution mitigation.

Uncertainty of nitrogen budget in China

The accuracy of the nitrogen (N) budget is of great importance for evidence-based decision-making to address both food security and environmental protection challenges. This study attempts to advance understanding of uncertainties in China's N budget using the Coupled Human And Natural Systems (CHANS) model and Monte Carlo simulation from 1980 to 2018. Results show that the spatial and temporal variations in agricultural and industrial activities and insufficient knowledge on N cycling parameterization are the two dominant causes of uncertainties in the N budget in China. Uncertainties of N inputs generally are <10%, while they are <30% for N outputs and >30% for N accumulations. Uncertainty of nitrogen oxides emission is more sensitive to energy consumption due to the large contributions from industry and transportation. While the uncertainty of ammonia emission is predominantly affected by agricultural activity. Combining surface measurements, satellite observations, and atmospheric simulation models enables cross-check of N fluxes in multiple systems and reduces uncertainties of N budget.

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

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

Atmospheric nitrogen deposition: A review of quantification methods and its spatial pattern derived from the global monitoring networks

Atmospheric nitrogen (N) deposition is a vital component of the global N cycle. Excessive N deposition on the Earth's surface has adverse impacts on ecosystems and humans. Quantification of atmospheric N deposition is indispensable for assessing and addressing N deposition-induced environmental issues. In the present review, we firstly summarized the current methods applied to quantify N deposition (wet, dry, and total N deposition), their advantages and major limitations. Secondly, we illustrated the long-term N deposition monitoring networks worldwide and the results attained via such long-term monitoring. Results show that China faces heavier N deposition than the United States, European countries, and other countries in East Asia. Next, we proposed a framework for estimating the atmospheric wet and dry N deposition using a combined method of surface monitoring, modeling, and satellite remote sensing. Finally, we put forth the critical research challenges and future directions of the atmospheric N deposition. CAPSULE: A review of quantification methods and the global data on nitrogen deposition and a systematic framework was proposed for quantifying nitrogen deposition.

A high-resolution map of reactive nitrogen inputs to China

To feed an increasingly affluent population, reactive nitrogen (Nr) inputs to China’s lands and waters have substantially increased over the past century. Today, China’s Nr emissions account for over one third of global total emissions, leading to serious environmental pollution and health damages. Quantifying the spatial variability of Nr inputs is crucial for the identification of intervention points to mitigate Nr pollution, which, however, is not well known. Here, we present a database describing Nr inputs to China for the year 2017 with a 1 km × 1 km resolution, considering land use and Nr sources, compiled by using the CHANS model. Results show that the North China Plain, the Sichuan Basin and the Middle-Lower Yangtze River Plain are hotspots of Nr inputs, where per hectare Nr input is an order of magnitude higher than that in other regions. Cropland and surface water bodies receive much higher Nr inputs than other land use types. This unique database will provide basic data for research on environmental health and global change modelling.

Measurement(s)	reactive nitrogen species
Technology Type(s)	computational modeling technique
Factor Type(s)	land use • geographic location • reactive nitrogen source
Sample Characteristic - Location	China

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12971939

Changes of nitrogen deposition in China from 1980 to 2018

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 NH₄⁺-N deposition offset by a continuous increase in dry NH₃ deposition. Elevated NH₃ 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 NH₃, built on broader regional emission control strategies.

Enhanced atmospheric ammonia (NH3) pollution in China from 2008 to 2016: Evidence from a combination of observations and emissions

The increase of gaseous ammonia (NH₃) concentration in the atmosphere significantly impacts the regional air quality, human health, and the nitrogen cycle of ecosystems. This study aims to verify the reanalyzed product of IASI NH₃ (the ANNI-NH₃-v2.1R-I, hereafter referred to as IASI_NH₃_R) and to analyze the spatial and temporal characteristics of atmospheric NH₃ during 2008-2016 and its underlying influencing factors. Our results show a good agreement between spatial pattern and temporal (annual and monthly) trend of the satellite-derived surface NH₃ concentrations and the measured near-ground NH₃ measurements over different land covers in Eastern China, suggesting the IASI_NH₃_R product can be used to investigate spatial and temporal trends of atmospheric NH₃ concentration. The annual mean NH₃ column concentrations peaked in the North China Plain (averaged 12 × 10¹⁵ mol cm^-2 yr^-1) and showed a significant increasing trend at a rate of 0.6 × 10¹⁵ mol cm^-2 yr^-1 during the entire period, which can be ascribed to densely populated, intensive agricultural activities and substantial reduction of SO₂ and NO₂ emissions since 2011. The NH₃ column concentrations show a slight increase in winter in most regions of China, probably due to less precipitation amount and increased uncertainty for lower NH₃ columns and the thermal contrast (TC). A large seasonal variation of NH₃ column concentrations was observed, with the highest values in summer and the lowest in autumn. Such seasonal variation is mainly affected by seasonal differences in NH₃ emissions and meteorological conditions. Our results suggest that the current control measures effectively decreased SO₂ and NO₂ pollution but are not yet apparent in the mitigation of atmospheric NH₃ pollution, which also merits more attention considering that no effective measures are being implemented for NH₃ emission control at a regional or national scale in China.

Dry deposition of reactive nitrogen to different ecosystems across eastern China: A comparison of three community models

The assessment of nitrogen ecosystem loads mostly use the method of sampling observation combined with numerical model to estimate the spatial distribution pattern of nitrogen dry deposition flux. The selection of models is important which directly affects the reliability of the deposition flux results. In this study, the performance of three widely used models (WRF-Chem, EMEP, CMAQ) are compared. The dry deposition fluxes of typical active nitrogen components over eastern China showed uncertainties by a factor of 0.5 ~ 2 between the oxidized nitrogen (OXN) results of the three models and the observation network while the reduced nitrogen (RDN) simulation results are underestimated by a quarter of the observation reports. These three models show different results on four typical ecosystems: simulation of EMEP got the highest for OXN dry deposition flux on each ecosystem (urban 14.94 ± 4.92kgN ⋅ ha^-1 ⋅ yr^-1, cropland/grassland 5.53 ± 5.11kgN ⋅ ha^-1 ⋅ yr^-1, forest 4.75 ± 4.32kgN ⋅ ha^-1 ⋅ yr^-1, water bodies 1.48 ± 1.53kgN ⋅ ha^-1 ⋅ yr^-1); WRF-Chem has the highest value of RDN on the urban (8.91 ± 6.44kgN ⋅ ha^-1 ⋅ yr^-1) and water bodies (1.01 ± 1.44kgN ⋅ ha^-1 ⋅ yr^-1) while EMEP is highest in cropland/grassland (3.42 ± 3.43kgN ⋅ ha^-1 ⋅ yr^-1) and forest (2.34 ± 1.94kgN ⋅ ha^-1 ⋅ yr^-1). CMAQ is in medium range for both OXN and RDN simulations on each ecosystem. Compare with the critical loads, CMAQ generates more exceeded critical load areas than WRF-Chem and EMEP on cropland/grassland and forests ecosystem. For water bodies, WRF-chem and CMAQ showed higher exceeding critical load areas than EMEP. In summary, EMEP generally underestimates while the CMAQ and WRF-Chem model would overestimate the impacts on the ecosystems. So, policy implementation needs special attention accounting the difference of simulation effect with different models.

Improved Inversion of Monthly Ammonia Emissions in China Based on the Chinese Ammonia Monitoring Network and Ensemble Kalman Filter

Ammonia (NH₃) emission inventories are an essential input in chemical transport models and are helpful for policy-makers to refine mitigation strategies. However, current estimates of Chinese NH₃ emissions still have large uncertainties. In this study, an improved inversion estimation of NH₃ emissions in China has been made using an ensemble Kalman filter and the Nested Air Quality Prediction Modeling System. By first assimilating the surface NH₃ observations from the Ammonia Monitoring Network in China at a high resolution of 15 km, our inversion results have provided new insights into the spatial and temporal patterns of Chinese NH₃ emissions. More enhanced NH₃ emission hotspots, likely associated with industrial or agricultural sources, were captured in northwest China, where the a posteriori NH₃ emissions were more than twice the a priori emissions. Monthly variations of NH₃ emissions were optimized in different regions of China and exhibited a more distinct seasonality, with the emissions in summer being twice those in winter. The inversion results were well-validated by several independent datasets that traced gaseous NH₃ and related atmospheric processes. These findings highlighted that the improved inversion estimation can be used to advance our understanding of NH₃ emissions in China and their environmental impacts.

Chemical compositions of fog and precipitation at Sejila Mountain in the southeast Tibetan Plateau, China

Chemical compositions of fog and rain water were measured between July 2017 and September 2018 at Sejila Mountain, southeast Tibet, where fog events frequently occurred in original fir forests at altitude 3950 m. Fog water samples were collected using a Caltech Active Strand Cloud Collector (CASCC), and rain samples were collected using a precipitation gauge. Differences were observed between fog water and rain composition for most analyzed ions. Ion abundance in fog water was Ca²⁺ > Cl^- > Na⁺ > SO₄^2- > Mg²⁺ > NH₄⁺ >K⁺ > NO₃^- whereas an order of Ca²⁺ > Na⁺ > Cl^- > Mg²⁺ > SO₄^2- > NO₃^- > K⁺ > NH₄⁺ was observed for rain water. All ion concentrations were higher in fog water than in rain water. Additionally, Ca²⁺ was the dominant cation in both fog and rain samples, accounting for more than half of all measured cations. NH₄⁺ and SO₄^2- concentrations were notable for being higher in fog than rain water when compared with other ions. For trace elements, Al, As, Mn and Se were the most abundant elements in fog water; only Al and As were detected in rain water. Seventy-two hour back-trajectory analysis showed that air masses during fog and/or rain events mainly came from the south of Sejila Mountain. Spearman correlation analysis and source contribution calculations indicated that both marine and terrestrial sources contributed to the observed ion concentrations. Considering the higher concentrations of NH₄⁺ and higher ratio of NH₄⁺/NO₃^- measured in fog than in rain, we suggest that quantification of fog nitrogen deposition and its ecological effect in this area should be given more attention.

A database of atmospheric nitrogen concentration and deposition from the nationwide monitoring network in China

Atmospheric nitrogen (N) deposition has increased substantially across China since 1980; however, data for N deposition fluxes since the 2000s has been very limited. Understanding and mitigating the impacts of N deposition requires long-term quantification of dry as well as wet deposition of key reactive nitrogen (Nr) species. Here we present a dataset for inorganic N concentrations and deposition for the period 2010-2015 in China, compiled from the nationwide deposition monitoring network. The dataset comprises information from 32 monitoring sites on concentrations and bulk deposition (wet plus part of dry deposition) fluxes of NH₄⁺-N and NO₃^--N, air concentrations and dry deposition fluxes of the major Nr species NH₃, NO₂, HNO₃, and particulate NH₄⁺ and NO₃^-. This unique database is available inter alia to advance understanding of the spatial patterns of inorganic N concentrations and deposition in China and its associated effects, constrain primary Nr (e.g., NH₃, NO_x) emission inventories, and validate outputs of atmospheric chemistry and transport models.