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Oxygenated organic molecules (OOMs) are dominated by the N-containing species in polluted urban environments. As N-containing OOMs, especially those with more than one nitrogen atom, prevail in the high m/z (mass-to-charge) range (m/z> 350 Th), unambiguous identification of N-containing OOMs is highly desirable for understanding of their formation processes, precursors and influencing factors. To achieve this, we applied an ultra-high-resolution chemical-ionization Orbitrap (CI-Orbitrap) in a field campaign and found that OOMs contain one (1N-OOMs), two (2N-OOMs) and three (3N-OOMs) nitrogen atoms comprised 50 %, 26 % and 4 %, respectively, of total OOMs. More interestingly, the fraction of 2N-OOMs increased with the increase in carbon number (nC) and was dominated by the ones derived from aliphatic precursors (2N-OOMAli, 64.2 %), indicating the importance of multistep oxidation. Plausible precursors of 2N-OOMs were aliphatics (2N-OOMAli, 64.2 %), aromatics (2N-OOMAro, 16 %) and monoterpenes (2N-OOMMT, 15.4 %). The absolute concentrations of 2N-OOMs were greatly affected by the pollution level for most cases. The 2N-OOMAli was the most abundant 2N-OOM, and its fraction even increased on the polluted day with an enhanced proportion of the ones with nC >10. While 2N-OOMAli and 2N-OOMAro were dominated by daytime photochemical production, nighttime NO3-initiated oxidation played a comparable role to the daytime photochemistry in the formation of 2N-OOMMT. The 2N-OOMAro species were of the highest oxygenation level, followed by 2N-OOMMT and 2N-OOMAli, which were affected by photochemistry and NOx concentrations. These results highlight the significant formation of 2N-OOMs and the influencing factors on their formation in polluted urban environments, where various volatile organic compound (VOC) precursors and atmospheric oxidants are present.

Preterm birth remains the leading cause of neonatal morbidity and mortality today. Genetic, immunological, and infectious substrates are suspected. Preterm infants are at higher risk of severe neonatal infections and the main cause of bacterial infection in this population isEscherichia coliK1. Unfortunately, women with history of preterm birth have a high risk of recurrence. Therefore, these women constitute a target population for a vaccine, to date non-existent, againstE. coliK1 to prevent these infections.In this study, we characterize the immunological and microbiological properties in adult female mice of a live attenuated vaccine candidate and the protection it conferred to newborn mice against severe infection caused byE. coliK1. We show that ourE. coliK1 ΔaroA vaccine induces a strong immunity driven by polyclonal bactericidal antibodies. In our model of meningitis, pups born from mothers immunized before conception were strongly protected against different strains ofE. coliK1 both in early-onset and late-onset diseases.Given the very high rate of mortality and neurological sequalae in neonatal meningitis caused byE. coliK1, this pre-clinical study provides a proof-of-concept for the development of a vaccine strategy againstE. coliK1 severe infection in women at risk of preterm birth.

Interferon gamma (IFN-γ) plays central roles in the pathophysiology of inflammatory bowel disease (IBD), both activating inflammatory responses and immunosuppressive functions. However, the epigenetic mechanisms controlling the expression of IFN-γ responsive genes at the gut epithelial barrier are not well understood. In this study, we identified the epigenetic regulator HP1γ as a transcriptional repressor of the IFN-γ-responsive genes STAT1 (signal transducer and activator of transcription 1) and PD-L1 (Programmed Cell Death Ligand 1). Accordingly, HP1γ gene inactivation in the mouse gut epithelium resulted in an immunopathology with a long-lasting up-regulation of STAT1 and PD-L1.Colon organoids models and in vitrocell lines showed that HP1γ deficiency primed STAT1 and PD-L1 expressions, ultimately sensitizing epithelial cells to IFN-γ stimulation. Chromatin immunoprecipitation experiments suggest that HP1 promoter tethering is involved in the silencing of gene expression. Overall, these results identify HP1γ as an epigenetic silencing pathway controlling the IFN-γ response at the epithelial barrier.

Defects in RNA splicing have been linked to numerous human disorders, but remain poorly explored in inflammatory bowel disease (IBD). Here, we report that, in the gut epithelium of patients with ulcerative colitis (UC), the expression of the chromatin and alternative splicing regulator HP1γ is strongly reduced. Accordingly, inactivation of the HP1γ gene in the mouse gut triggered several IBD-like traits, including inflammation and dysbiosis. In parallel, we discovered that its loss of function broadly increased splicing noise, reducing requirement for canonical splicing consensus sequences, and favoring the usage of cryptic splice sites at numerous genes with key functions in gut biology. This notably resulted in the production of progerin, a noncanonical toxic splice variant of prelamin A mRNA, responsible for the Hutchinson Gilford Progeria Syndrome (HGPS) of premature aging. Likewise, production of progerin transcript was found to be a signature of colonic cells from UC patients. Thus, our study identifies HP1γ as a regulator of RNA metabolism in vivo, providing a unique mechanism linking anti-inflammation and accuracy of RNA splicing in the gut epithelium. HP1 defect may confer a general disturbance in RNA splicing precision to scrutinize in IBD and more generally in accelerating aging diseases.

The amine map over the Yangtze River Delta region based on mobile measurements

Atmospheric PM

China's unprecedented lockdown to contain the spread of the novel coronavirus disease (COVID-19) in early 2020, provided a tragic natural experiment to investigate the responses of atmospheric pollution to emission reduction at regional scale. Primarily driven by primary emissions, particulate trace elements is vitally important due to their disproportionally adverse impacts on human health and ecosystem. Here 14 trace elements in PM2.5 were selected for continuous measurement hourly in urban representative site of Shanghai, for three different phases: pre-control period (1–23 January 2020), control period (24 January-10 February 2020; overlapped with Chinese Lunar New Year holiday) and post control period (11–26 February 2020) the city's lockdown measures. The results show that all meteorological parameters (including temperature, RH, mixing layer height et al.) were generally consistent among different periods. Throughout the study period, the concentrations of most species displayed a “V-shaped” trend, suggesting significant effects by the restriction measures imposed during the lockdown period. While this is not the case for species like K, Cu and Ba, indicating their unusual origins. As a case study, the geographical origins of Cu were explored. Seven major sources, i.e., Vehicle-related emission (including road dust; indicative of Ca, Fe, Ba, Mn, Zn, Cu; accounting for 30.1%), shipping (Ni; 5.0%), coal combustion (As, Pb; 4.2%), Se and Cr industry (24.9%), nonferrous metal smelting (Au, Hg; 7.5%) and fireworks burning (K, Cu, Ba; 28.3%) were successfully pinpointed based on positive matrix factorization (PMF) analysis. Our source apportionment results also highlight fireworks burning was one of the dominant source of trace elements during the Chinese Lunar New Year holiday. It is worth noting that 56% of the total mass vehicular emissions are affiliated with non-exhaust sources (tire wear, brake wear, and road surface abrasion)., Graphical abstract Image 1

The circadian clock coordinates biological and physiological functions to day/night cycles. The perturbation of the circadian clock increases cancer risk and affects cancer progression. Here, we studied how BMAL1 knockdown (BMAL1-KD) by shRNA affects the epithelial–mesenchymal transition (EMT), a critical early event in the invasion and metastasis of colorectal carcinoma (CRC). In corresponding to a gene set enrichment analysis, which showed a significant enrichment of EMT and invasive signatures in BMAL1_high CRC patients as compared to BMAL1_low CRC patients, our results revealed that BMAL1 is implicated in keeping the epithelial–mesenchymal equilibrium of CRC cells and influences their capacity of adhesion, migration, invasion, and chemoresistance. Firstly, BMAL1-KD increased the expression of epithelial markers (E-cadherin, CK-20, and EpCAM) but decreased the expression of Twist and mesenchymal markers (N-cadherin and vimentin) in CRC cell lines. Finally, the molecular alterations after BMAL1-KD promoted mesenchymal-to-epithelial transition-like changes mostly appeared in two primary CRC cell lines (i.e., HCT116 and SW480) compared to the metastatic cell line SW620. As a consequence, migration/invasion and drug resistance capacities decreased in HCT116 and SW480 BMAL1-KD cells. Together, BMAL1-KD alerts the delicate equilibrium between epithelial and mesenchymal properties of CRC cell lines, which revealed the crucial role of BMAL1 in EMT-related CRC metastasis and chemoresistance.

Ammonia (NH3) is an important agent involved in atmospheric chemistry and nitrogen cycling. Current estimates of NH3 emissions from biomass burning (BB) differ by more than a factor of 2, impeding a reliable assessment of their environmental consequences. Combining highresolution satellite observations of NH3 columns with network measurements of the concentration and stable nitrogen isotope composition (δ15N) of NH3, we present coherent estimates of the amount of NH3 derived from BB in the heartland of Southeast Asia, a tropical monsoon environment. Our results reveal a strong variability in atmospheric NH3 levels in time and space across different landscapes. All of the evidence on hand suggests that anthropogenic activities are the most important modulating control with respect to the observed patterns of NH3 distribution in the study area. Nisotope balance considerations revealed that during the intensive fire period, the atmospheric input from BB accounts for no more than 21 ± 5 % (1s) of the ambient NH3, even at the rural sites and in the proximity of burning areas. Our N-isotope-based assessment of the variation in the relative contribution of BB-derived NH3 is further validated independently through the measurements of particulate KC, a chemical tracer of BB. Our findings underscore that BB-induced NH3 emissions in tropical monsoon environments can be much lower than previously anticipated, with important implications for future modeling studies to better constrain the climate and air quality effects of wildfires., SCOPUS: ar.j, info:eu-repo/semantics/published

Due to the local emissions and transportations of air pollution from the most polluted regions such as the North China Plain and Yangtze Delta metropolitan, Xuzhou is becoming one of the most polluted cities in East China. The sources and formation processes of nitrogen-containing aerosols are therefore very complex. Two continuous aerosol measurement campaigns were conducted in this industrial city during the wintertime and summertime of 2016, to investigate the chemical compositions and potential sources of total nitrogen (TN, including 89% inorganic nitrogen and 11% organic nitrogen) in PM2.5. Abrupt enhancements of nitrogen-containing aerosols (e.g., NO3− and NH4+) were found in the winter, and nitrate became as a dominant contributor in high pollution days (e.g., PM2.5 > 150 μg m−3). Nitrogen oxidation ratios (NOR) correlated significantly with aerosol liquid water content (ALWC), which was estimated by ISORPROPIA-II model. This suggested heterogeneous process might be an important pathway in nitrate formation during the high PM2.5 days. The nitrogen isotope composition (δ15N) in TN varied from −1.3 to +13.2‰ with a mean value of 6.9 ± 3.6‰ during the wintertime. An isotope-based source apportionment approach was then developed using a Bayesian isotope mixing model (SIAR) with chemical compositions as an important constrain, which improved accuracy and reduced the overall uncertainties in estimations of TN sources. From this optimized model, we identified six major sources including NH3 from combustion-related emissions (49%), NH3 derived from animal wastes (6%), NH3 from urban volatilization (3%), NOx derived from coal combustion (33%), NOx from biomass burning (5%) and NOx from vehicles (3%). Our results demonstrated that ambient NOx was dominated by coal combustion. Since NOx and NH3 are important precursors for ammonium nitrate aerosols, controlling of combustion related NOx and NH3 emissions might be an important way to reduce PM2.5 levels in this region.

Currently, haze events in winter occur more frequently than decades ago, especially in Eastern and Central China, including the Yangtze River Delta (YRD). WRF-Chem is applied in this study to explore the discrepancies of the simulated air pollutants induced by employing different emission inventories, particularly during haze events. Two inventories are involved in this study, MEIC (Multi-resolution Emission Inventory for China) and GlobEmission (inventory from GlobEmission project emission estimates), representing the emission inventories based on quite different ways. We first compared monthly emissions of SO2 and NOx in two inventories during January over YRD, and found the mean differences of SO2 (NOx) are 20% (7%), with the ranges of 0%–47% (−100%–100%). The largest differences are both found in Shanghai, with 47% for SO2 (MEIC in 2010, GlobEmission in 2014), and 45% for NOx (MEIC in 2010, GlobEmission in 2015), respectively, partly because the reduction of emissions was large during the 12th Five-Year Plan (2011–2015) in this area. By comparing the simulated air pollutants mass concentrations using two inventories with in-situ observations during January 2015, we found that the simulated SO2 using MEIC and GlobEmission are both higher than observations, with mean normalized biases of 207% and 121% over YRD, respectively, and much larger in the city cluster of Nanjing-Shanghai, where are nearly four (MEIC) and three times (GlobEmission) higher than observations. In contrast, NO2 simulations using GlobEmission are lower than observations (22%) and MEIC simulations (45%) over YRD. The largest biases of GlobEmission simulation are found in Zhejiang province (over 70%). The biases of simulated monthly mean PM2.5 are 38% (MEIC) and 30% (GlobEmission) over YRD, respectively. A case study during heavy haze event shows that the biases increase to 61% (MEIC) and 39% (GlobEmission), and spatial correlation coefficient in the simulation using GlobEmission increase to 0.69 over YRD. Temporal correlation coefficients in the city cluster of Nanjing-Shanghai increase from 0.5 (MEIC) to 0.7 (GlobEmission).

The North China Plain and the Yangtze River Delta are the two of the most heavily polluted regions in China. Observational studies revealed that ‘explosive’ PM2.5 mass concentration growths frequently occurred in the two regions. This study analyzed all the PM2.5 mass concentration growth processes from clean condition (i.e., 150 μg m−3) in Beijing (BJ) and Shanghai (SH), two representative cities of the two regions, using hourly monitored PM2.5 concentrations during 2013–2016. 173 and 76 growth processes were identified in BJ and SH, respectively. PM2.5 rising rates (PMRR) and dynamic growth durations were calculated to illustrate the characteristics of the growth processes. Hourly particulate chemical composition data and meteorological data in BJ and SH were further analyzed. The 4-year averaged PMRR of PM2.5 total mass were similarly of 7.11 ± 9.82 μg m−3 h−1 in BJ and 6.71 ± 6.89 μg m−3 h−1 in SH. A decreasing trend was found for the PM2.5 growth processes in two cities from 2013 to 2016, reflecting the effectiveness of emission controls implemented in the past years. The contributions of particulate components to the PM2.5 total mass growth were different in BJ and SH. Average PMRR value of PM1 organic aerosols (OA), SO42−, NO3−, and NH4+ in BJ was 1.90, 0.95, 0.82, and 0.53 μg m−3 h−1, respectively. Average PMRR of PM2.5 OA, SO42−, NO3−, and NH4+ in SH was 1.70, 1.18, 1.99 and 1.14 μg m−3 h−1, respectively. Based on the contributions of different components, the PM2.5 mass concentration growth processes in BJ and SH were proposed to be classified into ‘other components-dominant growth processes’, ‘all components-contributing growth processes’, ‘one or more explosive secondary components-dominant growth processes’, and ‘mixed-factor growth processes’. Potential source contribution function analysis and the meteorological condition analysis showed that source origins and prevailing wind for the two cities during different categories of growth processes had substantial difference. The important source areas included Hebei and Shandong for BJ, and Jiangsu and Anhui for SH. The dominant wind directions during growth processes were northeast, south and southwest in BJ, and were west to north in SH. The results suggested the contributing components, the prevailing wind conditions, and the formation processes were substantially different in the two cities, despite the similar PMRR of PM2.5 total mass during the growth processes between BJ and SH. Future research is needed to study the detailed formation mechanisms of the different PM2.5 mass concentration growth processes in the two cities.

Enhanced secondary aerosol formation was observed during the COVID-19 lockdown in Xi'an, especially for polluted episodes. More oxidized‑oxygenated organic aerosol (MO-OOA) and sulfate showed the dominant enhancements, especially in large particle-mode. Meanwhile, relative humidity (RH) showed a positive promotion on the formation of sulfate and MO-OOA during the lockdown, but had no obvious correlation with less oxidized‑oxygenated organic aerosol (LO-OOA) or nitrate. Organosulfurs (OS) displayed a higher contribution (~58%) than inorganic sulfate to total sulfate enhancement in the polluted episode during the lockdown. Although the total nitrate (TN) decreased during the lockdown ascribing to a larger reduction of inorganic nitrate, organic nitrate (ON) showed an obvious increase from pre-lockdown (0.5 ± 0.6 μg m

Agricultural sources and non-agricultural emissions contribute to gaseous ammonia (NH3) that plays a vital role in severe haze formation. Qualitative and quantitative contributions of these sources to ambient PM2.5 (particulate matter with an aerodynamic equivalent diameter below 2.5 µm) concentrations remains uncertain. Stable nitrogen isotopic composition (δ15N) of NH3 and NH4+ (δ15N(NH3) and δ15N(NH4+), respectively) can yield valuable information about its sources and associated processes. This review provides an overview of the recent progress in analytical techniques for δ15N(NH3) and δ15N(NH4+) measurement, sampling of atmospheric NH3 and NH4+ in the ambient air and their sources signature (e.g., agricultural vs. fossil fuel), and isotope-based source apportionment of NH3 in urban atmosphere. This study highlights that collecting sample that are fully representative of emission sources remains a challenge in fingerprinting δ15N(NH3) values of NH3 emission sources. Furthermore, isotopic fractionation during NH3 gas-to-particle conversion under varying ambient field conditions (e.g., relative humidity, particle pH, temperature) remains unclear, which indicates more field and laboratory studies to validate theoretically predicted isotopic fractionation are required. Thus, this study concludes that lack of refined δ15N(NH3) fingerprints and full understanding of isotopic fractionation during aerosol formation in a laboratory and field conditions is a limitation for isotope-based source apportionment of NH3. More experimental work (in chamber studies) and theoretical estimations in combinations of field verification are necessary in characterizing isotopic fractionation under various environmental and atmospheric neutralization conditions, which would help to better interpret isotopic data and our understanding on NHx (NH3 + NH4+) dynamics in the atmosphere.

The COVID-19 pandemic has drastically affected the economic and social activities, leading to large reductions in anthropogenic emissions on a global scale. Despite the reduction of primary emissions during the lockdown period, heavy haze pollution was observed unexpectedly in megacities in North and East China. In this study, we conducted online measurements of organic aerosol in a background site before and during the lockdown in Guanzhong basin, Northwest China. The oxygenated organic aerosol (OOA) increased from 24% of total OA (3.2 ± 1.6 μg m−3) before lockdown to 54% of total OA (4.5 ± 1.3 μg m−3) during lockdown, likely due to substantial decrease of NOx emissions during lockdown which resulted in large increase of O3 and thus atmospheric oxidizing capacity. OOA showed higher mass concentrations and fractional contributions during lockdown than before lockdown, and increased with the increase of Ox in both periods. In comparison, aqueous secondary organic aerosol (aqSOA) showed high mass concentrations and fractional contributions in both polluted periods before and during lockdown with the increase of aerosol liquid water content (ALWC). The increase of aqSOA under high ALWC conditions is very likely the reason of pollution events during lockdown. Combined with trajectory analysis, the absence of Guanzhong cluster in polluted period during lockdown may play a key role in the OA variations between two polluted periods. In addition, when comparing the clusters from the same transmission directions between before lockdown and during lockdown, the OA fractions showed similar variations during lockdown in all clusters, suggesting the OA variations are widespread in northwest China., Graphical abstract Unlabelled Image

Agricultural sources and non-agricultural emissions contribute to gaseous ammonia (NH3) that plays a vital role in severe haze formation. Qualitative and quantitative contributions of these sources to ambient PM2.5 (particulate matter with an aerodynamic equivalent diameter below 2.5 µm) concentrations remains uncertain. Stable nitrogen isotopic composition (δ15N) of NH3 and NH4+ (δ15N(NH3) and δ15N(NH4+), respectively) can yield valuable information about its sources and associated processes. This review provides an overview of the recent progress in analytical techniques for δ15N(NH3) and δ15N(NH4+) measurement, sampling of atmospheric NH3 and NH4+ in the ambient air and their sources signature (e.g., agricultural vs. fossil fuel), and isotope-based source apportionment of NH3 in urban atmosphere. This study highlights that collecting sample that are fully representative of emission sources remains a challenge in fingerprinting δ15N(NH3) values of NH3 emission sources. Furthermore, isotopic fractionation during NH3 gas-to-particle conversion under varying ambient field conditions (e.g., relative humidity, particle pH, temperature) remains unclear, which indicates more field and laboratory studies to validate theoretically predicted isotopic fractionation are required. Thus, this study concludes that lack of refined δ15N(NH3) fingerprints and full understanding of isotopic fractionation during aerosol formation in a laboratory and field conditions is a limitation for isotope-based source apportionment of NH3. More experimental work (in chamber studies) and theoretical estimations in combinations of field verification are necessary in characterizing isotopic fractionation under various environmental and atmospheric neutralization conditions, which would help to better interpret isotopic data and our understanding on NHx (NH3 + NH4+) dynamics in the atmosphere. Electronic Supplementary Material Supplementary material is available in the online version of this article at 10.1007/s11783-021-1414-6 and is accessible for authorized users. Supplementary material includes supplementary tables on summary of recent isotope-based source apportionment studies on ambient NH3 derived from δ15N(NH3) values (Table A1); and summary of recent isotope-based source apportionment studies on particulate NH4+ derived from δ15N(NH4+) values (Table A2).

Air pollution has been recognized as a threat to human health since the time of Hippocrates, ca 400 BC. Successive written accounts of air pollution occur in different countries through the following two millennia until measurements, from the eighteenth century onwards, show the growing scale of poor air quality in urban centres and close to industry, and the chemical characteristics of the gases and particulate matter. The industrial revolution accelerated both the magnitude of emissions of the primary pollutants and the geographical spread of contributing countries as highly polluted cities became the defining issue, culminating with the great smog of London in 1952. Europe and North America dominated emissions and suffered the majority of adverse effects until the latter decades of the twentieth century, by which time the transboundary issues of acid rain, forest decline and ground-level ozone became the main environmental and political air quality issues. As controls on emissions of sulfur and nitrogen oxides (SO 2 and NO x ) began to take effect in Europe and North America, emissions in East and South Asia grew strongly and dominated global emissions by the early years of the twenty-first century. The effects of air quality on human health had also returned to the top of the priorities by 2000 as new epidemiological evidence emerged. By this time, extensive networks of surface measurements and satellite remote sensing provided global measurements of both primary and secondary pollutants. Global emissions of SO 2 and NO x peaked, respectively, in ca 1990 and 2018 and have since declined to 2020 as a result of widespread emission controls. By contrast, with a lack of actions to abate ammonia, global emissions have continued to grow. This article is part of a discussion meeting issue ‘Air quality, past present and future’.

The Guanzhong basin is a part of the three top priority regions in China's blue sky action as of 2019. Understanding the chemical composition, sources, and atmospheric process of aerosol in this region is therefore imperative for improving air quality. In this study, we present, for the first time, the seasonal variations of organic aerosol (OA) in Xi'an, the largest city in the Guanzhong basin. Biomass burning OA (BBOA) and oxidized OA (OOA) contributed >50% of OA in both autumn and winter. The average concentrations of BBOA in autumn (14.8 ± 5.1 μg m-3) and winter (11.6 ± 6.8 μg m-3) were similar. The fractional contribution of BBOA to total OA, however, decreased from 31.9% in autumn to 15.3% in winter, because of enhanced contributions from other sources in winter. The OOA fraction in OA increased largely from 20.9% in autumn to 34.9% in winter, likely due to enhanced emissions of precursors and stagnant meteorological conditions which facilitate the accumulation and secondary formation. A large increase in OOA concentration was observed during polluted days, by a factor of ~4 in autumn and ~6 in winter compared to clean days. In both seasons, OOA formation was most likely dominated by photochemical oxidation when aerosol liquid water content was

Whether mudflats are an important source of atmospheric ammonia (NH3) remains an open question, despite the fact that over half of the world's population live within 3 km of surface water bodies. Here, we established three sites (lake, river, and sea) that are representative of tide-influenced mudflats across the Yangtze Delta in eastern China. Online field measurements of NH3 and auxiliary hydrometeorological parameters were simultaneously performed over a 9 month period. Surprisingly, the average NH3 concentrations measured at these locations are as low as regional background levels. No pulses of increased NH3 were found at these sites when mudflats were exposed due to receding water levels. High atmospheric NH3 concentrations are persistently associated with high temperatures, but their geographical origins have no overlap with the locations of water bodies. The potential mechanism is also discussed. Collectively, we provide the first direct observational evidence concerning mudflat as a source of NH3, SCOPUS: ar.j, info:eu-repo/semantics/published

It is a puzzle as to why more severe haze formed during the New Year Holiday in 2020 (NYH‐20), when China was in an unprecedented state of shutdown to contain the coronavirus (COVID‐19) outbreak, than in 2019 (NYH‐19). We performed a comprehensive measurement and modeling analysis of the aerosol chemistry and physics at multiple sites in China (mainly in Shanghai) before, during, and after NYH‐19 and NYH‐20. Much higher secondary aerosol fraction in PM2.5 were observed during NYH‐20 (73%) than during NYH‐19 (59%). During NYH‐20, PM2.5 levels correlated significantly with the oxidation ratio of nitrogen (r 2 = 0.77, p < 0.01), and aged particles from northern China were found to impede atmospheric new particle formation and growth in Shanghai. A markedly enhanced efficiency of nitrate aerosol formation was observed along the transport pathways during NYH‐20, despite the overall low atmospheric NO2 levels., Key Points Higher concentrations and distinct compositions of aerosol particles were observed during the COVID‐19 shutdownFast formation of secondary inorganic aerosol contributed to high aerosol mass loadingLonger‐range, regional transport facilitated and enhanced particulate nitrate formation

Atmospheric reactive nitrogen (N r ) has been a cause of serious environmental pollution in China. Historically, China used too little N r in its agriculture to feed its population. However, with the rapid increase in N fertilizer use for food production and fossil fuel consumption for energy supply over the last four decades, increasing gaseous N r species (e.g. NH 3 and NO x ) have been emitted to the atmosphere and then deposited as wet and dry deposition, with adverse impacts on air, water and soil quality as well as plant biodiversity and human health. This paper reviews the issues associated with this in a holistic way. The emissions, deposition, impacts, actions and regulations for the mitigation of atmospheric N r are discussed systematically. Both NH 3 and NO x make major contributions to environmental pollution but especially to the formation of secondary fine particulate matter (PM 2.5 ), which impacts human health and light scattering (haze). In addition, atmospheric deposition of NH 3 and NO x causes adverse impacts on terrestrial and aquatic ecosystems due to acidification and eutrophication. Regulations and practices introduced by China that meet the urgent need to reduce N r emissions are explained and resulting effects on emissions are discussed. Recommendations for improving future N management for achieving ‘win-win’ outcomes for Chinese agricultural production and food supply, and human and environmental health, are described. This article is part of a discussion meeting issue ‘Air quality, past present and future’.

Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate (pNO3−), the end product of the oxidation of NOx gases (NO + NO2) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO3− to constrain NOx source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the δ15N values of fresh pNO3− (δ15N–pNO3−) in PM2.5 at a rural site in northern China, where atmospheric pNO3− can be attributed exclusively to biomass burning. The observed δ15N–pNO3− (12.17±1.55 ‰; n = 8) was much higher than the N isotopic source signature of NOx from biomass burning (1.04±4.13 ‰). The large difference between δ15N–pNO3− and δ15N–NOx (Δ(δ15N)) can be reconciled by the net N isotope effect (εN) associated with the gas–particle conversion from NOx to NO3−. For the biomass burning site, a mean εN( ≈ Δ(δ15N)) of 10.99±0.74 ‰ was assessed through a newly developed computational quantum chemistry (CQC) module. εN depends on the relative importance of the two dominant N isotope exchange reactions involved (NO2 reaction with OH versus hydrolysis of dinitrogen pentoxide (N2O5) with H2O) and varies between regions and on a diurnal basis. A second, slightly higher CQC-based mean value for εN (15.33±4.90 ‰) was estimated for an urban site with intense traffic in eastern China and integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates for NOx at this site. Based on the δ15N values (10.93±3.32 ‰; n = 43) of ambient pNO3− determined for the urban site, and considering the location-specific estimate for εN, our results reveal that the relative contribution of coal combustion and road traffic to urban NOx is 32 % ± 11 % and 68 %± 11 %, respectively. This finding agrees well with a regional bottom-up emission inventory of NOx. Moreover, the variation pattern of OH contribution to ambient pNO3− formation calculated by the CQC module is consistent with that simulated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), further confirming the robustness of our estimates. Our investigations also show that, without the consideration of the N isotope effect during pNO3− formation, the observed δ15N–pNO3− at the study site would erroneously imply that NOx is derived almost entirely from coal combustion. Similarly, reanalysis of reported δ15N–NO3− data throughout China and its neighboring areas suggests that NOx emissions from coal combustion may be substantively overestimated (by > 30 %) when the N isotope fractionation during atmospheric pNO3− formation is neglected.

Predicting tropospheric cloud formation and subsequent nutrient deposition relies on understanding the sources and processes affecting aerosol constituents of the atmosphere that are preserved in cloud water. However, this challenge is difficult to address quantitatively based on the sole use of bulk chemical properties. Nitrogenous aerosols, mainly ammonium (NH4+) and nitrate (NO3-), play a particularly important role in tropospheric cloud formation. While dry and wet (mainly rainfall) deposition of NH4+ and NO3- are regularly assessed, cloud water deposition is often underappreciated. Here we collected cloud water samples at the summit of Mt. Tai (1545 m above sea level) in eastern China during a long-lasting biomass burning (BB) event and simultaneously measured for the first time the isotopic compositions (mean ±1σ) of cloud water nitrogen species (δ15N-NH4+ = −6.53 ‰ ± 4.96 ‰, δ15N-NO3- = −2.35 ‰ ± 2.00 ‰, δ18O-NO3- = 57.80 ‰ ± 4.23 ‰), allowing insights into their sources and potential transformation mechanism within the clouds. Large contributions of BB to the cloud water NH4+ (32.9 % ± 4.6 %) and NO3- (28.2 % ± 2.7 %) inventories were confirmed through a Bayesian isotopic mixing model, coupled with our newly developed computational quantum chemistry module. Despite an overall reduction in total anthropogenic NOx emission due to effective emission control actions and stricter emission standards for vehicles, the observed cloud δ15N-NO3- values suggest that NOx emissions from transportation may have exceeded emissions from coal combustion. δ18O-NO3- values imply that the reaction of OH with NO2 is the dominant pathway of NO3- formation (57 % ± 11 %), yet the contribution of heterogeneous hydrolysis of dinitrogen pentoxide was almost as important (43 % ± 11 %). Although the limited sample set used here results in a relatively large uncertainty with regards to the origin of cloud-associated nitrogen deposition, the high concentrations of inorganic nitrogen imply that clouds represent an important source of nitrogen, especially for nitrogen-limited ecosystems in remote areas. Further simultaneous and long-term sampling of aerosol, rainfall, and cloud water is vital for understanding the anthropogenic influence on nitrogen deposition in the study region.

Enhanced secondary aerosol formation was observed during the COVID-19 lockdown in Xi'an, especially for polluted episodes. More oxidized‑oxygenated organic aerosol (MO-OOA) and sulfate showed the dominant enhancements, especially in large particle-mode. Meanwhile, relative humidity (RH) showed a positive promotion on the formation of sulfate and MO-OOA during the lockdown, but had no obvious correlation with less oxidized‑oxygenated organic aerosol (LO-OOA) or nitrate. Organosulfurs (OS) displayed a higher contribution (~58%) than inorganic sulfate to total sulfate enhancement in the polluted episode during the lockdown. Although the total nitrate (TN) decreased during the lockdown ascribing to a larger reduction of inorganic nitrate, organic nitrate (ON) showed an obvious increase from pre-lockdown (0.5 ± 0.6 μg m-3 and 1 ± 2% of TN) to lockdown (5.3 ± 3.1 μg m-3 and 17 ± 9% of TN) in the polluted case (P < 0.05). In addition, RH also displayed a positive promotion on the formation of ON and OS, and the increases of both OS and ON were much efficient in the nighttime than in the daytime. These results suggest that higher RH and stagnant meteorology might facilitate the sulfate and MO-OOA enhancement, especially in the nighttime, which dominated the secondary aerosol enhancement in haze pollution during the lockdown.