Spatial and Temporal Trends in Global Emissions of Nitrogen Oxides from 1960 to 2014

Unintentional emissions of polychlorinated naphthalenes in China: Sources, composition, and historical trends

Polychlorinated naphthalenes (PCNs) are detrimental to human health and the environment. With the commercial production of PCNs banned, unintentional releases have emerged as a significant environmental source. However, relevant information is still scarce. In this study, provincial emissions for eight PCNs homologues from 37 sources in the Chinese mainland during the period of 1960-2019 were estimated based on a source-specific and time-varying emission factor database. The results showed that the total PCNs emissions in 2019 reached 757.0 kg with Hebei ranked at the top among all the provinces and iron & steel industry as the biggest source. Low-chlorinated PCNs comprised 90% of emissions by mass, while highly chlorinated PCNs dominated in terms of toxicity, highlighting divergent priorities for mitigating emissions and safeguarding human health. The emissions showed an overall upward trend from 1960 to 2019 driven by emission increase from iron & steel industry in terms of source, and from North China and East China in terms of geographic area. Per-capita emissions followed an inverted U-shaped environmental Kuznets curve while emission intensities decreased with increasing per-capita Gross Domestic Product (GDP) following a nearly linear pattern when log-transformed.

Quantifying instantaneous nitrogen oxides emissions from power plants based on space observations

Thermal power plants are significant contributors to nitrogen oxides (NOx), impacting global atmospheric conditions and human health. Satellite observations, known for their continuity and global coverage, have become an effective means of quantifying power plant emissions. Previous studies, often accumulating long temporal data into integrated plumes, resulted in substantial errors in annual emissions at the individual power plant level due to neglecting variations in emissions and diffusion conditions. This study presents, for the first time, the quantification of instantaneous NOx emissions based on single overpass observations from the Tropospheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor satellite. By addressing the temporal variability of power plant emissions, it effectively reduces annual estimation errors. Comparative analysis between the Exponentially-Modified Gaussian (EMG) and Gaussian Plume Model (GPM) simulations demonstrates the capability of EMG to provide instantaneous emission estimates based on actual plumes, exhibiting closer proximity to actual monitoring values than GPM. Applying the EMG method, we quantify the instantaneous emission rates of six power plants in the United States. Comparing annual emission estimations at individual power plants with traditional integrated plume results, our method demonstrates a 63.7 % improvement in annual emission estimations. This study offers more detailed data on power plant emissions, providing a new avenue for better understanding the emission behavior of thermal power plants.

Impacts of land cover changes on summer surface ozone in China during 2000-2019

China implemented continuous forestation and experienced significant greening tendency in the past several decades. While the ecological project brings benefits to regional carbon assimilation, it also affects surface ozone (O3) pollution level through perturbations in biogenic emissions and dry deposition. Here, we use a coupled chemistry-vegetation model to assess the impacts of land use and land cover change (LULCC) on summertime surface O3 in China during 2000-2019. The LULCC is found to enhance O3 by 1-2 ppbv in already-polluted areas. In contrast, moderate reductions of -0.4 to -0.8 ppbv are predicted in southern China where the largest forest cover changes locate. Such inconsistency is attributed to the background chemical regimes with positive O3 changes over VOC-limited regions but negative changes in NOx-limited regions. The net contribution of LULCC to O3 budget in China is 24.17 Kg/s, in which the positive contribution by more isoprene emissions almost triples the negative effects by the increased dry deposition. Although the LULCC-induced O3 perturbation is much lower than the effects of anthropogenic emissions, forest expansion has exacerbated regional O3 pollution in North China Plain and is expected to further enhance surface O3 with continuous forestation in the future.

Quantification of NOx sources contribution to ambient nitrate aerosol, uncertainty analysis and sensitivity analysis in a megacity

Dual isotopes of nitrogen and oxygen of NO3- are crucial tools for quantifying the formation pathways and precursor NOx sources contributing to atmospheric nitrate. However, further research is needed to reduce the uncertainty associated with NOx proportional contributions. The acquisition of nitrogen isotopic composition from NOx emission sources lacks regulation, and its impact on the accuracy of contribution results remains unexplored. This study identifies key influencing factors of source isotopic composition through statistical methods, based on a detailed summary of δ15N-NOx values from various sources. NOx emission sources are classified considering these factors, and representative means, standard deviations, and 95 % confidence intervals are determined using the bootstrap method. During the sampling period in Tianjin in 2022, the proportional nitrate formation pathways varied between sites. For suburban and coastal sites, the ranking was [Formula: see text] (NO2 + OH radical) > [Formula: see text] (N2O5 + H2O) > [Formula: see text] (NO3 + DMS/HC), while the rural site exhibited similar fractional contributions from all three formation pathways. Fossil fuel NOx sources consistently contributed more than non-fossil NOx sources in each season among three sites. The uncertainties in proportional contributions varied among different sources, with coal combustion and biogenic soil emission showing lower uncertainties, suggesting more stable proportional contributions than other sources. The sensitivity analysis clearly identifies that the isotopic composition of 15N-enriched and 15N-reduced sources significantly influences source contribution results, emphasizing the importance of accurately characterizing the localized and time-efficient nitrogen isotopic composition of NOx emission sources. In conclusion, this research sheds light on the importance of addressing uncertainties in NOx proportional contributions and emphasizes the need for further exploration of nitrogen isotopic composition from NOx emission sources for accurate atmospheric nitrate studies.

Health burden from food systems is highly unequal across income groups

Food consumption contributes to the degradation of air quality in regions where food is produced, creating a contrast between the health burden caused by a specific population through its food consumption and that faced by this same population as a consequence of food production activities. Here we explore this inequality within China's food system by linking air-pollution-related health burden from production to consumption, at high levels of spatial and sectorial granularity. We find that low-income groups bear a 70% higher air-pollution-related health burden from food production than from food consumption, while high-income groups benefit from a 29% lower health burden relative to their food consumption. This discrepancy largely stems from a concentration of low-income residents in food production areas, exposed to higher emissions from agriculture. Comprehensive interventions targeting both production and consumption sides can effectively reduce health damages and concurrently mitigate associated inequalities, while singular interventions exhibit limited efficacy.

Revealing varying relationships between wastewater mercury emissions and economic growth in Chinese cities

Mercury emission from industrial wastewater has a great impact on the aquatic environment but is not well studied. Inventory analysis, decoupling and decomposition methods have been conducted based on the China Pollution Source Census dataset, which combines industry removal efficiencies to calculate mercury emissions from industrial wastewater in 340 cities in China during 2000-2010. The results show that over these 11 years, total mercury emissions and per capita mercury emissions increased by approximately 5 times, while the emission intensity increased by only about 3%. From 2000 to 2010, only 0.59% of cities showed strong decoupling between economic growth and mercury emissions, and 37.65% of cities showed weak decoupling, whereas 38.82% of cities showed negative decoupling. We attribute the decoupling of economic development and emissions in individual cities to several socioeconomic factors and find that a decline in emission intensity is the main driver. The Gini coefficient indicates a significant imbalance between cities' emissions, but this situation improved during 2000-2010. The objective of this article is to provide a historical perspective on the situation of mercury emissions from wastewater in China, thereby contributing' to the broader understanding of industrial pollution.

Iron and Steel Industry Emissions: A Global Analysis of Trends and Drivers

The iron and steel industry (ISI) is important for socio-economic progress but emits greenhouse gases and air pollutants detrimental to climate and human health. Understanding its historical emission trends and drivers is crucial for future warming and pollution interventions. Here, we offer an exhaustive analysis of global ISI emissions over the past 60 years, forecasting up to 2050. We evaluate emissions of carbon dioxide and conventional and unconventional air pollutants, including heavy metals and polychlorinated dibenzodioxins and dibenzofurans. Based on this newly established inventory, we dissect the determinants of past emission trends and future trajectories. Results show varied trends for different pollutants. Specifically, PM2.5 emissions decreased consistently during the period 1970 to 2000, attributed to adoption of advanced production technologies. Conversely, NOx and SO2 began declining recently due to stringent controls in major contributors such as China, a trend expected to persist. Currently, end-of-pipe abatement technologies are key to PM2.5 reduction, whereas process modifications are central to CO2 mitigation. Projections suggest that by 2050, developing nations (excluding China) will contribute 52-54% of global ISI PM2.5 emissions, a rise from 29% in 2019. Long-term emission curtailment will necessitate the innovation and widespread adoption of new production and abatement technologies in emerging economies worldwide.

Development of an integrated machine-learning and data assimilation framework for NOx emission inversion

As major air pollutants, nitrogen oxides (NO_x, mainly comprising NO and NO₂) not only have adverse effects on human health but also contribute to the formation of secondary pollutants, such as ozone and particulate nitrate. To acquire reasonable NO_x simulation results for further analysis, a reasonable emission inventory is needed for three-dimensional chemical transport models (3D-CTMs). In this study, a comprehensive emission adjustment framework for NO_x emission, which integrates the simulation results of the 3D-CTM, surface NO₂ measurements, the three-dimensional variational data assimilation method, and an ensemble back propagation neural network, was proposed and applied to correct NO_x emissions over China for the summers of 2015 and 2020. Compared with the simulation using prior NO_x emissions, the root-mean-square error, normalized mean error, and normalized mean bias decreased by approximately 40 %, 40 %, and 60 % in NO₂ simulation using posterior NO_x emissions corrected by the framework proposed in this work. Compared with the emissions for 2015, the NO_x emission generally decreased by an average of 5 % in the simulation domain for 2020, especially in Henan and Anhui provinces, where the percentage reductions reached 24 % and 19 %, respectively. The proposed framework is sufficiently flexible to correct emissions in other periods and regions. The framework can provide reliable and up-to-date emission information and can thus contribute to both scientific research and policy development relating to NO_x pollution.

Satellite spectroscopy reveals the atmospheric consequences of the 2022 Russia-Ukraine war

With increasing geopolitical conflicts and climate change, the effects of war on the atmosphere remain unclear, especially the recent large-scale war between Russia and Ukraine. Here, we assess how war affects human emission activities by observing atmospheric nitrogen dioxide (NO₂) using high-resolution satellite spectroscopy. Spatial and temporal responses of atmospheric composition to armed conflict are characterized. Significant decreases in NO₂ concentrations of 10.7-27.3 % occurred in most Ukrainian cities at the beginning of the war, in contrast to dramatic increases in NO₂ concentrations in Russian cities outside the northern border. Anomalous changes in NO₂ were also found in transportation hubs. By excluding the effect of meteorology, the machine learning model indicates that war-induced changes in anthropogenic emissions may account for ∼40 % of the reduction in NO₂ pollution for major cities such as Kyiv. Our study demonstrates that satellites can provide a unique perspective on the atmospheric consequences of humanitarian disasters.

Association between exposure to combustion-related air pollution and multiple sclerosis risk

BACKGROUND

Smoking and occupational pulmonary irritants contribute to multiple sclerosis (MS) development. We aimed to study the association between ambient air pollution and MS risk and potential interaction with the human leukocyte antigen (HLA)-DRB1*15:01 allele.

METHODS

Exposure to combustion-related air pollution was estimated as outdoor levels of nitrogen oxides (NOx) at the participants' residence locations, by spatially resolved dispersion modelling for the years 1990-18. Using two population-based case-control studies (6635 cases, 8880 controls), NOx levels were associated with MS risk by calculating odds ratios (OR) with 95% confidence intervals (CI) using logistic regression models. Interaction between high NOx levels and the HLA-DRB1*15:01 allele regarding MS risk was calculated by the attributable proportion due to interaction (AP). In addition, a register study was performed comprising all MS cases in Sweden who had received their diagnosis between 1993 and 2018 (n = 22 173), with 10 controls per case randomly selected from the National Population register.

RESULTS

Residential air pollution was associated with MS risk. NOx levels (3-year average) exceeding the 90th percentile (24.6 µg/m3) were associated with an OR of 1.37 (95% CI 1.10-1.76) compared with levels below the 25th percentile (5.9 µg/m3), with a trend of increasing risk of MS with increasing levels of NOx (P <0.0001). A synergistic effect was observed between high NOx levels (exceeding the lower quartile among controls) and the HLA-DRB1*15:01 allele regarding MS risk (AP 0.26, 95% CI 0.13-0.29).

CONCLUSIONS

Our findings indicate that moderate levels of combustion-related ambient air pollution may play a role in MS development.

Strategic roadmap to assess forest vulnerability under air pollution and climate change

Although it is an integral part of global change, most of the research addressing the effects of climate change on forests have overlooked the role of environmental pollution. Similarly, most studies investigating the effects of air pollutants on forests have generally neglected the impacts of climate change. We review the current knowledge on combined air pollution and climate change effects on global forest ecosystems and identify several key research priorities as a roadmap for the future. Specifically, we recommend (1) the establishment of much denser array of monitoring sites, particularly in the South Hemisphere; (2) further integration of ground and satellite monitoring; (3) generation of flux-based standards and critical levels taking into account the sensitivity of dominant forest tree species; (4) long-term monitoring of N, S, P cycles and base cations deposition together at global scale; (5) intensification of experimental studies, addressing the combined effects of different abiotic factors on forests by assuring a better representation of taxonomic and functional diversity across the ~73,000 tree species on Earth; (6) more experimental focus on phenomics and genomics; (7) improved knowledge on key processes regulating the dynamics of radionuclides in forest systems; and (8) development of models integrating air pollution and climate change data from long-term monitoring programs.

Investigation of the nitrogen flows of the food supply chain in Beijing-Tianjin-Hebei region, China during 1978-2017

Reactive nitrogen (Nr) is an indispensable material for food production. However, it may cause serious environmental problems. The enhancement of nitrogen management in the food supply chain is an effective way to reduce Nr loss and increase Nr use efficiency. While Nr flows in association with the food chain have synergy in a mega-region, in-depth investigations at a cross-regional scale have remained relatively undocumented. This study developed a food-related Nr flow model based on a material flow analysis for the Beijing-Tianjin-Hebei region (BTH) during the years 1978-2017. A multi-regional input-output method was applied to investigate the Nr emissions embodied in the transboundary food supply. The results showed that the total Nr emissions from the food system during the years 1978-2017 in the BTH region increased until 2004 and subsequently decreased gradually. In 2017, Beijing exhibited the lowest Nr emissions per capita (2.3 kg N/cap) and per land use (3089 kg N/km²), while Hebei and Tianjin demonstrated the greatest Nr emissions intensity by capita (13.6 kg N/cap) and by land use (6392 kg N/km²), respectively. While farming and livestock husbandry dominated the regional Nr emissions (i.e., responsible for 90% of the total in 2017), food consumption and waste management have had an increasingly substantial role, as their shared percentage in the total increased by 22% over the study period. Nr emissions resulting from the inner-transboundary food supply chain decreased by 81% between 2012 and 2015 but dramatically increased by 231% between 2015 and 2017. This rebound effect partially resulted from the implementation of coordinated development planning for the BTH region in 2015. This study can facilitate the efficient regulation of regional nitrogen flows and the desired transition of food supply chain.

Foliar stable isotope ratios of carbon and nitrogen in boreal forest plants exposed to long-term pollution from the nickel-copper smelter at Monchegorsk, Russia

Long-term exposure to primary air pollutants, such as sulphur dioxide (SO2) and nitrogen oxides (NOx), alters the structure and functions of forest ecosystems. Many biochemical and biogeochemical processes discriminate against the heavier isotopes in a mixture; thus, the values of δ13C and δ15N (i.e. the ratio of stable isotopes 13C to 12C and that of 15 N to 14 N, respectively) may give insights into changes in ecosystem processes and identify the immediate drivers of these changes. We studied sources of variation in the δ13C and δ15N values in the foliage of eight boreal forest C3 plants at 10 sites located at the distance of 1-40 km from the Monchegorsk nickel-copper smelter in Russia. From 1939‒2019, this smelter emitted over 14,000,000 metric tons (t) of SO2, 250,000 t of metals, primarily nickel and copper, and 140,000 t of NOx. The δ13C value in evergreen plants and the δ15N value in all plants increased near the smelter independently of the plant mycorrhizal type. We attribute the pollution-related increase in the foliar δ13C values of evergreen species mainly to direct effects of SO2 on stomatal conductance, in combination with pollution-related water stress, which jointly override the potential opposite effect of increasing ambient CO2 concentration on δ13C values. Stomatal uptake of NOx and root uptake of 15N-enriched organic N compounds and NH4+ may explain the increased foliar δ15N values and elevated foliar N concentrations, especially in the evergreen trees (Pinus sylvestris), close to Monchegorsk, where the soil inorganic N supply is reduced due to the impact of long-term SO2 and heavy metal emissions on plant biomass. We conclude that, despite the uncertainties in interpreting δ13C and δ15N responses to pollution, the Monchegorsk smelter has imposed and still imposes a great impact on C and N cycling in the surrounding N-limited subarctic forest ecosystems.

Investigating the Relationship between Air Pollutants and Meteorological Parameters Using Satellite Data over Bangladesh

Understanding of the relationship between air pollutants and meteorological parameters on the regional scale is a prerequisite for setting up air pollution prevention and control strategies; however, there is a lack of methodical investigations, particularly in the context of Bangladesh’s deficiency of information on air pollution. This study represents the first attempt to investigate the relationship between air pollutants (NO2, O3, SO2, and CO) and meteorological parameters over Bangladesh using satellite data (OMI and MOPITT) during the period from 2015 to 2020. Geographically weighted regression (GWR) modelling was utilized to assess the relationship between air pollutants and weather variables. The spatial representation and average values of geographically varying coefficients showed that the column densities of air pollutants were affected by the meteorological parameters. For example, NO2 was positively associated with temperature in most of the studied regions, with an average geographically varying coefficient value of 0.12 Dobson units (DU, 1 DU = 2.687 × 1016 molecules/cm2), indicating that NO2 concentrations increase by 0.12 DU/year with every unit increase in temperature. The sources of NO2 and SO2 in Dhaka were identified through emission inventory analysis, and transportation and industry emissions were the most significant influencing factors for NO2 and SO2, respectively. Temperature and pressure showed a higher degree of relationship with all four air pollutants compared with other parameters. The results and discussion presented in this study can be of benefit for policy makers in developing air pollution control strategies in Bangladesh.

Considerable Unaccounted Local Sources of NOx Emissions in China Revealed from Satellite

High-resolution (e.g., 5 km) emission data of nitrogen oxides (NO_x = NO + NO₂) provide localized knowledge of pollution sources for targeted regulations, yet such data are lacking or inaccurate over most regions at present. Here we improve our PHLET-based inversion method to derive NO_x emissions in China at a 5-km resolution in summer 2019, based on the TROPOMI-POMINO satellite product of nitrogen dioxide (NO₂) columns. With low computational costs, our inversion explicitly accounts for the effects of horizontal transport and nonlinear chemistry. We find numerous small-to-medium sources related to minor roads and small human settlements at relatively low affluence levels, in addition to clear emission signals along major transportation lines, consistent with road line density and Tencent location data. Many small-to-medium sources and transportation emissions are unclear or missing in the spatial distributions of four widely used emission inventories. Our emissions offer a unique reference for targeted emission control.

Isotopic constraints confirm the significant role of microbial nitrogen oxides emissions from the land and ocean environment

Nitrogen oxides (NO_x, the sum of nitric oxide (NO) and N dioxide (NO₂)) emissions and deposition have increased markedly over the past several decades, resulting in many adverse outcomes in both terrestrial and oceanic environments. However, because the microbial NO_x emissions have been substantially underestimated on the land and unconstrained in the ocean, the global microbial NO_x emissions and their importance relative to the known fossil-fuel NO_x emissions remain unclear. Here we complied data on stable N isotopes of nitrate in atmospheric particulates over the land and ocean to ground-truth estimates of NO_x emissions worldwide. By considering the N isotope effect of NO_x transformations to particulate nitrate combined with dominant NO_x emissions in the land (coal combustion, oil combustion, biomass burning and microbial N cycle) and ocean (oil combustion, microbial N cycle), we demonstrated that microbial NO_x emissions account for 24 ± 4%, 58 ± 3% and 31 ± 12% in the land, ocean and global environment, respectively. Corresponding amounts of microbial NO_x emissions in the land (13.6 ± 4.7 Tg N yr⁻¹), ocean (8.8 ± 1.5 Tg N yr⁻¹) and globe (22.5 ± 4.7 Tg N yr⁻¹) are about 0.5, 1.4 and 0.6 times on average those of fossil-fuel NO_x emissions in these sectors. Our findings provide empirical constraints on model predictions, revealing significant contributions of the microbial N cycle to regional NO_x emissions into the atmospheric system, which is critical information for mitigating strategies, budgeting N deposition and evaluating the effects of atmospheric NO_x loading on the world.

Global Emissions of Hydrogen Chloride and Particulate Chloride from Continental Sources

Gaseous and particulate chlorine species play an important role in modulating tropospheric oxidation capacity, aerosol water uptake, visibility degradation, and human health. The lack of recent global continental chlorine emissions has hindered modeling studies of the role of chlorine in the atmosphere. Here, we develop a comprehensive global emission inventory of gaseous HCl and particulate Cl- (pCl), including 35 sources categorized in six source sectors based on published up-to-date activity data and emission factors. These emissions are gridded at a spatial resolution of 0.1° × 0.1° for the years 1960 to 2014. The estimated emissions of HCl and pCl in 2014 are 2354 (1661-3201) and 2321 (930-3264) Gg Cl a-1, respectively. Emissions of HCl are mostly from open waste burning (38%), open biomass burning (19%), energy (19%), and residential (13%) sectors, and the major sources classified by fuel type are combustion of waste (43%), biomass (32%), and coal (25%). Emissions of pCl are mostly from biofuel (29%) and open biomass burning processes (44%). The sectoral and spatial distributions of HCl and pCl emissions are very heterogeneous along the study period, and the temporal trends are mainly driven by the changes in emission factors, energy intensity, economy, and population.

A blessing in disguise: new insights on the effect of COVID-19 on the carbon emission, climate change, and sustainable environment

COVID-19, declared by the World Health Organization (WHO) to be a pandemic, has affected greenhouse gas emissions and contributed to the uncertainty of environmental activities. This study demonstrates the effect of lockdowns, the number of new confirmed cases, and the number of newly confirmed deaths due to COVID-19 on CO₂ emissions. The data series used are for the UK from 23 March 2020 to 31 December 2020 and for Spain from 14 March 2020 to 31 December 2020. This research adopted the Augmented Dickey-Fuller (ADF) test for a stationarity check of the data series, the Johansen cointegration test for determining cointegration among variables, and the vector error correction model (VEC) Granger causality test for directional cause and effect between exogenous and endogenous variables. The VEC model shows a bidirectional relationship between CO₂ emissions and lockdown and a unidirectional relationship with newly confirmed cases and deaths for the UK. The results of Spain confirmed the unidirectional relationship of CO₂ emissions, lockdown, new confirmed cases, and deaths. The Granger causality test reconfirms the relationship of variables except for newly confirmed deaths for the UK and newly confirmed cases for Spain. Conclusively, the pandemic breakout reduced the emission of CO₂. The directional relation of variables supported the short-run relationship of CO₂ emissions with newly confirmed cases and deaths, while a long- and short-run relationship was shown with lockdown. The directional and relational behavior of lockdown potentially linked the CO₂ emissions with daily life activities.

Inequality in historical transboundary anthropogenic PM2.5 health impacts

Atmospheric transport of fine particulate matter (PM_2.5), the leading environmental risk factor for public health, is estimated to exert substantial transboundary effects at present. During the past several decades, human-produced pollutant emissions have undergone drastic and regionally distinctive changes, yet it remains unclear about the resulting global transboundary health impacts. Here we show that between 1950 and 2014, global anthropogenic PM_2.5 has led to 185.7 million premature deaths cumulatively, including about 14% from transboundary pollution. Among four country groups at different affluence levels, on a basis of per capita contribution to transboundary mortality, a richer region tends to exert severer cumulative health externality, with the poorest bearing the worst net externality after contrasting import and export of pollution mortality. The temporal changes in transboundary mortality and cross-regional inequality are substantial. Effort to reduce PM_2.5-related transboundary mortality should seek international collaborative strategies that account for historical responsibility and inequality.

Atmospheric emissions of PCDDs and PCDFs in China from 1960 to 2014

Quantification of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) is a requirement of the Stockholm Convention on persistent organic pollutants (POPs), and essential to evaluate and understand their environmental fate and associated health risks. Unfortunately, works estimating the emission of PCDD/Fs in China are limited, especially in terms of historical trends and information on spatial distribution. In this study, provincial emissions of 17 toxic PCDD/Fs congeners from 79 sources were quantified from 1960 to 2014, and 0.1º × 0.1º gridded emissions for 2014 were obtained by applying a source-specific, annually varying emission factor (EF) dataset with similar time trends as measurements for China. Historical national PCDD/F emissions showed an increasing trend until around 1980, and then plateaued due to decreased emissions from cement production and waste burning. Decreased emissions from cement production and waste burning in northeast, east, and south China, and Taiwan province were the main causes for the stabilized national emissions after 1980. Spatially, highly positive correlations of emission densities with population and GDP densities were identified, but no clear temporal patterns were observed. Emission densities showed a decreasing trend in the order of cities, towns and rural areas, while the opposite was seen for per capita emissions.

Highly-resolved spatial-temporal variations of air pollutants from Chinese industrial boilers

Industrial boilers are a significant anthropogenic source of air pollutant emissions. In this study, a county-based atmospheric emission inventory of particulate matter (PM), PM₁₀, PM_2.5, SO₂, NOx, organic carbon (OC) and elemental carbon (EC) from industrial boilers over mainland China in 2017 was developed for the first time, based on county-level activity data from ~61,000 coal-fired industrial boilers (CFIBs), ~44,000 biomass-fired industrial boilers (BFIBs), ~71,000 gas-fired industrial boilers (GFIBs) and ~9300 oil-fired industrial boilers (OFIBs), updated emission factors (EFs) and air pollution control device (APCD) efficiencies. The total national PM, PM_2.5, PM₁₀, SO₂, NOx, OC and EC emissions from industrial boilers in 2017 were estimated to be 1,240, 347, 761, 1,648, 1,340, 13.1 and 15.8 kilotons (kt), respectively. Intensive air pollutant emissions from industrial boilers of more than 1000 kg/km² were predominantly in north-eastern, northern and eastern China. CFIBs contributed the most (77.6-94.0 %) to air pollutant emissions because of their high air pollutant EFs and the large amounts of coal consumed. BFIBs were the second-highest contributor to national air pollutant emissions, with the contribution of BFIBs to PM_2.5, OC and EC emissions in central and southern China reaching up to 42.1 %, 61.7 % and 45.5 %, respectively. There were seasonal peaks in monthly air pollutant emissions in heating regions. The overall uncertainty realting to the new emission inventory was estimated as -25.9 %-22.7 %. Significant air pollutant emission reductions were obtained from 2017 to 2030, and by 2030 the PM, PM₁₀, PM_2.5, SO₂ and NOx emissions were forecast to decrease by 40.1-84.0 %, 41.6-84.3 %, 44.5-75.2 %, 44.5-75.2 % and 19.5-46.8 % compared to 2017, respectively, under four proposed scenarios. The results of this study showed that differentiated industrial boiler management measures should be developed according to the actual emission characteristics. This work developed a county-based atmospheric emission inventory of PM, PM₁₀, PM_2.5, SO₂, NOx, OC and EC from Chinese industrial boilers in 2017 for the first time.

A robust approach to deriving long-term daily surface NO2 levels across China: Correction to substantial estimation bias in back-extrapolation

BACKGROUND

Long-term surface NO₂ data are essential for retrospective policy evaluation and chronic human exposure assessment. In the absence of NO₂ observations for Mainland China before 2013, training a model with 2013-2018 data to make predictions for 2005-2012 (back-extrapolation) could cause substantial estimation bias due to concept drift.

OBJECTIVE

This study aims to correct the estimation bias in order to reconstruct the spatiotemporal distribution of daily surface NO₂ levels across China during 2005-2018.

METHODS

On the basis of ground- and satellite-based data, we proposed the robust back-extrapolation with a random forest (RBE-RF) to simulate the surface NO₂ through intermediate modeling of the scaling factors. For comparison purposes, we also employed a random forest (Base-RF), as a representative of the commonly used approach, to directly model the surface NO₂ levels.

RESULTS

The validation against Taiwan's NO₂ observations during 2005-2012 showed that RBE-RF adequately corrected the substantial underestimation by Base-RF. The RMSE decreased from 10.1 to 8.2 µg/m³, 7.1 to 4.3 µg/m³, and 6.1 to 2.9 µg/m³ in predicting daily, monthly, and annual levels, respectively. For North China with the most severe pollution, the population-weighted NO₂ ([NO₂]_pw) during 2005-2012 was estimated as 40.2 and 50.9 µg/m³ by Base-RF and RBE-RF, respectively, i.e., 21.0% difference. While both models predicted that the national annual [NO₂]_pw increased during 2005-2011 and then decreased, the interannual trends were underestimated by >50.2% by Base-RF relative to RBE-RF. During 2005-2018, the nationwide population that lived in the areas with NO₂ > 40 µg/m³ were estimated as 259 and 460 million by Base-RF and RBE-RF, respectively.

CONCLUSION

With RBE-RF, we corrected the estimation bias in back-extrapolation and obtained a full-coverage dataset of daily surface NO₂ across China during 2005-2018, which is valuable for environmental management and epidemiological research.

Life Cycle Assessment of Nitrogen Circular Economy-Based NOx Treatment Technology

Humans are significantly perturbing the global nitrogen cycle, leading to excess reactive nitrogen in the environment. Nitrogen oxides as a key reactive nitrogen species are mainly controlled by selective non-catalytic reduction and selective catalytic reduction. Converting nitrogen oxides to ammonia, defined as ReNOx, emerges as an alternative method under a disparate design concept. However, little is known about its overall environmental performance. In this study, we conducted for the first time a life cycle assessment of ReNOx. Compared with the eco-index in the condition of 200 °C with a conversion rate of 95%, it would increase substantially in the condition of 160 °C with a conversion rate of 80% and in the case without a sound NH3 treatment. Feedstock format change, adsorption material performance deterioration, and recovery rate decline would increase the eco-index by 8%, 12%, and 18%, respectively. The eco-index was decreased by 31% in the optimized scenario with a renewable energy source and an increased conversion rate. The environmental impacts were compared with traditional methods at impact, damage, and eco-index levels. Finally, the implications on process arrangement in the flue gas system, the externality for power generation, and the contribution to the nitrogen circular economy were examined. The results can serve as a reference for its developers to improve the technology from the environmental perspective.

Updated Global Black Carbon Emissions from 1960 to 2017: Improvements, Trends, and Drivers

Accurate estimation of black carbon (BC) emissions is essential for assessing the health and climate impact of this pollutant. Past emission inventories were associated with high uncertainty due to data limitations, and recent information has provided a unique updating opportunity. Moreover, understanding the drivers that cause temporal emission changes is of research value. Here, we update the global BC emission estimates using new data on the activities and emission factors (EFs). The new inventory covers 73 detailed sources at 0.1° × 0.1° spatial resolution and monthly temporal resolution from 1960 to 2017. The estimated annual emissions were 32% higher than the average of several previous inventories, which was primarily due to field-measured EFs for residential stoves and differentiated EFs for motor vehicles. In addition, the updated emissions show an inverse U-shaped temporal trend, which was mainly driven by the interaction between the positive effects of population growth, per capita energy consumption, and vehicle fleet and the negative effects of residential energy switching, stove upgrading, phasing out of beehive coke ovens, and reduced EFs for vehicles and industrial processes. Urbanization caused a significant increase in urban emissions accompanied by a more significant decline in rural emissions.

The Warming Climate Aggravates Atmospheric Nitrogen Pollution in Australia

Australia is a warm country with well-developed agriculture and a highly urbanized population. How these specific features impact the nitrogen cycle, emissions, and consequently affect environmental and human health is not well understood. Here, we find that the ratio of reactive nitrogen (N r ) losses to air over losses to water in Australia is 1.6 as compared to values less than 1.1 in the USA, the European Union, and China. Australian N r emissions to air increased by more than 70% between 1961 and 2013, from 1.2 Tg N yr-1 to 2.1 Tg N yr-1. Previous emissions were substantially underestimated mainly due to neglecting the warming climate. The estimated health cost from atmospheric N r emissions in Australia is 4.6 billion US dollars per year. Emissions of N r to the environment are closely correlated with economic growth, and reduction of N r losses to air is a priority for sustainable development in Australia.

Coal Is Dirty, but Where It Is Burned Especially Matters

Coal abatement actions for pollution reduction often target total coal consumption. The health impacts of coal uses, however, vary extensively among sectors. Here, we modeled the sectorial contributions of coal uses to emissions, outdoor and indoor PM_2.5 (particulate matter with an aerodynamic diameter of less than 2.5 mm) concentrations, exposures, and health outcomes in China from 1970 to 2014. We show that in 2014, residential coal accounted for 2.9% of total energy use but 34% of premature deaths associated with PM_2.5 exposure, showing that effects were magnified substantially along the causal path. The number of premature deaths attributed to unit coal consumption in the residential sector was 40 times higher than that in the power and industrial sectors. Emissions of primary PM_2.5 were more important than secondary aerosol precursors in terms of health consequences, and indoor exposure accounted for 97% and 91% of total premature deaths attributable to PM_2.5 from coal combustion in 1974 and 2014, respectively. Our assessment raises a critical challenge in the switching of residential coal uses to effectively mitigate PM_2.5 exposure in the Chinese population.

Effect of Calcination Temperature on the Activation Performance and Reaction Mechanism of Ce-Mn-Ru/TiO2 Catalysts for Selective Catalytic Reduction of NO with NH3

In this study, anatase TiO₂-supported cerium, manganese, and ruthenium mixed oxides (CeO _x -MnO _x -RuO _x /TiO₂; CMRT catalysts) were synthesized at different calcination temperatures via conventional impregnation methods and used for selective catalytic reduction (SCR) of NO _x with NH₃. The effect of calcination temperature on the structure, redox properties, activation performance, surface-acidity properties, and catalytic properties of the CMRT catalysts was investigated. The results show that the CMRT catalyst calcined at 350 °C exhibits the most efficient low-temperature (<120 °C) denitration activity. Moreover, the selective catalytic oxidation (SCO) reaction of ammonia is intensified when the reaction temperature is >200 °C, which leads to a decrease in the N₂ selectivity of the CMRT catalysts and further results in an increase in the production of NO and N₂O byproducts. X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy show that the CMRT catalyst calcined at 350 °C contains more Ce⁴⁺, Mn⁴⁺, Ru⁴⁺, and lattice oxygen, which greatly improve the catalyst's ability to activate NO that promotes the NH₃-SCR reaction. The Ru ⁿ⁺ sites of the CMRT catalyst calcined at 250 °C are the competitive adsorption sites of NO and NH₃ molecules, while those of the CMRT catalyst calcined at 350 and 450 °C are active sites. Both the Langmuir-Hinshelwood (L-H) mechanism and the Eley-Rideal (E-R) mechanism occur on the surface of the CMRT catalyst at the low reaction temperature (100 °C).

Impact of the COVID-19 Pandemic Lockdown on Air Pollution in 20 Major Cities around the World

In order to fight against the spread of COVID-19, the most hard-hit countries in the spring of 2020 implemented different lockdown strategies. To assess the impact of the COVID-19 pandemic lockdown on air quality worldwide, Air Quality Index (AQI) data was used to estimate the change in air quality in 20 major cities on six continents. Our results show significant declines of AQI in NO2, SO2, CO, PM2.5 and PM10 in most cities, mainly due to the reduction of transportation, industry and commercial activities during lockdown. This work shows the reduction of primary pollutants, especially NO2, is mainly due to lockdown policies. However, preexisting local environmental policy regulations also contributed to declining NO2, SO2 and PM2.5 emissions, especially in Asian countries. In addition, higher rainfall during the lockdown period could cause decline of PM2.5, especially in Johannesburg. By contrast, the changes of AQI in ground-level O3 were not significant in most of cities, as meteorological variability and ratio of VOC/NOx are key factors in ground-level O3 formation.

Differentiated-Rate Clean Heating Strategy with Superior Environmental and Health Benefits in Northern China

Residential heating using solid fuels contributes significantly to air pollution and has subsequent health impacts in China. To mitigate emissions, a clean heating campaign (CHC-1) covering 28 municipalities has been implemented. Although only a single penetration rate was initially planned by CHC-1 for all municipalities, outcomes in the different municipalities varied considerably. Recently, a second phase (CHC-2) has been launched for the remaining 128 municipalities in northern China with once again a fixed penetration rate set. Here, we quantified factors that affected the penetration rates of CHC-1, developed an intervention scheme with differentiated targets for CHC-2, and compared the environmental and health benefits of the fixed- and differentiated-rate strategies. We found that the penetration rates of CHC-1 depended on per capita income, terrain slope, and population density and that such relationships could be quantified using a piecewise regression model. This model was applied to develop a differentiated-rate strategy for CHC-2. It clearly evidenced that a differentiated scheme would be more environmentally beneficial. Although the same number of rural households can achieve clean heating under both intervention scenarios, the proposed differentiated strategy can prevent 30 000 (23 000-34 000) premature deaths associated with residential heating annually compared to the 26 000 (21 000-31 000) premature deaths prevented under the fixed-rate scheme. Differences among gender and age groups and the effects of urbanization and aging are also discussed.

Estimations of Long-Term nss-SO42- and NO3- Wet Depositions over East Asia by Use of Ensemble Machine-Learning Method

Wet deposition of non-sea-salt sulfate (nss-SO₄^2-) and nitrate (NO₃^-), derived from anthropogenic emissions of SO₂ and NO_x, exerts adverse effects on ecosystems. In this work, an ensemble back-propagation neural network was proposed to estimate the long-term wet depositions of nss-SO₄^2- (2005-2017) and NO₃^- (2001-2014) over East Asia in 10 km resolution. The R² values for the 10-fold cross-validation of annual wet depositions of nss-SO₄^2- and NO₃^- were 0.90 and 0.85, respectively. The hotspots of the wet deposition of these two acidic species span southwestern, central, and eastern China. The molar ratio of NO₃^- to nss-SO₄^2- increased in 10 out of 12 analyzed East Asian countries from 2005 to 2014, which indicates that the acidity in rainwater shifts from the sulfur type to nitrogen type over most of the regions. The wet deposition on the four ecosystems (forest, grassland, cropland, and freshwater body) was also analyzed. Results showed that the nss-SO₄^2- wet deposition on 25.5% of freshwater bodies in 2015 and NO₃^- wet deposition on 21.7% of grassland in 2014 exceeded the ecosystem empirical critical loads (25 kg/ha sulfate and 2 kg N/ha) in East Asia. Thus, more stringent and regionally collaborative sulfur and nitrogen emission-control measures are urgently needed to protect the ecosystem of East Asia.

An inter-comparative evaluation of PKU-FUEL global SO2 emission inventory

PKU-FUEL is a recently developed gridded global emission inventory for multiple air pollutants that uses a bottom-up approach. The inventory includes data collected monthly for the period of 1960 to 2014 and at a 0.1° × 0.1° latitude/longitude resolution. In an effort to evaluate and improve this emission inventory, the PKU-FUEL Sulfur Dioxide (SO₂) emission inventory was compared to other currently available and widely used global SO₂ emission inventories constructed based on bottom-up and top-down approaches, including CEDS and OMI-HTAP. While PKU-FUEL is capable of capturing SO₂ emissions across the globe and particularly in Asia, it misses 41 industrial point sources globally, accounting for 9.3% of Ozone Monitoring Instrument (OMI) remote sensing-measured industrial point sources. Most of these missing point sources are identified in Latin America, the Middle East (~60%), and some remote places. To improve the PKU-FUEL SO₂ inventory, we applied OMI-measured emissions to sources missing from PKU-FUEL. GEOS-Chem model simulations were performed to evaluate original and improved PKU-FUEL SO₂ inventories against measured SO₂ concentrations across the world. Results were further compared to GEOS-Chem modeled SO₂ concentrations using the CEDS inventory. We show that the modeled SO₂ concentrations determined using both CEDS and improved PKU-FUEL inventories to a large extent corroborate sampled data and that the improved PKU-FUEL performs better for those regions lacking monitoring data.

Global Sulfur Dioxide Emissions and the Driving Forces

The presence of sulfur dioxide (SO₂) in the air is a global concern because of its severe environmental and public health impacts. Recent evidence from satellite observations shows rapid changes in the spatial distribution of global SO₂ emissions, but such features are generally missing in global emission inventories that use a bottom-up method due to the lack of up-to-date information, especially in developing countries. Here, we rely on the latest data available on emission activities, control measures, and emission factors to estimate global SO₂ emissions for the period 1960-2014 on a 0.1° × 0.1° spatial resolution. We design two counterfactual scenarios to isolate the contributions of emission activity growth and control measure deployment on historical SO₂ emission changes. We find that activity growth has been the major factor driving global SO₂ emission changes overall, but control measure deployment is playing an increasingly important role. With effective control measures deployed in developed countries, the predominant emission contributor has shifted from developed countries in the early 1960s (61%) to developing countries at present (83%). Developing countries show divergency in mitigation strategies and thus in SO₂ emission trends. Stringent controls in China are driving the recent decline in global emissions. A further reduction in SO₂ emissions would come from a large number of developing nations that currently lack effective SO₂ emission controls.

Similarities and Differences in the Temporal Variability of PM2.5 and AOD Between Urban and Rural Stations in Beijing

Surface particulate matter with an aerodynamic diameter of <2.5 μm (PM2.5) and column-integrated aerosol optical depth (AOD) exhibits substantial diurnal, daily, and yearly variabilities that are regionally dependent. The diversity of these temporal variabilities in urban and rural areas may imply the inherent mechanisms. A novel time-series analysis tool developed by Facebook, Prophet, is used to investigate the holiday, seasonal, and inter-annual patterns of PM2.5 and AOD at a rural station (RU) and an urban station (UR) in Beijing. PM2.5 shows a coherent decreasing tendency at both stations during 2014–2018, consistent with the implementation of the air pollution action plan at the end of 2013. RU is characterized by similar seasonal variations of AOD and PM2.5, with the lowest values in winter and the highest in summer, which is opposite that at UR with maximum AOD, but minimum PM2.5 in summer and minimum AOD, but maximum PM2.5 in winter. During the National Day holiday (1–7 October), both AOD and PM2.5 holiday components regularly shift from negative to positive departures, and the turning point generally occurs on October 4. AODs at both stations steadily increase throughout the daytime, which is most striking in winter. A morning rush hour peak of PM2.5 (7:00–9:00 local standard time (LST)) and a second peak at night (23:00 LST) are observed at UR. PM2.5 at RU often reaches minima (maxima) at around 12:00 LST (19:00 LST), about four hours later (earlier) than UR. The ratio of PM2.5 to AOD (η) shows a decreasing tendency at both stations in the last four years, indicating a profound impact of the air quality control program. η at RU always begins to increase about 1–2 h earlier than that at UR during the daytime. Large spatial and temporal variations of η suggest that caution should be observed in the estimation of PM2.5 from AOD.

The impact of urban agglomeration on ozone precursor conditions: A systematic investigation across global agglomerations utilizing multi-source geospatial datasets

Urbanization significantly influences ozone via two conditions of its formation: 1) precursor concentration; and 2) chemical regime. Recently, there has been raised concern about the influence of urban agglomerations on these two conditions. Although valuable efforts have been made, some contrary viewpoints exist. Meanwhile, urban agglomerations in developed and developing regions are experiencing different urbanization processes, so a systematic comparison between these two regions is warranted. In this context, by leveraging multi-source geospatial datasets, this paper systematically gauges the influence of urban agglomerations on ozone precursor conditions and further investigates the spatiotemporal variations. Based on the analysis of 71 global agglomerations during 2005-2016, it is found that: 1) not all urban agglomerations have a positive effect on ozone precursor conditions; 2) the negative effects of urban agglomerations can be attributed to the low latitudes and the ecological areas (p < 0.05); 3) the agglomeration influence intensifies with the increase of built-up area, population, and latitude (p < 0.05); 4) the anthropogenic nitrogen oxide (NO_x) emission from all sectors can aggravate the magnitude of the urban agglomeration influence (p < 0.05), while for volatile organic compounds (VOC_s), only the contribution of industrial emissions is significant (p < 0.05); and 5) in view of the temporal dynamics, the influence of urban agglomeration on ozone precursor condition is opposite in developed and developing regions. This study will provide important insights for future urban agglomeration studies and ozone pollution monitoring with geospatial datasets.

Biochar effects on crop yields and nitrogen loss depending on fertilization

Biochar (BC) application to low fertility soils is a promising approach to increase crop yield, improve soil quality, and mitigate climate change simultaneously. Only few studies evaluated the combined effects of BC and nitrogen (N) fertilization rates on crop productivity and N losses under field conditions. The objectives were to investigate combined effects of BC (2 rates) and N (5 rates) fertilization on crop productivity and N losses in a long-term field experiment started in 2008 in a winter wheat/summer maize rotation system in the North China Plain. Linear-plateau models best described the responses of wheat and maize yields to N rates. N₂O fluxes, NH₃ volatilization, and soil mineral N contents increased exponentially with N fertilization rates. Despite the effect of BC on wheat or maize yields was negligible, BC retains of mineral N at 240 kg N ha^-1 yr^-1. BC application increased NH₃ volatilization by 31% in wheat season and 26% in maize season because of pH increase. BC reduced N₂O emissions by 8-23% in the wheat season and by 24% at lower N rates (≤60 kg ha^-1) in the maize season, due to BC induced complete denitrification to N₂. BC stimulated N₂O emissions by 18-26% compared to soils without BC in maize season at N rates higher than 60 kg ha^-1. The combination of increased mineral N retention and C availability with BC addition increased nitrification and/or denitrification rates, leading to increased N₂O emissions. For the wheat/maize rotation system, BC application decreased N₂O emissions at lower N rates (≤120 kg ha^-1 yr^-1) but had no effects at higher N rates.

A WRF-Chem model-based future vehicle emission control policy simulation and assessment for the Beijing-Tianjin-Hebei region, China

Using 2025 as the target year, we quantitatively assessed the reduction potentials of emissions of primary pollutants (including CO, HC, NO_x, PM_2.5 and PM₁₀) under different vehicle control policies and the impacts of vehicle emission control policies in the BTH region on the regional PM_2.5 concentration in winter and the surface ozone (O₃) concentration in summer. Comparing the different scenarios, we found that (1) vehicle control policies will bring significant reductions in the emissions of primary pollutants. Among the individual policies, upgrading new vehicle emission standards and fuel quality in Beijing, Tianjin, and Hebei will be the most effective policy, with emission reductions of primary pollutants of 26.3%-54.7%, 38.0%-70.3% and 46.0%-81.6% in 2025, respectively; (2) for PM_2.5 in winter, the Combined Scenario (CS) will lead to a reduction of 0.5-3.9 μg m^-3 (3.5%-11.6%) for the monthly average PM_2.5 concentrations in most areas. The monthly nitrate and ammonium concentrations would reduce by 5.8% and 5.3%, respectively, in the whole BTH region, indicating that vehicle emission control policies may play an important role in the reduction of PM_2.5 concentrations in winter, especially for nitrate aerosols; and (3) for O₃ concentrations in summer, vehicle emission control policies will lead to significant decreases. Under the CS scenario, the maximum reduction of monthly average O₃ concentrations in the summer is approximately 3.6 ppb (5.9%). Most areas in the BTH region have a decrease of 15 ppb (7.5%) in peak values compared to the base scenario. However, in some VOC-sensitive areas in the BTH region, such as the southern urban areas, significant reductions in NO_x may lead to increases in ozone concentrations. Our results highlight that season- and location-specific vehicle emission control measures are needed to alleviate ambient PM_2.5 and O₃ pollution effectively in this region due to the complex meteorological conditions and atmospheric chemical reactions.

Toward the improvement of total nitrogen deposition budgets in the United States

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

Evolution of the life cycle primary PM2.5 emissions in globalized production systems

Production system-related air pollution emissions are dominant components in global emission reduction targets and in realizing relevant sustainable development goals (SDGs). To better understand the air pollution emissions induced by globalized production systems through a life cycle perspective, environmental extended multiregional input-output (EE-MRIO) analysis was applied to calculate the primary product-based emissions and the final product-based emissions embodied in the global production systems. Combined with two types of linkage analysis, named the hypothetical extraction method (HEM) and the emissions pure backward linkage (EPBL), emissions were analysed at three scopes at the sector level from macro sector linkage perspectives. An illustrative analysis was presented based on the global EXIOBASE MRIO database and primary PM_2.5 emissions from 1995 to 2011. The results show that from 1995 to 2011, the primary PM_2.5 emissions in the global production systems increased by 35%. In 2011, China's production system generated the highest primary product-based and final product-based primary PM_2.5 emissions, which accounted for 30.7% and 29.6% of the global total, respectively. The emission flows balance between primary product-based emissions and final product-based emissions revealed that most developing countries are sources of emissions and that developed countries are sinks of emissions in production systems. An approximately U-shaped relationship was found in the primary PM_2.5 emissions embodied in final products, while the opposite relationship was found embodied in primary products. Meanwhile, sector-specific protocols for controlling the high indirect emissions sectors can make the supply chain cleaner. Our findings further indicated that focusing more on industries can help relevant emissions control policymaking processes.

Energy and air pollution benefits of household fuel policies in northern China

In addition to many recent actions taken to reduce emissions from energy production, industry, and transportation, a new campaign substituting residential solid fuels with electricity or natural gas has been launched in Beijing, Tianjin, and 26 other municipalities in northern China, aiming at solving severe ambient air pollution in the region. Quantitative analysis shows that the campaign can accelerate residential energy transition significantly, and if the planned target can be achieved, more than 60% of households are projected to remove solid fuels by 2021, compared with fewer than 20% without the campaign. Emissions of major air pollutants will be reduced substantially. With 60% substitution realized, emission of primary PM2.5 and contribution to ambient PM2.5 concentration in 2021 are projected to be 30% and 41% of those without the campaign. With 60% substitution, average indoor PM2.5 concentrations in living rooms in winter are projected to be reduced from 209 (190 to 230) μg/m3 to 125 (99 to 150) μg/m3 The population-weighted PM2.5 concentrations can be reduced from 140 μg/m3 in 2014 to 78 μg/m3 or 61 μg/m3 in 2021 given that 60% or 100% substitution can be accomplished. Although the original focus of the campaign was to address ambient air quality, exposure reduction comes more from improved indoor air quality because ∼90% of daily exposure of the rural population is attributable to indoor air pollution. Women benefit more than men.

Impacts of air pollutants from rural Chinese households under the rapid residential energy transition

Rural residential energy consumption in China is experiencing a rapid transition towards clean energy, nevertheless, solid fuel combustion remains an important emission source. Here we quantitatively evaluate the contribution of rural residential emissions to PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm) and the impacts on health and climate. The clean energy transitions result in remarkable reductions in the contributions to ambient PM2.5, avoiding 130,000 (90,000-160,000) premature deaths associated with PM2.5 exposure. The climate forcing associated with this sector declines from 0.057 ± 0.016 W/m2 in 1992 to 0.031 ± 0.008 W/m2 in 2012. Despite this, the large remaining quantities of solid fuels still contributed 14 ± 10 μg/m3 to population-weighted PM2.5 in 2012, which comprises 21 ± 14% of the overall population-weighted PM2.5 from all sources. Rural residential emissions affect not only rural but urban air quality, and the impacts are highly seasonal and location dependent.

Satellite-derived PM2.5 concentration trends over Eastern China from 1998 to 2016: Relationships to emissions and meteorological parameters

Fine particulate matter (PM_2.5) pollution in Eastern China (EC) has raised concerns due to its adverse effects on air quality, climate, and human health. This study investigated the long-term variation trend in satellite-derived PM_2.5 concentrations and how it was related to pollutant emissions and meteorological parameters over EC and seven regions of interest (ROIs) during 1998-2016. Over EC, the annual mean PM_2.5 increased before 2006 due to the enhanced emissions of primary PM_2.5, NO_x and SO₂, but decreased with the reduced SO₂ emissions after 2006 evidently in response to China's clean air policies. In addition, results from statistical analyses indicated that in the North China Plain (NCP), Northeast China (NEC), Sichuan Basin (SCB) and Central China (CC) planetary boundary layer height (PBLH) was the dominant meteorological driver for the PM_2.5 decadal changes, and in the Pearl River Delta (PRD) wind speed is the leading factor. Overall, the variation in meteorological parameters accounted for 48% of the variances in PM_2.5 concentrations over EC. The population-weighted PM_2.5 over EC increased from 36.4 μg/m³ in 1998-2004 (P1) to 49.4 μg/m³ in 2005-2010 (P2) then decreased to 46.5 μg/m³ in 2011-2016 (P3). In the NCP and NEC, the percentages of the population living above the World Health Organization (WHO) Interim Target-1 (IT-1, 35 μg/m³) have risen steadily over the past 20 yr, reaching maxima of 97.3% and 78.8% in P3, respectively, but decreases of ∼30% from P2 to P3 were found for the SCB and PRD.

Origin and Radiative Forcing of Black Carbon Aerosol: Production and Consumption Perspectives

Air pollution, a threat to air quality and human health, has attracted ever-increasing attention in recent years. In addition to having local influence, air pollutants can also travel the globe via atmospheric circulation and international trade. Black carbon (BC), emitted from incomplete combustion, is a unique but representative particulate pollutant. This study tracked down the BC aerosol and its direct radiative forcing to the emission sources and final consumers using the global chemical transport model (MOZART-4), the rapid radiative transfer model for general circulation simulations (RRTM), and a multiregional input-output analysis (MRIO). BC was physically transported (i.e., atmospheric transport) from western to eastern countries in the midlatitude westerlies, but its magnitude is near an order of magnitude higher if the virtual flow embodied in international trade is considered. The transboundary effects on East and South Asia by other regions increased from about 3% (physical transport only) to 10% when considering both physical and virtual transport. The influence efficiency on East Asia was also large because of the comparatively large emission intensity and emission-intensive exports (e.g., machinery and equipment). The radiative forcing in Africa imposed by consumption from Europe, North America, and East Asia (0.01 Wm^-2) was even larger than the total forcing in North America. Understanding the supply chain and incorporating both atmospheric and virtual transport may improve multilateral cooperation on air pollutant mitigation both domestically and internationally.

Trends in onroad transportation energy and emissions

Globally, 1.3 billion on-road vehicles consume 79 quadrillion BTU of energy, mostly gasoline and diesel fuels, emit 5.7 gigatonnes of CO₂, and emit other pollutants to which approximately 200,000 annual premature deaths are attributed. Improved vehicle energy efficiency and emission controls have helped offset growth in vehicle activity. New technologies are diffusing into the vehicle fleet in response to fuel efficiency and emission standards. Empirical assessment of vehicle emissions is challenging because of myriad fuels and technologies, intervehicle variability, multiple emission processes, variability in operating conditions, and varying capabilities of measurement methods. Fuel economy and emissions regulations have been effective in reducing total emissions of key pollutants. Real-world fuel use and emissions are consistent with official values in the United States but not in Europe or countries that adopt European standards. Portable emission measurements systems, which uncovered a recent emissions cheating scandal, have a key role in regulatory programs to ensure conformity between "real driving emissions" and emission standards. The global vehicle fleet will experience tremendous growth, especially in Asia. Although existing data and modeling tools are useful, they are often based on convenience samples, small sample sizes, large variability, and unquantified uncertainty. Vehicles emit precursors to several important secondary pollutants, including ozone and secondary organic aerosols, which requires a multipollutant emissions and air quality management strategy. Gasoline and diesel are likely to persist as key energy sources to mid-century. Adoption of electric vehicles is not a panacea with regard to greenhouse gas emissions unless coupled with policies to change the power generation mix. Depending on how they are actually implemented and used, autonomous vehicles could lead to very large reductions or increases in energy consumption. Numerous other trends are addressed with regard to technology, emissions controls, vehicle operations, emission measurements, impacts on exposure, and impacts on public health.

IMPLICATIONS

Without specific policies to the contrary, fossil fuels are likely to continue to be the major source of on-road vehicle energy consumption. Fuel economy and emission standards are generally effective in achieving reductions per unit of vehicle activity. However, the number of vehicles and miles traveled will increase. Total energy use and emissions depend on factors such as fuels, technologies, land use, demographics, economics, road design, vehicle operation, societal values, and others that affect demand for transportation, mode choice, energy use, and emissions. Thus, there are many opportunities to influence future trends in vehicle energy use and emissions.

Satellite-Based Estimates of Daily NO2 Exposure in China Using Hybrid Random Forest and Spatiotemporal Kriging Model

A novel model named random-forest-spatiotemporal-kriging (RF-STK) was developed to estimate the daily ambient NO₂ concentrations across China during 2013-2016 based on the satellite retrievals and geographic covariates. The RF-STK model showed good prediction performance, with cross-validation R² = 0.62 (RMSE = 13.3 μg/m³) for daily and R² = 0.73 (RMSE = 6.5 μg/m³) for spatial predictions. The nationwide population-weighted multiyear average of NO₂ was predicted to be 30.9 ± 11.7 μg/m³ (mean ± standard deviation), with a slowly but significantly decreasing trend at a rate of -0.88 ± 0.38 μg/m³/year. Among the main economic zones of China, the Pearl River Delta showed the fastest decreasing rate of -1.37 μg/m³/year, while the Beijing-Tianjin Metro did not show a temporal trend ( P = 0.32). The population-weighted NO₂ was predicted to be the highest in North China (40.3 ± 10.3 μg/m³) and lowest in Southwest China (24.9 ± 9.4 μg/m³). Approximately 25% of the population lived in nonattainment areas with annual-average NO₂ > 40 μg/m³. A piecewise linear function with an abrupt point around 100 people/km² characterized the relationship between the population density and the NO₂, indicating a threshold of aggravated NO₂ pollution due to urbanization. Leveraging the ground-level NO₂ observations, this study fills the gap of statistically modeling nationwide NO₂ in China, and provides essential data for epidemiological research and air quality management.

Impacts of rural worker migration on ambient air quality and health in China: From the perspective of upgrading residential energy consumption

In China, rural migrant workers (RMWs) are employed in urban workplaces but receive minimal resources and welfare. Their residential energy use mix (REM) and pollutant emission profiles are different from those of traditional urban (URs) and rural residents (RRs). Their migration towards urban areas plays an important role in shaping the magnitudes and spatial patterns of pollutant emissions, ambient PM_2.5 (fine particulate matter with a diameter smaller than 2.5 μm) concentrations, and associated health impacts in both urban and rural areas. Here we evaluate the impacts of RMW migration on REM pollutant emissions, ambient PM_2.5, and subsequent premature deaths across China. At the national scale, RMW migration benefits ambient air quality because RMWs tend to transition to a cleaner REM upon arrival at urban areas-though not as clean as urban residents'. In 2010, RMW migration led to a decrease of 1.5 μg/m³ in ambient PM_2.5 exposure concentrations (C_ex) averaged across China and a subsequent decrease of 12,200 (5700 to 16,300, as 90% confidence interval) in premature deaths from exposure to ambient PM_2.5. Despite the overall health benefit, large-scale cross-province migration increased megacities' PM_2.5 levels by as much as 10 μg/m³ due to massive RMW inflows. Model simulations show that upgrading within-city RMWs' REMs can effectively offset the RMW-induced PM_2.5 increase in megacities, and that policies that properly navigate migration directions may have potential for balancing the economic growth against ambient air quality deterioration. Our study indicates the urgency of considering air pollution impacts into migration-related policy formation in the context of rapid urbanization in China.

Characteristics of ambient ozone (O3) pollution and health risks in Zhejiang Province

Troposphere ozone, which is from secondary formation processes, has been increasing dramatically during the last decades in China, inducing high health risks. In this study, temporal and spatial distribution of O₃ was studied among 13 sites of three cities during 2014-2016. The objectives were to clarify the characteristics of the ambient pollution of O₃ under the influence from other pollutants and meteorological parameters and the health outcomes from exposure to O₃. The concentrations of O₃ during summer were much higher than those during winter, and the concentrations in downtown areas were higher than in rural or mountain areas. PM_2.5, NO₂, SO₂, and wind speed (WS) were negatively correlated with O₃, and CO, temperature (T), and relative humidity (RH) were positively correlated with O₃. In multivariable analysis, two separate factors-solar radiation and atmospheric diffusion status, affected the O₃ levels. The concentrations of O₃ reached the highest level at 15:00 and the lowest value at about 6:00-8:00, with the similar trend to T and WS, and opposite to RH. According to the dose-response model, relative risks (RRs) and population attributable fractions (PAFs) with confidence intervals (CIs) for chronic obstructive pulmonary disease (COPD) from exposure to O₃ were 1.0612 (CI 1.0607-1.0616) and 5.32% (CI 5.29-5.36%), respectively, attributable to 2000 deaths in Zhejiang Province in 2014.