Attribution of Tropospheric Ozone to NO x and VOC Emissions: Considering Ozone Formation in the Transition Regime

Ozone sensitivity regimes vary at different heights in the planetary boundary layer

The sensitivity of tropospheric ozone (O3) to its precursors volatile organic compounds (VOCs) and nitrogen oxides (NOX) determines the emission reduction strategy for O3 mitigation. Due to the lack of comprehensive vertical measurements of VOCs, the vertical distribution of O3 sensitivity regimes has not been well understood. O3 precursor sensitivity determined by ground-level measurements has been generally used to guide O3 control strategy. Here, to precisely diagnose O3 sensitivity regimes at different heights in the planetary boundary layer (PBL), we developed a vertical measurement system based on an unmanned aerial vehicle platform to conduct comprehensive vertical measurements of VOCs, NOX and other relevant parameters. Our results suggest that the O3 precursor sensitivity shifts from a VOC-limited regime at the ground to a NOX-limited regime at upper layers, indicating that the ground-level O3 sensitivity cannot represent the situation of the whole PBL. We also found that the state-of-the-art photochemical model tends to underestimate oxygenated VOCs at upper layers, resulting in overestimation of the degree of VOCs-limited regime. Therefore, thorough vertical measurements of VOCs to accurately diagnose O3 precursor sensitivity is in urgent need for the development of effective O3 control strategies.

Catalytic oxidation of toluene by manganese oxides: Effect of K+ doping on oxygen vacancy

Alkali metal potassium was beneficial to the electronic regulation and structural stability of transition metal oxides. Herein, K ions were introduced into manganese oxides by different methods to improve the degradation efficiency of toluene. The results of activity experiments indicated that KMnO4-HT (HT: Hydrothermal method) exhibited outstanding low-temperature catalytic activity, and 90% conversion of toluene can be achieved at 243°C, which was 41°C and 43°C lower than that of KNO3-HT and Mn-HT, respectively. The largest specific surface area was observed on KMnO4-HT, facilitating the adsorption of toluene. The formation of cryptomelane structure over KMnO4-HT could contribute to higher content of Mn3+ and lattice oxygen (Olatt), excellent low-temperature reducibility, and high oxygen mobility, which could increase the catalytic performance. Furthermore, two distinct degradation pathways were inferred. Pathway Ⅰ (KMnO4-HT): toluene → benzyl → benzoic acid → carbonate → CO2 and H2O; Pathway ⅠⅠ (Mn-HT): toluene → benzyl alcohol → benzoic acid → phenol → maleic anhydride → CO2 and H2O. Fewer intermediates were detected on KMnO4-HT, indicating its stronger oxidation capacity of toluene, which was originated from the doping of K+ and the interaction between KOMn. More intermediates were observed on Mn-HT, which can be attributed to the weaker oxidation ability of pure Mn. The results indicated that the doping of K+ can improve the catalytic oxidation capacity of toluene, resulting in promoted degradation of intermediates during the oxidation of toluene.

Observation-Based Diagnostics of Reactive Nitrogen Recycling through HONO Heterogenous Production: Divergent Implications for Ozone Production and Emission Control

Understanding of nitrous acid (HONO) production is crucial to photochemical studies, especially in polluted environments like eastern China. In-situ measurements of gaseous and particulate compositions were conducted at a rural coastal site during the 2018 spring Ozone Photochemistry and Export from China Experiment (OPECE). This data set was applied to investigate the recycling of reactive nitrogen through daytime heterogeneous HONO production. Although HONO levels increase during agricultural burning, analysis of the observation data does not indicate more efficient HONO production by agricultural burning aerosols than other anthropogenic aerosols. Box and 1-D modeling analyses reveal the intrinsic relationships between nitrogen dioxide (NO2), particulate nitrate (pNO3), and nitric acid (HNO3), resulting in comparable agreement between observed and simulated HONO concentrations with any one of the three heterogeneous HONO production mechanisms, photosensitized NO2 conversion on aerosols, photolysis of pNO3, and conversion from HNO3. This finding underscores the uncertainties in the mechanistic understanding and quantitative parametrizations of daytime heterogeneous HONO production pathways. Furthermore, the implications for reactive nitrogen recycling, ozone (O3) production, and O3 control strategies vary greatly depending on the HONO production mechanism. On a regional scale, the conversion of HONO from pNO3 can drastically enhance O3 production, while the conversion from NO2 can reduce O3 sensitivity to NOx changes in polluted eastern China.

Assessment of ozone pollution on rice yield reduction and economic losses in Sichuan province during 2015-2020

In recent years, there has been a significant increase in surface ozone (O3) concentrations in the troposphere. Ozone pollution has significant adverse effects on ecosystems, human health, and climate change, particularly on crop growth and yield. This study utilized the observational hourly O3 data, cumulative O3 concentration over 40 ppb per h (AOT40), and the mean daytime 7-h O3 concentration (M7) to analyze the spatiotemporal distributions of relative yield losses (RYLs) and evaluate the yield reduction and economic losses of rice in Sichuan province from 2015 to 2020. The results indicated that the average O3 concentration during the growing rice season ranged from 55.4 to 69.3 μg/m3, with the highest O3 concentration observed in 2017, and the AOT40 ranged from 4.5 to 8.7 ppm h from 2015 to 2020. At the county level, the O3 concentration, AOT40, and the relative yield loss (RYL) of rice based on AOT40 exhibited clear spatiotemporal differences in Sichuan. The RYLs of AOT40 were 4.9-9.2% from 2015 to 2020. According to AOT40 and M7 metrics, the yield loss and economic losses attributed to O3 pollution amounted to 78.75-150.36 (9.74-21.54) ten thousand tons, and 2079.08-4149.89 (257.25-594.45) million Yuan, respectively. Rice yield and economic losses were relatively large in the Chengdu Plain, southern Sichuan, and northeast Sichuan. These findings will contribute to a deeper understanding of the detrimental effects of elevated surface O3 concentrations on rice crops. It is imperative to implement more stringent O3 reduction measures aimed at lowering O3 concentrations, enhancing rice quality, and safeguarding food security in Sichuan.

Dominant contribution of combustion-related ammonium during haze pollution in Beijing

Aerosol ammonium (NH4+), mainly produced from the reactions of ammonia (NH3) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH4+ can provide scientific insights into air quality improvements. However, the sources of NH3 in urban areas are not well understood, and few studies focus on NH3/NH4+ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH4+. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH3 emission sources and vertically resolved NH4+ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH3 emissions, including fossil fuel sources, NH3 slip, and biomass burning, are important sources of aerosol NH4+ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH3 sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH3 is mostly local from Beijing, and biomass burning is likely an important NH3 source (∼15%-20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO2 and NH3) control policy should be considered to improve air quality.

Exposure to BTEX is associated with cardiovascular disease, dyslipidemia and leukocytosis in national US population

BACKGROUND

Comprehensive research on the effects of individual benzene, toluene, ethylbenzene, and xylenes (BTEX) and their mixture measured in blood samples, on cardiovascular diseases (CVD) and related risk factors among the general population is limited.

OBJECTIVES

To investigate the effects of blood individual and mixed BTEX on total CVD and its subtypes, lipid profiles, and white blood cell (WBC) count.

METHODS

Survey-weighted multivariate logistic regression was used to examine the associations between blood individual and mixed BTEX with CVD and its subtypes in 17,007 participants from NHANES 1999-2018. The combined effect of BTEX mixture on CVD was estimated using weighted quantile sum modeling and quantile g-computation. Weighted multivariate linear regression assessed the effects of BTEX on lipid profiles and WBC, including its five-part differential count.

RESULTS

In comparison to the reference quartile of BTEX mixture, individuals in the highest quartile had a significantly increased adjusted odds ratio of CVD risk (1.64, 95 % CI: 1.23 to 2.19, P for trend = 0.008). Positive associations were observed for benzene, toluene, ethylbenzene, and m-/p-xylene, demonstrating a monotonically increasing exposure-response relationship. Mixed BTEX was associated with congestive heart failure (CHF), angina pectoris, and heart attack. Individual benzene, toluene, and ethylbenzene were associated with CHF, while toluene, ethylbenzene, and all xylene isomers were linked to angina pectoris. Benzene, toluene, and o-xylene were associated with heart attack. Both mixed and individual BTEX showed positive associations with triglycerides, cholesterol, low-density lipoprotein, and WBC, including its five-part differential count, but a negative relationship with high-density lipoprotein. Subgroup analyses identified modifying effects of smoking, drinking, exercise, BMI, hypertension, and diabetes on the associations between specific toxicants and CVD risk.

CONCLUSIONS

Exposure to BTEX was associated with cardiovascular diseases and cardiovascular risk factors. These findings emphasize the importance of considering blood BTEX levels when assessing cardiovascular health risks.

Long-term trajectory of ozone impact on maize and soybean yields in the United States: A 40-year spatial-temporal analysis

Understanding the long-term change trends of ozone-induced yield losses is crucial for formulating strategies to alleviate ozone damaging effects, aiming towards achieving the Zero Hunger Sustainable Development Goal. Despite a wealth of experimental research indicating that ozone's influence on agricultural production exhibits marked fluctuations and differs significantly across various geographical locations, previous studies using global statistical models often failed to capture this spatial-temporal variability, leading to uncertainties in ozone impact estimation. To address this issue, we conducted a comprehensive assessment of the spatial-temporal variability of ozone impacts on maize and soybean yields in the United States (1981-2021) using a geographically and temporally weighted regression (GTWR) model. Our results revealed that over the past four decades, ozone pollution has led to average yield losses of -3.5% for maize and -6.1% for soybean, translating into an annual economic loss of approximately $2.6 billion. Interestingly, despite an overall downward trend in ozone impacts on crop yields following the implementation of stringent ozone emission control measures in 1997, our study identified distinct peaks of abnormally high yield reduction rates in drought years. Significant spatial heterogeneity was detected in ozone impacts across the study area, with ozone damage hotspots located in the Southeast Region and the Mississippi River Basin for maize and soybean, respectively. Furthermore, we discovered that hydrothermal factors modulate crop responses to ozone, with maize showing an inverted U-shaped yield loss trend with temperature increases, while soybean demonstrated an upward trend. Both crops experienced amplified ozone-induced yield losses with rising precipitation. Overall, our study highlights the necessity of incorporating spatiotemporal variability into assessments of crop yield losses attributable to ozone pollution. The insights garnered from our findings can contribute to the formulation of region-specific pollutant emission policies, based on the distinct profiles of ozone-induced agricultural damage across different regions.

Contributions of local emissions and regional background to summertime ozone in central China

Source contributions and regional transport of maximum daily average 8-h (MDA8) O3 during a high O3 month (June 2019) in Henan province in central China are explored using a source-oriented Community Multiscale Air Quality (CMAQ) model. The monthly average MDA8 O3 exceeds ∼70 ppb in more than half of the areas and shows a clear spatial gradient, with lower O3 concentrations in the southwest and higher in the northeast. Significant contributions of anthropogenic emissions to monthly average MDA8 O3 concentrations of more than 20 ppb are predicted in the provincial capital Zhengzhou, mostly due to emissions from the transportation sector (∼50%) and in the areas in the north and northeast regions where industrial and power generation-related emissions are high. Biogenic emissions in the region only contribute to approximately 1-3 ppb of monthly average MDA8 O3. In industrial areas north of the province, their contributions reach 5-7 ppb. Two CMAQ-based O3-NOx-VOCs sensitivity assessments (the local O3 sensitivity ratios based on the direct decoupled method and the production ratio of H2O2 to HNO3) and the satellite HCHO to NO2 column density ratio consistently show that most of the areas in Henan are in NOx-limited regime. In contrast, the high O3 concentration areas in the north and at the city centers are in the VOC-limited or transition regimes. The results from this study suggest that although reducing NOx emissions to reduce O3 pollution in the region is desired in most areas, VOC reductions must be applied to urban and industrial regions. Source apportionment simulations with and without Henan anthropogenic emissions show that the benefit of local anthropogenic NOx reduction might be lower than expected from the source apportionment results because the contributions of Henan background O3 increase in response to the reduced local anthropogenic emissions due to less NO titration. Thus, collaborative O3 controls in neighboring provinces are needed to reduce O3 pollution problems in Henan effectively.

Drivers of Increasing Ozone during the Two Phases of Clean Air Actions in China 2013-2020

In response to the severe air pollution issue, the Chinese government implemented two phases (Phase I, 2013-2017; Phase II, 2018-2020) of clean air actions since 2013, resulting in a significant decline in fine particles (PM_2.5) during 2013-2020, while the warm-season (April-September) mean maximum daily 8 h average ozone (MDA8 O₃) increased by 2.6 μg m^-3 yr^-1 in China during the same period. Here, we derived the drivers behind the rising O₃ concentrations during the two phases of clean air actions by using a bottom-up emission inventory, a regional chemical transport model, and a multiple linear regression model. We found that both meteorological variations (3.6 μg m^-3) and anthropogenic emissions (6.7 μg m^-3) contributed to the growth of MDA8 O₃ from 2013 to 2020, with the changes in anthropogenic emissions playing a more important role. The anthropogenic contributions to the O₃ rise during 2017-2020 (1.2 μg m^-3) were much lower than that in 2013-2017 (5.2 μg m^-3). The lack of volatile organic compound (VOC) control and the decline in nitrogen oxides (NO_x) emissions were responsible for the O₃ increase in 2013-2017 due to VOC-limited regimes in most urban areas, while the synergistic control of VOC and NO_x in Phase II initially worked to mitigate O₃ pollution during 2018-2020, although its effectiveness was offset by the penalty of PM_2.5 decline. Future mitigation efforts should pay more attention to the simultaneous control of VOC and NO_x to improve O₃ air quality.

Understanding the nocturnal ozone increase in Nanjing, China: Insights from observations and numerical simulations

Surface ozone (O₃) is mainly photochemically formed by nitrogen oxides (NO_X) and volatile organic compounds (VOCs), and therefore O₃ usually has a distinct diurnal variation with high concentrations in the afternoon and low values at night. However, eight nocturnal O₃ increase (NOI) events were identified in Nanjing in June 2021. To understand the mechanism of NOI events, we selected two events (June 6-7, and 24-25) for observational data analysis. The Community Multiscale Air Quality (CMAQ) model was employed for the process analysis (PA) and regional transport of O₃. By analyzing the O₃ observation data and meteorological factors, we found that there were clear southeastward O₃ transport paths. The O₃ peak clearly moved from the upwind to the downwind cities in both events. Model simulations showed that when nocturnal O₃ enhancement occurred, horizontal transport resulted in a negative to positive net O₃ production rate. O₃ continued to get accumulated in Nanjing. Nocturnal O₃ in the first event was dominated by long-range transport, with the top two contributing cities being Huzhou (5.6 %) and Jiaxing (4.7 %). NOI during the second event was from the nearby upwind cities. The top three contributing cities were Shanghai (18.3 %), Wuxi (9.1 %), and Suzhou (8.8 %). We conclude that the June NOI events in Nanjing were mainly driven by the horizontal transport of southeasterly winds. This study provides scientific support for O₃ prevention and control in Nanjing in the summer.

Expediting Toluene Combustion by Harmonizing the Ce-O Strength over Co-Doped CeZr Oxide Catalysts

Low-temperature catalytic degradation of volatile organic compounds (VOCs) by enhancing the activity of non-precious metal catalysts has always been the focus of attention. The mineralization of aromatic VOCs requires the participation of a large number of oxygen atoms, so the activation of oxygen species is crucial in the degradation reaction. Herein, we originally adjust the Ce-O bond strength in CeZr oxide catalysts by cobalt doping to promote the activation of oxygen species, thus improving the toluene degradation performance while maintaining high stability. Subsequent characterizations and theoretical calculations demonstrate that the weakening of the Ce-O bond strength increases the oxygen vacancy content, promotes the activation of oxygen species, and enhances the redox ability of the catalysts. This strategy also promotes the activation of toluene and accelerates the depletion of intermediate species. This study will contribute a strategy to enhance the activation ability of oxygen species in non-noble metal oxide catalysts, thereby enhancing the degradation performance of VOCs.

Combined exposure of heat stress and ozone enhanced cognitive impairment via neuroinflammation and blood brain barrier disruption in male rats

BACKGROUND

Heat stress (HS) exposure has been linked to cognitive dysfunction. In reality, high temperature does not occur alone in environment, and ozone (O₃) and heatwaves usually co-exist in atmospheric environment. However, whether O₃ exposure exacerbates HS-induced cognitive impairment and the potential underlying mechanisms have not been explored experimentally. The aim of this study was to determine the co-effects and mechanisms of HS and O₃ on the cognitive dysfunction.

METHODS

48 Sprague Dawley male rats were randomly divided into 4 groups: control, HS, O₃ and HS plus O₃ (HO₃) groups. Rats in HS and HO₃ group were exposed to 40 °C every morning from 9:00 to 12:00 for 15 consecutive days. While rats in O₃ and HO₃ groups were exposed to 0.7 ppm O₃ the same day from 14:00 to 17:00 for 15 days. Cognitive performance was examined with Morris water maze test. Neurodegeneration, glial activation, neuroinflammation, blood brain barrier (BBB) disruption and apoptosis were evaluated by Western blot, Elisa, immunohistochemistry and immunofluorescence staining.

RESULTS

HS induced cognitive decline and neuronal damage in rats. Further studies showed that exposure of rats to HS could also induce glial activation, neuroinflammation and neuronal apoptosis in hippocampus, and decrease in the expressions of ZO-1, claudin-5 and occluding, indicative of BBB disruption. Impressively, the neuronal effects induced by HS, as depicted above, could be worsened by co-exposure to O₃ in rats.

CONCLUSIONS

Co-exposure to O₃ promotes HS-induced cognitive impairment in rats possibly through glial-mediated neuroinflammation and BBB disruption.

Maize yield reduction and economic losses caused by ground-level ozone pollution with exposure- and flux-response relationships in the North China Plain

Ground-level ozone (O₃) has negative effects on agricultural crops. Maize is an important grain crop in China. The North China Plain (NCP) serves as the major crops' production area of China and experiences severe ozone pollution. Using the ground-level ozone simulated by an atmospheric chemistry transport model (WRF-Chem), we quantified the yield reduction and economic losses of maize during 2015-2018 over NCP based on exposure-response AOT40 (accumulation of hourly O₃ concentration exceed 40 ppb) and flux-response POD₆ (phytotoxic dose of ozone over 6 nmol m^-2 s^-1). Results showed that the ozone concentration, AOT40, and POD₆ clearly increased from 2015 to 2018 in growing season of maize over NCP. The four-year annual mean ozone concentration, AOT40, and POD₆ were 0.055 ppm, 18.02 ppm h, and 5.02 mmol m^-2, respectively. At county level, the relative loss of maize yield (MRYL) based on AOT40 and POD₆ had clearly spatio-temporal differences in NCP. The average MRYLs of AOT40 and of POD₆ from 2015 to 2018 were 10.4% and 21.4%, respectively, and these reductions were associated with 2399 million and 5637 million US dollars, respectively. This study suggests that surface ozone increased the yield losses of maize, and indicates that further reductions in ozone concentrations can enhance the food security in China.

Palladium-Doped Single-Walled Carbon Nanotubes as a New Adsorbent for Detecting and Trapping Volatile Organic Compounds: A First Principle Study

Volatile organic compounds (VOCs) are in the vapor state in the atmosphere and are considered pollutants. Density functional theory (DFT) calculations with the wb97xd exchange correlation functional and the 6-311+G(d,p) basis set are carried out to explore the potential possibility of palladium-doped single-walled carbon nanotubes (Pd/SWCNT-V), serving as the resource for detecting and/or adsorbing acetonitrile (ACN), styrene (STY), and perchloroethylene (PCE) molecules as VOCs. The suggested adsorbent in this study is discussed with structural parameters, frontier molecular orbital theory, molecular electrical potential surfaces (MEPSs), natural bond orbital (NBO) analyses, and the density of states. Furthermore, following the Bader theory of atoms in molecules (AIM), the topological properties of the electron density contributions for intermolecular interactions are analyzed. The obtained results show efficient VOC loading via a strong chemisorption process with a mean adsorption energy of −0.94, −1.27, and −0.54 eV for ACN, STY, and PCE, respectively. Our results show that the Pd/SWCNT-V can be considered a good candidate for VOC removal from the environment.

Low-Temperature Combustion of Toluene over Cu-Doped SmMn2O5 Mullite Catalysts via Creating Highly Active Cu2+-O-Mn4+ Sites

Catalytic combustion of volatile organic compounds (VOCs) at low temperatures is still an urgent issue to be solved. Herein, low-temperature combustion of toluene over Cu-doped SmMn₂O₅ mullite catalysts via creating highly active Cu²⁺-O-Mn⁴⁺ sites has been originally demonstrated. Cu-doped SmMn₂O₅ mullite catalysts exhibited 90% conversion of toluene at 206 °C and displayed robust stability even in the presence of water. It has been demonstrated that Cu doping created Cu²⁺-O-Mn⁴⁺ active composite sites that were more exposed after removing surface Sm species via acid-etching. Benefiting from this, the redox and oxygen activation ability of catalysts was significantly enhanced. The consumption of benzaldehyde and benzoic acid as intermediate species and the CO₂ generation ability were apparently promoted, which were the direct reasons for the enhanced low-temperature combustion of toluene. This work provides novel ideas for the development of high-performance catalysts for low-temperature VOC combustion, which has great industrial application prospects.

Modulating the Electronic Metal-Support Interactions in Single-Atom Pt1 -CuO Catalyst for Boosting Acetone Oxidation

The development of highly active single-atom catalysts (SACs) and identifying their intrinsic active sites in oxidizing industrial hazardous hydrocarbons are challenging prospects. Tuning the electronic metal-support interactions (EMSIs) is valid for modulating the catalytic performance of SACs. We propose that the modulation of the EMSIs in a Pt₁ -CuO SAC significantly promotes the activity of the catalyst in acetone oxidation. The EMSIs promote charge redistribution through the unified Pt-O-Cu moieties, which modulates the d-band structure of atomic Pt sites, and strengthens the adsorption and activation of reactants. The positively charged Pt atoms are superior for activating acetone at low temperatures, and the stretched Cu-O bonds facilitate the activation of lattice oxygen atoms to participate in subsequent oxidation. We believe that this work will guide researchers to engineer efficient SACs for application in hydrocarbon oxidation reactions.

Ozone Pollution in Chinese Cities: Spatiotemporal Variations and Their Relationships with Meteorological and Other Pollution Factors (2016–2020)

With the acceleration of urbanization, ozone (O3) pollution has become increasingly serious in many Chinese cities. This study analyzes the temporal and spatial characteristics of O3 based on monitoring and meteorological data for 366 cities and national weather stations throughout China from 2016 to 2020. Least squares linear regression and Spearman’s correlation coefficient were computed to investigate the relationships of O3 with various pollution factors and meteorological conditions. Global Moran’s I and the Getis–Ord index Gi* were adopted to reveal the spatial agglomeration of O3 pollution in Chinese cities and characterize the temporal and spatial characteristics of hot and cold spots. The results show that the national proportion of cities with an annual concentration exceeding 160 μg·m−3 increased from 21.6% in 2016 to 50.9% in 2018 but dropped to 21.5% in 2020; these cities are concentrated mainly in Central China (CC) and East China (EC). Throughout most of China, the highest seasonal O3 concentrations occur in summer, while the highest values in South China (SC) and Southwest China (SWC) occur in autumn and spring, respectively. The highest monthly O3 concentration reached 200 μg·m−3 in North China (NC) in June, while the lowest value was 60 μg·m−3 in Northeast China (NEC) in December. O3 is positively correlated with the ground surface temperature (GST) and sunshine duration (SSD) and negatively correlated with pressure (PRS) and relative humidity (RHU). Wind speed (WIN) and precipitation (PRE) were positively correlated in all regions except SC. O3 concentrations are significantly differentiated in space: O3 pollution is high in CC and EC and relatively low in the western and northeastern regions. The concentration of O3 exhibits obvious agglomeration characteristics, with hot spots being concentrated mainly in NC, CC and EC.

Is atmospheric oxidation capacity better in indicating tropospheric O3 formation?

Tropospheric ozone (O₃) concentration is increasing in China along with dramatic changes in precursor emissions and meteorological conditions, adversely affecting human health and ecosystems. O₃ is formed from the complex nonlinear photochemical reactions from nitrogen oxides (NO _x = NO + NO₂) and volatile organic compounds (VOCs). Although the mechanism of O₃ formation is rather clear, describing and analyzing its changes and formation potential at fine spatial and temporal resolution is still a challenge today. In this study, we briefly summarized and evaluated different approaches that indicate O₃ formation regimes. We identify that atmospheric oxidation capacity (AOC) is a better indicator of photochemical reactions leading to the formation of O₃ and other secondary pollutants. Results show that AOC has a prominent positive relationship to O₃ in the major city clusters in China, with a goodness of fit (R ²) up to 0.6. This outcome provides a novel perspective in characterizing O₃ formation and has significant implications for formulating control strategies of secondary pollutants.

Achieving acetone efficient deep decomposition by strengthening reactants adsorption and activation over difunctional Au(OH)Kx/hierarchical MFI catalyst

Realizing the simultaneous adsorption and activation of O₂ and reactants over supported noble metal catalysts is crucial for the oxidation of organic hydrocarbons. Herein, we report a facile one-step ethylene glycol reduction method to synthesize difunctional Au(OH)K_x sites, which were anchored on a hierarchical hollow MFI support and adopted for acetone decomposition. The alkali ion-associated adjacent surface hydroxyl groups were coordinated with Au nanoparticles, resulting in partially oxidized Au¹⁺ sites with improved dispersion. The results obtained from exclusive ex situ and in situ experiments illustrated that the proper content of K and hydroxyl groups significantly enhanced the adsorption of surface O₂ and acetone molecules around the Au sites simultaneously, whereas the excess K species inhibited the catalytic performance by blocking the pore structure and decreasing the acidity of catalysts. The Au(OH)K_0.7/h-MFI catalyst exhibited the highest efficiency for acetone oxidation, over which 1500 ppm acetone can be completely oxidized at just 280 °C with an extremely low activation energy of 32.5 kJ mol^-1. The carbonate species were detected as the main intermediates during acetone decomposition over the difunctional Au(OH)K_x sites through a Langmuir - Hinshelwood (L - H) mechanism. This finding paves the way for designing and constructing efficient functional active sites for the complete oxidation of hydrocarbons.

Pollution and Cleaning of PDMS Pervaporation Membranes after Recovering Ethyl Acetate from Aqueous Saline Solutions

The removal of volatile organic compounds (VOCs) from wastewater containing nonvolatile salts has become an important and interesting case of the application of the pervaporation (PV) process. The aim of this study was to evaluate the influence of salts on the PV removal of ethyl acetate from wastewater using a polydimethylsiloxane (PDMS) membrane. The fouled membrane was then characterized via scanning electron microscopy–energy-dispersive X-ray analysis (SEM–EDX) to investigate salt permeation. The membrane backflushing process was carried out by periodically flushing the permeate side of the tubular membrane. The results demonstrated that salts (NaCl and CaCl₂) could permeate through the PDMS membrane and were deposited on the permeate side. The presence of salts in the feed solution caused a slight increase in the membrane selectivity and a decrease in the permeate flux. The flux decreased with increasing salt concentration, and a notable effect occurred at higher feed-salt concentrations. A permeate flux of up to 98.3% of the original flux was recovered when the permeation time and backflushing duration were 30 and 5 min, respectively, indicating that the effect of salt deposition on flux reduction could be mitigated. Real, organic, saline wastewater was treated in a pilot plant, which further verified the feasibility of wastewater PV treatment.

Global Perspective of Drought Impacts on Ozone Pollution Episodes

Ozone (O₃) pollution threatens global public health and damages ecosystem productivity. Droughts modulate surface O₃ through meteorological processes and vegetation feedbacks. Unraveling these influences is difficult with traditional chemical transport models. Here, using an atmospheric chemistry-vegetation coupled model in combination with a suite of existing measurements, we investigate the drought impacts on global surface O₃ and explore the main driving processes. Relative to the mean state, accelerated photochemical rates dominate the surface O₃ enhancement during droughts except for eastern U.S. and western Europe, where reduced stomatal uptakes make comparable contributions. During 1990-2012, the simulated frequency of O₃ pollution episodes in western Europe decreases greatly with a negative trend of -5.5 ± 6.6 days per decade following the reductions in anthropogenic emissions if meteorology is fixed. However, such decreased trend is weakened to -2.1 ± 3.8 days per decade, which is closer to the observed trend of -2.9 ± 1.1 days per decade when year-to-year meteorology is applied because increased droughts alone offset 43% of the effects from air pollution control. Our results highlight that more stringent controls of O₃ precursors are necessary to mitigate the higher risks of O₃ pollution episodes by more droughts in a warming world.

Modeling of spatial and temporal variations of ozone-NOx-VOC sensitivity based on photochemical indicators in China

Comprehensive air quality model with extensions (CAMx)-decoupled direct method (DDM) was used to simulate ozone-NO_x-VOCs sensitivity of for May-November in 2016-2018 in China. Based on the relationship between the simulated ozone (O₃) sensitivity values and the ratio of formaldehyde (HCHO) to NO₂ (FNR) and the ratio of production rate of hydrogen peroxide (H₂O₂) to production rate of nitric acid (HNO₃) ( [Formula: see text] ), the localized range of FNR and [Formula: see text] thresholds in different regions in China were obtained. The overall simulated FNR values are about 1.640-2.520, and [Formula: see text] values are about 0.540-0.830 for the transition regime. Model simulated O₃ sensitivities or region specific FNR or [Formula: see text] thresholds should be applied to ensure the accurate local O₃ sensitivity regimes. Using the tropospheric column FNR values from ozone monitoring instrument (OMI) satellite data as an indicator with the simulated threshold values, the spatial distributions of O₃ formation regimes in China are determined. The O₃ sensitivity regimes from eastern to central China are gradually from VOC-limited, transition to NO_x-limited spatially, and moving toward to transition or NO_x-limited regime from 2005 to 2019 temporally.

Evaluation of a highly condensed SAPRC chemical mechanism and two emission inventories for ozone source apportionment and emission control strategy assessments in China

The response of summertime O₃ to changes in the nitrogen oxides (NO_x) and volatile organic compounds (VOC) emissions, and contributions of different NO_x and VOC sources to O₃ in China are studied using a highly condensed photochemical mechanism in the Statewide Air Pollution Research Center (SAPRC) family (CS07A) and two popular anthropogenic emission inventories, the Multi-resolution Emission Inventory for China (MEIC) and Regional Emission inventory in ASia (REAS). Although CS07A predicts slightly lower O₃ concentrations than the standard fix-parameter version of the SAPRC-11 mechanism, the two mechanisms predict almost identical relative responses to daily maximum 8-hour O₃ (O₃-8h) due to NO_x and VOC emission reductions. A source-oriented version of the CS07A is applied to determine source contributions of NO_x and VOCs to O₃ using MEIC and REAS. The two inventories lead to similar model performance of O₃, with MEIC predicting higher O₃ in Beijing and Shanghai, especially on high O₃ days. Source apportionment results show that industry and transportation are the top two contributors to non-background O₃ for both inventories, followed by power and biogenic emissions. In general, the two inventories lead to similar source contribution estimations to O₃ attributable to NO_x. However, their estimations of relative contributions to VOC-related O₃ differ for the industrial and transportation sectors. Differences in the source apportionment results are more significant in some urban areas, although both emissions capture the spatial variations in the source contributions. Our results suggest that future emission control policies should be assessed using multiple emission inventories, and the condensed CS07A is suitable for policy applications when a large number of simulations are needed.

Diagnostic analysis of regional ozone pollution in Yangtze River Delta, China: A case study in summer 2020

A regional ozone (O₃) pollution event occurred in the Yangtze River Delta region during August 17-23, 2020 (except on August 21). This study aims to understand the causes of O₃ pollution during the event using an emission-based model (i.e., the Community Multiscale Air Quality (CMAQ) model) and an observation-based model (OBM). The OBM was used to investigate O₃ sensitivity to its precursors during the O₃ pollution, concluding that O₃ formation was limited by volatile organic compounds (VOCs) on August 19, but was co-limited by VOCs and nitrogen oxides (NOx) on other polluted days. Aromatics and alkenes were the two main VOC groups contributing to the O₃ formation, with trans-2-butene and m/p-xylene as the key species among the VOCs measured at the Nanjing urban site. The source apportionment results estimated using the source-oriented CMAQ model suggest that the transportation and industry sources dominated the non-background O₃ production in Nanjing, which were responsible for 52% and 24.7%, respectively. The O₃ concentration attributed to NOx (~70%) was significantly higher than that attributed to VOCs (approximately 30%). The process analysis revealed that vertical mixing increased the O₃ concentrations in the early morning, and photochemical reactions promoted O₃ formation and accumulation during the daytime within the planetary boundary layer. At night, outflow from horizontal transport and nocturnal chemistry jointly resulted the O₃ depletion. The contributions of inter-city transport during the O₃ pollution period in Nanjing were also estimated. The predicted O₃ concentration was largely recorded from long-distance regions, reaching 46%, followed by local sources (38%) and surrounding cities (16%). The results indicate that both NOx and VOCs contributed significantly to O₃ pollution during this event, and the emissions controls of NOx and the key VOC species of aromatics and alkenes from a cooperative regional perspective should be considered to mitigate O₃ pollution.

Effect of COVID-19 pandemic on ambient air quality and excess risk of particulate matter in Turkey

The COVID-19 pandemic, which has reached 4 million global cases as of March 10, 2020, has become a worldwide problem. Turkey is one of the most affected (9th in the world) country with 139 771 cases. An intermittent curfew policy that differ for three age groups, and an intercity travel ban varying within the country have been implemented. The effects of changes in social life and industrial activity in terms of environmental pollution are not yet known. The short-term effects on PM2.5, PM10, SO2, NO2, NO, NOx, O3 and CO concentrations measured at 51 air quality measurement stations (AQMS) in 11 cities in March - April period of 2020 were statistically compared with that of the previous year. While PM2.5 (9/14 AQMS) and PM10 (29/35 AQMS) concentrations were not significantly affected, NO (12/24 AQMS), NO2 (20/29 AQMS), NOX (17/25 AQMS) concentrations were decreased, SO2 concentrations at half of the AQMSs (11/25) did not show a significant change. There were stations at which higher pollutant concentrations were measured in the study period in 2020 compared to that of 2019. Excess risks associated with PM2.5 and PM10 were estimated to be variable, albeit with a small difference. In conclusion, the heterogeneous actions taken in response to the COVID-19 pandemic resulted in mixed effects on ambient air quality.

A new approach for health-oriented ozone control strategy: Adjoint-based optimization of NOx emission reductions using metaheuristic algorithms

While levels of particulate matters in the Pearl River Delta Region (PRD) show a significant reduction, ozone (O₃) has an opposite increasing trend, becoming the critical air quality target in this decade. Emission control strategies are typically formulated sector by sector, spatial variability in emissions reductions and health impacts of air pollutants may not be taken into account, affecting the overall effectiveness of control strategies. This study proposes an adjoint-based optimization framework to facilitate health-oriented O₃ control over PRD. The location-specific adjoint sensitivity coefficients, which reflect the spatiotemporal influences from emissions of nitrogen dioxide (NO_x) on O₃ health impacts, are combined with metaheuristic algorithms to minimize the O₃-related premature mortalities over receptor regions. Using the proposed optimization methodology, the regional O₃ health benefits under current emission reduction policy can be increased by 16-27%. The results show that relatively larger NO_x emissions reductions occurred at highly developed and populated areas. Particularly, significant reductions in NO_x emissions are observed at Shenzhen and urban Guangzhou. Furthermore, implementing regional NO_x emissions abatement has advantages to achieve an overall O₃ health benefits for all cities. The interregional influences of NO_x emissions abatement between cities indicate a promising strategy of health-oriented O₃ control in PRD.

Emission factors and characteristics of volatile organic compounds (VOCs) from adhesive application in indoor decoration in China

Adhesive application in indoor decoration is an important anthropogenic volatile organic compound (VOC) emission source of both indoors and outdoors. However, few studies have been conducted on VOC emission factors and characteristics from indoor decorating adhesives. In this study, the VOC emission factors were obtained by measurement of VOCs in 210 adhesives. The results showed that the VOC emission factors were 41.23 g/L for wall and ground solidify, 33.49 g/L for tile adhesive, 76.88 g/L for white glue, 52.36 g/L for wallcovering adhesive, 132.28 g/L for sealant glue, 49.33 g/kg for foaming adhesive, 654.23 g/L for all-purpose adhesive, 251.93 g/L for free nails adhesive, 152.01 g/L for marble glue, and 136.79 g/L for beautiful sealant. Methodology for calculating activity data of decorating adhesive consumptions was developed and a VOC emission inventory from adhesive application in indoor decoration was developed using a bottom-up estimation methodology. The VOC emissions from 2012 to 2017 in China were 235,987.76, 246,230.47, 250,981.62, 249,849.48, 227,150.33 and 212, 433.07 t, respectively. The beautiful sealant, wall and ground solidify, sealant glue and all-purpose adhesive contributed the most of the total emissions, collectively accounting for 78.14%. Shandong, Jiangsu, Zhejiang, Sichuan and Guangdong ranked as the top five provinces for VOC emissions, together contributing 39.10% to the national total emissions. Shandong and Jiangsu reached up to 17,057.95 t/year and 15,207.92 t/year, respectively. Priority should be given to four types of adhesives with pretty high VOC contents for designing effective VOC control measures, including solvent-based all-purpose adhesive, solvent-based free nails adhesive, solvent-based sealant glue, and solvent-based beautiful sealant. Future emission trends are projected through 2030 based on current emission control policies and real estate trend. It may be possible to reduce VOC emissions by 60.81% and 69.37% by 2030 under the two scenarios, respectively, compared with the VOC emissions in 2017.

Quantifying the impacts of inter-city transport on air quality in the Yangtze River Delta urban agglomeration, China: Implications for regional cooperative controls of PM2.5 and O3

The Yangtze River Delta (YRD) urban agglomeration is one of the most developed regions in China. During recent decades, this region has experienced severe regional haze and photochemical smog pollution problems. In this study, we used a source-oriented chemical transport model to quantitatively estimate the effects of inter-city transport on fine particulate matter (PM_2.5) and ozone (O₃) among the 41 cities in the YRD urban agglomeration during the EXPLORE-YRD (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in the Yangtze River Delta) campaign (May 17 to June 17, 2018). The results show that inter-city transport is very significant in the YRD region. On average, the emissions from the local city, the other YRD cities, and the regions outside of the YRD contribute 25.3%, 49.9%, and 24.8% to the PM_2.5, respectively, and they contribute 33.7%, 46.8%, and 19.5% of the non-background O₃, respectively. On PM_2.5 or O₃ pollution days, the transport contribution from the non-local YRD cities becomes much more important, while the local emissions and the transport from non-YRD emissions become less important. The results also suggest that the cities within a distance of 184 km and 94 km contribute 60% of the PM_2.5 and O₃, respectively. Therefore, we recommend that regional cooperative control programs in the YRD consider emission controls over cities within these ranges. The range for primary PM_2.5 (92 km) is very different from that for secondary PM_2.5 (515 km)_. Cooperative emission controls of SO₂ and NO_x on a much larger regional scale are required to reduce the secondary PM_2.5 in the YRD.

Emission source-based ozone isopleth and isosurface diagrams and their significance in ozone pollution control strategies

In the past decade, ozone (O₃) pollution has been continuously worsening in most developing countries. The accurate identification of the nonlinear relationship between O₃ and its precursors is a prerequisite for formulating effective O₃ control measures. At present, precursor-based O₃ isopleth diagrams are widely used to infer O₃ control strategy at a particular location. However, there is frequently a large gap between the O₃-precursor nonlinearity delineated by the O₃ isopleths and the emission source control measures to reduce O₃ levels. Consequently, we developed an emission source-based O₃ isopleth diagram that directly illustrates the O₃ level changes in response to synergistic control on two types of emission sources using a validated numerical modeling system and the latest regional emission inventory. Isopleths can be further upgraded to isosurfaces when co-control on three types of emission sources is investigated. Using Guangzhou and Foshan as examples, we demonstrate that similar precursor-based O₃ isopleths can be associated with significantly different emission source co-control strategies. In Guangzhou, controlling solvent use emissions was the most effective approach to reduce peak O₃ levels. In Foshan, co-control of on-road mobile, solvent use, and fixed combustion sources with a ratio of 3:1:2 or 3:1:3 was best to effectively reduce the peak O₃ levels below 145 ppbv. This study underscores the importance of using emission source-based O₃ isopleths and isosurface diagrams to guide a precursor emission control strategy that can effectively reduce the peak O₃ levels in a particular area.

Comprehensive Insights Into O3 Changes During the COVID-19 From O3 Formation Regime and Atmospheric Oxidation Capacity

Economic activities and the associated emissions have significantly declined during the 2019 novel coronavirus (COVID-19) pandemic, which has created a natural experiment to assess the impact of the emitted precursor control policy on ozone (O3) pollution. In this study, we utilized comprehensive satellite, ground-level observations, and source-oriented chemical transport modeling to investigate the O3 variations during the COVID-19 pandemic in China. Here, we found that the significant elevated O3 in the North China Plain (40%) and Yangtze River Delta (35%) were mainly attributed to the enhanced atmospheric oxidation capacity (AOC) in these regions, associated with the meteorology and emission reduction during lockdown. Besides, O3 formation regimes shifted from VOC-limited regimes to NOx-limited and transition regimes with the decline of NOx during lockdown. We suggest that future O3 control policies should comprehensively consider the effects of AOC on the O3 elevation and coordinated regulations of the O3 precursor emissions.

Unraveling the effects of potassium incorporation routes and positions on toluene oxidation over α-MnO2 nanorods: Based on experimental and density functional theory (DFT) studies

Alkali metal potassium is conducive to structure promotion and electronic modulation in metal oxides. Here, K species was successfully introduced into α-MnO₂via in situ synthesis (Pre-K/MnO₂) and hydrothermal impregnation method (Post-K/MnO₂) with target to boost the low-temperature reactivity and deep destruction efficiency for toluene oxidation. Results reveal that Post-K/MnO₂ possesses the highest catalytic activity with toluene (1000 ppm) totally mineralized at just 258 °C, achieving over 70 °C of temperature reduction than that of Pre-K/MnO₂. K specie shows obvious charge transfer balance ability in MnO₂, forming MnO₆-K-MnO₆ bridging bond and leading to more uniform energy of Mn-O bonds. High electron density of K⁺ can promote the activation of oxygen molecules, resulting in a better catalytic performance of toluene. Abundant Brønsted acid sites are beneficial for toluene adsorption and regeneration of hydroxyl on the surface, which promote the degradation of intermediates during toluene oxidation. Moreover, Post-K/MnO₂ shows satisfied catalytic performance under different space velocities and initial concentrations and humid condition. Density functional theory (DFT) calculation revealed the situation of oxygen vacancy and toluene/oxygen adsorption energy in catalysts with different K doping locations. Results showed that the adsorption energy is stronger when K located in large tunnel (0.46 × 0.46 nm), and it is easier to form oxygen vacancy while K entered the small tunnel (0.33 × 0.33 nm). The present work paves new insights into the designing of efficient transition metal oxide catalyst for VOC deep purification.

Anomalous behaviour of ozone under COVID-19 and explicit diagnosis of O3-NOx-VOCs mechanism

Air pollution is linked to higher rates of human mortality especially those infected with COVID 19. Ozone is a harmful pollutant and is responsible for many health issues. However, some reports suggest that ozone is a strong disinfectant, and can kill the viruses. We hereby, report on the vulnerability of ozone due to COVID-19 lockdown whose levels flutter from surging to saturation in a highly polluted Indian capital, due to significant decline in anthropogenic emissions of ozone precursors. Average observed levels stabilized at 30 ppb, 12 ppb, 740 ppb, and 900 ppb for ozone (O₃), nitrogen dioxide (NO₂), carbon monoxide (CO) and volatile organic compounds (VOCs) respectively during lockdown period from 27^th March to 10^th April 2020. The NO₂, CO and VOC declined by 50 %, 37 %, 38 % respectively during the lockdown period of 2020 as compared to similar period in 2019. The anomalous response of ozone during the lockdown is explained by resolving the poorly known complex O₃-NO_x-VOCs mechanism with the help of data from air monitoring stations in Delhi, India. The data obtained from this study advances the fundamental understanding of ozone chemistry that may lead to improved ozone parameterization in chemical transport models and better planning of ozone risk management strategies for any global mega cities.

Effect of emission control measures on ozone concentrations in Hangzhou during G20 meeting in 2016

The effect of emission control measures on ozone (O₃) concentrations in Hangzhou during G20 (The Group of Twenty Finance Ministers and Central Bank Governors) meeting during 24 August to 6 September of 2016 was evaluated using the nested version of a global chemical transport model. During G20, observed concentrations of PM₁₀, PM_2.5, SO₂, NO₂, and CO were all below national air quality standards, whereas those of MDA8 O₃ were above national standard (with an averaged value of 160.2 μg m^-3) but had a decreasing trend. Model sensitivity studies show that, MDA8 O₃ concentrations in Hangzhou during G20 were reduced by 11.3 μg m^-3 (6.8%), 14.8 μg m^-3 (8.9%), and 19.5 μg m^-3 (11.7%) with emission control measures in the core area, Zhejiang province, and the Yangtze River Delta (YRD) region, respectively, indicating that control measures were the most effective when carried out jointly in YRD. Considering the ratios of NO_x to VOCs during G20, Hangzhou and most areas of Zhejiang province were in transitional regime; reductions in either NO_x or VOCs could reduce O₃ concentrations. We also quantified how sensitive O₃ concentrations respond to emission reductions in sectors of industry, power, residential and transportation in the whole of YRD during G20. The removal of emissions in industry and transportation sectors would lead to the largest reductions of 17.6 μg m^-3 (10.5%) and 12.3 μg m^-3 (7.4%) in MDA8 O₃ concentrations in Hangzhou during G20, respectively. This study has important implications for the control of high O₃ levels in eastern China.

Persistent ozone pollution episodes in North China exacerbated by regional transport

Summertime ozone (O₃) concentrations over China continue to increase although the governmental Clear Air Actions have been carried out since 2013. The worst O₃ pollution is confronted over North China Plain. Ozone polluted days (with observed regionally-averaged maximum daily 8-h average (MDA8) O₃ concentrations exceeding 80 ppbv) in May-July in North China increased from 35 days in year 2014 to 56 days in year 2018, and persistent O₃ pollution episodes that lasted for 5 days or longer (OPEs5) contributed 14.3% and 66.1% to those O₃ polluted days in 2014 and 2018, respectively. Model simulations suggest that O₃ transport from central eastern China (including Shandong, Henan, Jiangsu and Anhui Provinces) contributes 36% of the enhanced O₃ concentrations in North China during OPEs5 relative to the seasonal mean. We find that emission control of volatile organic compounds in central eastern China is most effective to alleviate intensity of OPEs5 in North China.

Boosting Toluene Combustion by Engineering Co-O Strength in Cobalt Oxide Catalysts

Exploring active and low-cost transition metal oxides (TMOs) based catalysts for volatile organic compounds (VOCs) abatement is vital for air pollution control technologies. Since 18 oxygen atoms are required for the complete mineralization of a toluene molecule, the participation of a large amount of active oxygen is a key requirement for the catalytic oxidation of toluene. Here, toluene degradation was improved by weakening the Co-O bond strength on the surface of cobalt oxide, so as to increase the amount of active oxygen species, while maintaining the high stability of the catalyst for toluene combustion. The bond strength of Co-O and the amount of surface active O₂ was regulated by tuning the pyrolysis temperature. The catalyst's redox ability and surface oxygen species activity are improved due to the weakening of the Co-O bond strength. It has been demonstrated that active oxygen plays a crucial role in boosting toluene combustion by engineering Co-O strength in cobalt oxide catalysts. This work provides a new understanding of the exploration and development of high-performance TMO catalysts for VOCs abatement.

Contrasting trends of PM2.5 and surface-ozone concentrations in China from 2013 to 2017

Although much attention has been paid to investigating and controlling air pollution in China, the trends of air-pollutant concentrations on a national scale have remained unclear. Here, we quantitatively investigated the variation of air pollutants in China using long-term comprehensive data sets from 2013 to 2017, during which Chinese government made major efforts to reduce anthropogenic emission in polluted regions. Our results show a significant decreasing trend in the PM_2.5 concentration in heavily polluted regions of eastern China, with an annual decrease of ∼7% compared with measurements in 2013. The measured decreased concentrations of SO₂, NO₂ and CO (a proxy for anthropogenic volatile organic compounds) could explain a large fraction of the decreased PM_2.5 concentrations in different regions. As a consequence, the heavily polluted days decreased significantly in corresponding regions. Concentrations of organic aerosol, nitrate, sulfate, ammonium and chloride measured in urban Beijing revealed a remarkable reduction from 2013 to 2017, connecting the decreases in aerosol precursors with corresponding chemical components closely. However, surface-ozone concentrations showed increasing trends in most urban stations from 2013 to 2017, which indicates stronger photochemical pollution. The boundary-layer height in capital cities of eastern China showed no significant trends over the Beijing–Tianjin–Hebei, Yangtze River Delta and Pearl River Delta regions from 2013 to 2017, which confirmed the reduction in anthropogenic emissions. Our results demonstrated that the Chinese government was successful in the reduction of particulate matter in urban areas from 2013 to 2017, although the ozone concentration has increased significantly, suggesting a more complex mechanism of improving Chinese air quality in the future.

Spatial-temporal variations and source contributions to forest ozone exposure in China

As surface ozone (O₃) concentrations have significantly increased in many regions of China, it is concerned that O₃ may cause negative impacts on forests in the country. To estimate the risks of O₃ exposure to forest health, several frequently used O₃ exposure indices (M7, M24, N100, SUM60, W126, and AOT40f) were calculated for the entire year of 2013 and the source contributions to exposure in April and August were quantified using a source-oriented version of the Community Multiscale Air Quality (CMAQ) model. Critical loads for natural ecosystems (12 ppm-h for SUM60) and for moderately sensitive plant species (23.8 ppm-h for W126) are exceeded in 85% and 75% of the forest areas in China, respectively. About 90% of the forest areas have AOT40f higher than the critical load of 10 ppm-h. Forests in the western rim of the Sichuan Basin (WSCB), parts of the southern rim of the Qinghai-Tibetan Plateau (SQTP), Yunnan Province, and the North China Plain (NCP) have higher O₃ exposure than that of other areas. In spring, transport of O₃ and precursors from Southeast Asia have significant contributions to O₃ exposure for forests in Yunnan, SQTP and to a less extent in WSCB. In both spring and summer, industries and transportation sectors have large contributions to O₃ exposure along the WSCB and in the NCP. A higher priority in future field investigations to assess O₃ impacts on forests in China should be given to these regions. The results also suggest that O₃ production in China is much more due to nitrogen oxides (NO_x) than due to volatile organic compounds (VOCs). In order to protect China's forest from O₃, it would be most efficient to reduce NO_x emissions from industries, transportation, and other countries in general.