Increasing O2 Concentration Research Articles

Unraveling the O3-NOX-VOCs relationships induced by anomalous ozone in industrial regions during COVID-19 in Shanghai

The COVID-19 pandemic promoted strict restrictions to human activities in China, which led to an unexpected increase in ozone (O3) regarding to nitrogen oxides (NOx) and volatile organic compounds (VOCs) co-abatement in urban China. However, providing a quantitative assessment of the photochemistry that leads to O3 increase is still challenging. Here, we evaluated changes in O3 arising from photochemical production with precursors (NOX and VOCS) in industrial regions in Shanghai during the COVID-19 lockdowns by using machine learning models and box models. The changes of air pollutants (O3, NOX, VOCs) during the COVID-19 lockdowns were analyzed by deweathering and detrending machine learning models with regard to meteorological and emission effects. After accounting for effects of meteorological variability, we find increase in O3 concentration (49.5%). Except for meteorological effects, model results of detrending the business-as-usual changes indicate much smaller reduction (−0.6%), highlighting the O3 increase attributable to complex photochemistry mechanism and the upward trends of O3 due to clear air policy in Shanghai. We then used box models to assess the photochemistry mechanism and identify key factors that control O3 production during lockdowns. It was found that empirical evidence for a link between efficient radical propagation and the optimized O3 production efficiency of NOX under the VOC-limited conditions. Simulations with box models also indicate that priority should be given to controlling industrial emissions and vehicle exhaust while the VOCs and NOX should be managed at a proper ratio in order to control O3 in winter. While lockdown is not a condition that could ever be continued indefinitely, findings of this study offer theoretical support for formulating refined O3 management in industrial regions in Shanghai, especially in winter.

Sniffing Drones: A Promising Solution for Measuring Railroad Emissions in Urban Environments

Locomotive emissions from railroads can particularly impact air pollution, making it crucial to understand their impacts on human health and the environment and develop strategies to reduce them. The potential of drone technology equipped with a “sniffing” system for detecting air pollution emissions is promising and can be a valuable tool for assessing dynamic emissions. This research utilized sensor-equipped drones to measure gaseous emissions from cargo and passenger trains on a railway in Curitiba, Brazil. Reference equipment evaluated the accuracy of NO2, SO2, and O3 concentrations. The results showed that before the passage of trains, the average SO2 concentration was 20 µg/m³, with a maximum concentration of 110 µg/m³ detected during transit. The average increase in NO2 concentrations was from 30 µg/m³ to 120 µg/m³, and the average increase in O3 concentrations was from 80 µg/m³ to 135 µg/m³. The vertical profiles were evaluated before and after the passage of locomotives, indicating an accumulation of pollutants above the railroad. These findings demonstrate the potential of sniffing drones to measure railroad emissions in urban environments. They also highlight the need to regulate emissions from diesel-powered locomotives to minimize atmospheric pollution and its negative impact on public health in emerging and developing countries.

Strategic Design of Conquering Hypoxia in Tumor for Advanced Photodynamic Therapy.

Photodynamic therapy (PDT), with its advantages of high targeting, minimally invasive, and low toxicity side effects, has been widely used in the clinical therapy of various tumors, especially superficial tumors. However, the tumor microenvironment (TME) presents hypoxia due to the low oxygen (O2 ) supply caused by abnormal vascularization in neoplastic tissues and high O2 consumption induced by the rapid proliferation of tumor cells. The PDT process generally relies on the presence of O2 , so the efficacy of PDT can be hampered by a hypoxic TME. To address this problem, researchers have been developing advanced nanoplatforms and strategies to enhance the therapeutic effect of PDT in tumor treatment. This review summarizes recent advanced PDT therapeutic strategies to against the hypoxic TME during the PDT treatment, thus enhancing PDT efficacy, including increasing O2 content in TME through delivering O2 to the tumors and in situ generations of O2 ; decreasing the O2 consumption during PDT by design of type I photosensitizers. To boost the PDT therapeutic effect, recent synergistically combination therapy of PDT and other therapeutic methods such as chemotherapy, photothermal therapy (PTT), immunotherapy, gas therapy is accounted by addressing the challenging problems of mono PDT in hypoxic environments, including tumor resistance, proliferation, and metastasis. Finally, we provide perspectives of the opportunities and challenges of PDT in future clinical research and translations. This article is protected by copyright. All rights reserved.

The interactive effects of operating parameters on ash slagging during oxy-biomass combustion based on response surface methodology

Oxy-biomass combustion is one of the most promising carbon-negative technologies for achieving carbon neutrality, while ash slagging heavily hinders its application, which is affected by operating parameters significantly. In this study, the interactive effects of reaction temperature, O2 concentration, and recycled SO2 on the tendency of ash slagging were systematically explored by response surface methodology (RSM). The results showed that O2 concentration had the most important influence on the changes in ash softening temperature (ST), followed by reaction temperature and recycled SO2. Besides, there were strong interactions among these operating parameters, of which increasing O2 concentration dramatically amplified the effects of reaction temperature on ST. Meanwhile, O2 also had the dominant influence on the ash slagging index (RAFI), while it was dramatically weakened with increasing reaction temperature or SO2 concentration. Moreover, a novel method of ternary phase diagram was proposed for the simultaneous prediction of ST and RAFI, which revealed that the ash slagging tendency could be modified to the medium zone by adjusting reaction temperature, O2 concentration, and recycled SO2 to the ranges of 700 °C–730 °C, 40%–45%, 1500 ppm–1900 ppm, respectively.

How does COVID-19 lockdown affect air quality: Evidence from Lanzhou, a large city in Northwest China

Coronavirus disease (COVID-19) has disrupted health, economy, and society globally. Thus, many countries, including China, have adopted lockdowns to prevent the epidemic, which has limited human activities while affecting air quality. These affects have received attention from academics, but very few studies have focused on western China, with a lack of comparative studies across lockdown periods. Accordingly, this study examines the effects of lockdowns on air quality and pollution, using the hourly and daily air monitoring data collected from Lanzhou, a large city in Northwest China. The results indicate an overall improvement in air quality during the three lockdowns compared to the average air quality in the recent years, as well as reduced PM2.5, PM10, SO2, NO2, and CO concentrations with different rates and increased O3 concentration. During lockdowns, Lanzhou's “morning peak” of air pollution was alleviated, while the spatial characteristics remained unchanged. Further, ordered multi-classification logistic regression models to explore the mechanisms by which socioeconomic backgrounds and epidemic circumstances influence air quality revealed that the increment in population density significantly aggravated air pollution, while the presence of new cases in Lanzhou, and medium- and high-risk areas in the given district or county both increase the likelihood of air quality improvement in different degrees. These findings contribute to the understanding of the impact of lockdown on air quality, and propose policy suggestions to control air pollution and achieve green development in the post-epidemic era.

Rapid increase in tropospheric ozone over Southeast Asia attributed to changes in precursor emission source regions and sectors

Observations indicate that tropospheric ozone (O3) concentrations over Southeast Asia have been increasing rapidly since the 1990s. Here, we quantify source contributions from geographical regions and emission sectors of the two distinct types of O3 precursors, i.e., nitrogen oxides (NOx) and volatile organic compounds (VOCs), to the increase in tropospheric O3 in Southeast Asia during 1990–2019 using an O3 source tagging technique implemented in a global chemistry-climate model. The results show that although local anthropogenic emission of NOx in Southeast Asia only contributes 18% of the annual averaged near-surface O3 concentration, the increase in local NOx emission dominates the increasing trend of O3 concentration in Southeast Asia, accounting for 107% of the regional averaged trend of 1.07 ppb decade−1. Increases in NOx emissions from East Asia and South Asia explain 29% of the increasing trend, but 9% is offset by the emission reduction in North America. The vertical O3 trends in the troposphere contributed by individual source regions are consistent with those near the surface. Ground transportation is responsible for 79% of the rapid O3 increase, followed by 39% contribution from international shipping. Because an increase in anthropogenic NOx emissions enhances the O3 production efficiency by VOCs, the increase in O3 concentrations in Southeast Asia is thereby largely contributed by methane and biogenic VOCs.

Experimental and kinetic study of N2O thermal decomposition in pressurized oxy-combustion

Pressurized oxy-combustion is regarded as a promising carbon capture technology for mitigating global warming. In addition, N2O is a significant greenhouse gas in pressurized oxy-combustion at moderate temperatures, with a Global Warming Potential 310 times that of CO2. It is important to study the formation and control of N2O in pressurized oxy-combustion. In this paper, the effects of CO2 atmosphere, pressure, temperature, residence time, and O2 concentration on N2O decomposition were investigated in a pressurized plug flow reactor. The results show that CO2 atmosphere significantly promotes N2O decomposition compared to N2 atmosphere. Increasing temperature accelerates the decomposition of N2O, with 1023–1223 K being the main decomposition temperature range. The decomposition efficiency of N2O increases with increasing pressure, while its increase becomes slow at above 5 bar. N2O decomposition efficiency in CO2 atmosphere decreases slightly as O2 concentration increases. The experimental results were compared to the kinetic modeling results calculated using GRI3.0 and GLA2018 mechanism, respectively. The modeling results show that GRI3.0 mechanism accurately predicts the decomposition of N2O at 1 bar while overestimates the decomposition at elevated pressures, while GLA2018 mechanism significantly underestimates the decomposition at all tested pressures. Further Rate-of-production analysis shows that the main reaction of N2O decomposition is N2O(+M) = N2 + O + M, which becomes the dominant reaction especially at elevated pressures. An enhanced third-body efficiency of 3 for CO2 was incorporated into the reaction N2O(+M) = N2 + O(+M) in GLA2018, which could predict the decomposition of N2O at elevated pressure accurately. The above results indicate that the decomposition of N2O is significantly enhanced in pressurized oxy-combustion.

Numerical simulation of municipal waste and food digestate blending combustion and NOx reduction under oxygen-enriched atmospheres

The paper aims at exploring an approach to clean combustion of municipal solid waste (MSW) with a focus on NOx emissions, in response to a global strategy for clean energy. The objective is to use an orthogonal approach to assess the NOx reduction potential of co-combustion of anaerobic food digestate (AFW) and MSW in an oxygen-enriched atmosphere. The first step is the combustion of MSW in different oxygen concentration atmospheres (21%, 26%, 31%) to find the most efficient oxygen-rich atmosphere. Three AFW blending ratios of 10%, 15% and 20% were then selected on a mass basis to investigate co-combustion with MSW. The results show that the O2/CO2 atmosphere is lower in temperature but more uniformly distributed for the same 21% oxygen concentration. Meanwhile, oxygen-enriched combustion reduces NOx emissions and decreases with increasing O2 concentration, reaching a minimum of 26.36% of NOx production under air atmosphere. As the proportion of food digestate blending increases, the gas phase conversion of N tends to decrease and then increase, reaching a minimum at 15% blending, when the temperature distribution in the furnace chamber is optimal and the outlet NOx emissions are the lowest.

Surface ozone pollution in China: Trends, exposure risks, and drivers.

Within the context of the yearly improvement of particulate matter (PM) pollution in Chinese cities, Surface ozone (O3) concentrations are increasing instead of decreasing and are becoming the second most important air pollutant after PM. Long-term exposure to high concentrations of O3 can have adverse effects on human health. In-depth investigation of the spatiotemporal patterns, exposure risks, and drivers of O3 is relevant for assessing the future health burden of O3 pollution and implementing air pollution control policies in China. Based on high-resolution O3 concentration reanalysis data, we investigated the spatial and temporal patterns, population exposure risks, and dominant drivers of O3 pollution in China from 2013 to 2018 utilizing trend analysis methods, spatial clustering models, exposure-response functions, and multi-scale geographically weighted regression models (MGWR). The results show that the annual average O3 concentration in China increased significantly at a rate of 1.84 μg/m3/year from 2013 to 2018 (160 μg/m3) in China increased from 1.2% in 2013 to 28.9% in 2018, and over 20,000 people suffered premature death from respiratory diseases attributed to O3 exposure each year. Thus, the sustained increase in O3 concentrations in China is an important factor contributing to the increasing threat to human health. Furthermore, the results of spatial regression models indicate that population, the share of secondary industry in GDP, NOx emissions, temperature, average wind speed, and relative humidity are important determinants of O3 concentration variation and significant spatial differences are observed. The spatial differences of drivers result in the spatial heterogeneity of O3 concentration and exposure risks in China. Therefore, the O3 control policies adapted to various regions should be formulated in the future O3 regulation process in China.

Localised corrosion failure of an L245N pipeline in a CO2–O2–Cl− environment

ABSTRACT Multicomponent thermal fluid flooding has become a widely used and very effective heavy-oil recovery technology, but the harsh CO2–O2–Cl− environment can lead to serious corrosion perforation failure of the pipeline. Therefore, the influence of O2 content and main controlling factors on corrosion perforation were investigated through a weight-loss experiment, corrosion defect test, characterization and grey correlation method. The results showed that, with the increase in O2 content, the general corrosion rate and localized corrosion rate increased. The uneven protection performance of the product after the addition of O2 was found to be the key cause of localized corrosion. In addition, the synergistic effect of O2–Cl− promoted the lateral and longitudinal expansion of pits. The results of the grey correlation showed that the temperature (30–120 °C) had a greater impact on localized corrosion compared with the content of O2 (0–0.045 MPa) and Cl− (0–16,000 mg/L).

Ambient ozone – New threat to birds in mountain ecosystems?

Mountain ecosystems are inhabited by species with specific characteristics enabling survival at high altitudes, which make them at risk from various pressures. In order to study these pressures, birds represent excellent model organisms due to their high diversity and position at the top of food chains. The pressures upon mountain bird populations include climate change, human disturbance, land abandonment, and air pollution, whose impacts are little understood. Ambient ozone (O3) is one of the most important air pollutants occurring in elevated concentrations in mountain conditions. Although laboratory experiments and indirect course-scale evidence suggest its negative effects on birds, population-level impacts remain unknown. To fill this knowledge gap, we analysed a unique 25-years long time series of annual monitoring of bird populations conducted at fixed sites under constant effort in a Central European mountain range, the Giant Mountains, Czechia. We related annual population growth rates of 51 bird species to O3 concentrations measured during the breeding season and hypothesized (i) an overall negative relationship across all species, and (ii) more negative O3 effects at higher altitudes due to increasing O3 concentration along altitudinal gradient. After controlling for the influence of weather conditions on bird population growth rates, we found an indication of the overall negative effect of O3 concentration, but it was insignificant. However, the effect became stronger and significant when we performed a separate analysis of upland species occupying the alpine zone above treeline. In these species, populations growth rates were lower after the years experiencing higher O3 concentration indicating an adverse impact of O3 on bird breeding. This impact corresponds well to O3 behaviour and mountain bird ecology. Our study thus represents the first step towards mechanistic understanding of O3 impacts on animal populations in nature linking the experimental results with indirect indications at the country-level.

Experimental and numerical study on ignition and combustion characteristics of boron-magnesium composite powders

A high-pressure laser ignition and combustion system with adjustable oxidizer gas atmosphere is established to investigate the ignition and combustion characteristics of boron-magnesium (BM) composite powders. An ignition and combustion model of BM powders is established and validated in the present study. The results show that increasing water content, O2 content and Mg content all result in shorter ignition delay time of BM powders, among which the effect of water content is the most obvious. However, ignition delay time increases as pressure increases. The combustion time decreases with increasing Mg content and ambient pressure but increases with water content. With the increase of O2 content, combustion time of BM powders first increases and then decreases, which means a critical O2 content exists above which combustion time decreases. The results show that there exists a trade-off between ignition and combustion performance of BM composite powders.

Influence of Oxygen/Steam Addition on the Quality of Producer Gas during Direct (Air) Gasification of Residual Forest Biomass

Biomass gasification is a relevant option to produce a gaseous fuel, it faces, however, several barriers regarding its quality for energetic applications. Therefore, in this study, air-steam and O2-enriched air mixtures were used as gasification agents during the gasification of residual biomass from eucalyptus to improve the producer gas quality. The steam addition promoted an increase in CO2 and H2 concentrations, whilst decreasing the CO and CH4 concentrations. The steam addition had no evident impact on the lower heating value of the dry producer gas and a positive effect on gas yield and the H2:CO molar ratio, attaining the later values up to 1.6 molH2∙mol−1CO. The increase in O2 concentration in the gasification agent (φ) promoted an increase in all combustible species and CO2 concentrations. The lower heating value of the dry producer gas underwent an increase of 57%, reaching a value of 7.5 MJ∙Nm−3dry gas, when the φ increased from 20 to 40 %vol.O2, dry GA. The gas yield had a significant decrease (33%) with φ increase. This work showed that the addition of steam or O2 during air gasification of residual biomass improved producer gas quality, overcoming some of the barriers found in conventional air gasification technology.

Effect of Ar:O2 ratio on magnetic properties of ZnO:Y thin films

4 at.% Y-doped ZnO (4YZO) thin film was fabricated by RF magnetron sputtering, on Si(111) substrate upon varying Ar:O2 ratios (75:25, 50:50, and 40:60). The effect of Ar:O2 ratio on morphological, structural, magnetic, and optical properties of these films have been investigated in detail. The films orientated along the (002) diffraction plane were confirmed by X-ray diffraction pattern. The defect centres in the prepared films were analysed using visible band emission from Photoluminescence spectra which showed the presence of oxygen vacancy (VO+) and zinc interstitial (Zni+). The number of donor defects (VO+ and Zni+) in the films were reduced with an increase in O2 content. The vibrating sample magnetometer results indicated that 4 at.% Y-doped ZnO thin film deposited at an Ar:O2 ratio of 75:25 exhibited ferromagnetic property with a saturation magnetization value of 2.01 emu/cm3. The observed magnetic coercivity value of 500 Oe, makes it suitable for non-volatile magnetic memory storage applications. The films prepared using an Ar:O2 ratio of 50:50 and 40:60 showed a blend of paramagnetic and weak ferromagnetic behaviour. The donor defects present in the films were responsible for the ferromagnetism in 4YZO thin films. The magnetic properties of 4YZO thin films have been explained using the bound magnetic polaron (BMP) model.

Influence of Urbanization on the Spatial Distribution of Associations Between Air Pollution and Mortality in Beijing, China.

This study investigated the influence of urbanization on the intra-city spatial distribution of associations between air pollution and mortality in Beijing, China. First, we utilized the generalized additive model to establish the exposure-response associations of PM2.5, O3, with nonaccidental and cardiorespiratory mortality between urban and suburban areas. Second, we assessed district-specific air pollution-related mortality and analyzed how these associations were affected by the degree of urbanization. Finally, we analyzed the changes in air pollution-related mortality before and after the enforcement of the Air Pollution Prevention and Control Action Plan (referred to as the Action Plan). The effect estimates of PM2.5 for nonaccidental mortality were 0.20% (95% CI: 0.12-0.28) in urban areas and 0.46% (95% CI: 0.35-0.58) in suburban areas per 10μg/m3 increase in PM2.5 concentrations. The corresponding estimates of O3 were 0.13% (95% CI: -0.04-0.29) in urban areas and 0.34% (95% CI: 0.12-0.56) in suburban areas per 10μg/m3 increase in O3 concentrations; however, the difference between the estimates of O3 in urban and suburban areas was not statistically significant. The district-specific results suggested that the estimated risks increased along with urban vulnerability levels for the effects of PM2.5. Implementing the Action Plan reduced the mortality risks of PM2.5, but the risks of O3 increased in some districts. However, the difference in the estimates between the pre- and post-emission reductions was not statistically significant. Our study indicated that populations living in less urbanized areas are more vulnerable to the adverse effects of air pollution in Beijing, particularly for PM2.5.

Air Quality Index (AQI) Did Not Improve during the COVID-19 Lockdown in Shanghai, China, in 2022, Based on Ground and TROPOMI Observations

The lockdowns from the coronavirus disease of 2019 (COVID-19) have led to a reduction in anthropogenic activities and have hence reduced primary air pollutant emissions, which were reported to have helped air quality improvements. However, air quality expressed by the air quality index (AQI) did not improve in Shanghai, China, during the COVID-19 outbreak in the spring of 2022. To better understand the reason, we investigated the variations of nitrogen dioxide (NO2), ozone (O3), PM2.5 (particular matter with an aerodynamic diameter of less than 2.5 μm), and PM10 (particular matter with an aerodynamic diameter of less than 10 μm) by using in situ and satellite measurements from 1 March to 31 June 2022 (pre-, full-, partial-, and post-lockdown periods). The results show that the benefit of the significantly decreased ground-level PM2.5, PM10, and NO2 was offset by amplified O3 pollution, therefore leading to the increased AQI. According to the backward trajectory analyses and multiple linear regression (MLR) model, the anthropogenic emissions dominated the observed changes in air pollutants during the full-lockdown period relative to previous years (2019–2021), whereas the long-range transport and local meteorological parameters (temperature, air pressure, wind speed, relative humidity, and precipitation) influenced little. We further identified the chemical mechanism that caused the increase in O3 concentration. The amplified O3 pollution during the full-lockdown period was caused by the reduction in anthropogenic nitrogen oxides (NOx) under a VOC-limited regime and high background O3 concentrations owing to seasonal variations. In addition, we found that in the downtown area, ground-level PM2.5, PM10, and NO2 more sensitively responded to the changes in lockdown measures than they did in the suburbs. These findings provide new insights into the impact of emission control restrictions on air quality and have implications for air pollution control in the future.

Impact Analysis of Super Typhoon 2114 ‘Chanthu’ on the Air Quality of Coastal Cities in Southeast China Based on Multi-Source Measurements

The northward typhoon configuration along the southeast coast of China (TCN-SEC) is one of the key circulation patterns influencing the coastal cities in southeast China (CCSE). Here, we analyzed the air quality in CCSE during the high-incidence typhoon period from 2019 to 2021. Multi-source measurements were carried out to explore the impact of super typhoon 2114 ‘Chanthu’ on the air quality in CCSE. The results showed that the TCN-SEC and its surrounding weather situation had a favorable impact on the increase in pollutant concentration in CCSE, especially on the increase in O3 concentration. From 13 September to 17 September 2021, affected by the cyclonic shear in the south of super typhoon 2114 ‘Chanthu,’ the strong wind near the ground, stable relative humidity, strong precipitation, and the significantly reduced wind speed had a substantial effect on PM10, PM2.5, SO2, and NO2 concentrations. Calm and light air near the ground, weak precipitation, high daily maximum temperatures, and minimum relative humidity may provide favorable meteorological conditions for the accumulation of O3 precursors and photochemical reactions during the day, resulting in the daily peak values of O3 exceeding 160 μg/m3. The evolution of wind, relative humidity, and boundary layer height could play an important role in the variations in PM10 and PM2.5 concentrations by influencing pollutant accumulation or diffusion. It was suggested that the atmospheric structure of horizontal stability and vertical mixing below 1500 m could play a significant role in the accumulation and vertical distribution of ozone. The results highlight the important role of typhoons in the regional environment and provide a scientific basis for further application of multi-source observation data, as well as air pollution control.

Stratospheric Aerosol and Ozone Responses to the Hunga Tonga‐Hunga Ha'apai Volcanic Eruption

AbstractThe Hunga Tonga‐Hunga Ha'apai (HTHH) eruption on 15 January 2022 was one of the most explosive volcanic events of the 21st century so far. According to satellite‐based measurements, 0.4 Tg of sulfur dioxide (SO2) was injected into the stratosphere during the eruption. By using observations and model simulations, here we investigate changes in the chemical compositions of the stratosphere 1 year after the HTHH eruption and examine the key physical and chemical processes that influence the ozone (O3) concentrations. Injected SO2 was oxidized into sulfate during the first 2 months, and transported from the tropics to the Antarctic by the Brewer‐Dobson circulation within 1 year. In mid‐to‐low latitudes, enhanced sulfate aerosol increased O3 concentrations in the middle stratosphere but declined in the lower stratosphere. In addition to the chemical processes, sulfate aerosols also reduced polar low‐stratospheric O3 concentrations through enhanced Antarctic upwelling anomalies.

Combined Effects of Photochemical Processes, Pollutant Sources and Urban Configuration on Photochemical Pollutant Concentrations

Rapid urbanization, dense urban configuration and increasing traffic emissions have caused severe air pollution, resulting in severe threats to public health. Particularly, photochemical pollution is associated with chemical transformation introducing more complexity. The understanding of the combined effects of pollutant sources, urban configuration and chemical transformation is still insufficient because most previous studies focused on non-reactive pollutant dispersions. In this study, we adopt a simplified street network model including complex photochemical reactions, i.e., the Model of Urban Network of Intersecting Canyons and Highways (MUNICH), with the real traffic and street data of a region in Guangzhou to investigate the combined effects of the three factors above on photochemical pollution. Our simulations show that the overall reduction in traffic emissions decreases NOx pollution while increasing O3 concentration. Controlling VOC emission can effectively mitigate O3 pollution. Moreover, irregular building heights and arrangements can lead to certain hot spots of air pollution. High-rise buildings will obstruct ventilation and exacerbate pollution. If higher buildings have lower vehicle use, the deep canyon can offset the effect of lower emissions. In conclusion, urban planners and policy makers should avoid deep canyons and irregular street networks to achieve better pollutant dispersion and pay attention to controlling VOC emissions.

Impacts of Emission and Meteorological Conditions on Air Pollutants at Various Sites Around the COVID-19 Lockdown in Wuhan

The random forest algorithm was used to separate the mass concentrations of six air pollutants (SO2, NO2, CO, PM10, PM2.5, and O3) contributed by emissions and meteorological conditions. Their variations for five types of sites including Wuhan's central urban, suburb, industrial, the third ring road traffic, and urban background sites were investigated. The results showed that the values of PM2.5/CO, PM10/CO, and NO2/CO during the lockdown period decreased by 10.8-21.7, 9.34-24.7, and 14.4-22.1 times compared with the period before the lockdown, indicating that the contributions of emissions to PM2.5, PM10, and NO2 were reduced. O3/CO increased by 50.1-61.5 times, implying that the secondary formation increased obviously. The contributions of emissions to various types of pollutants all increased after the lockdown. During the lockdown period, affected by the operation of some uninterrupted industrial processes, PM2.5 concentrations in industrial areas dropped the least (20.5%). Compared with the lockdown period, residential activities, transportation, and industrial production were basically restored after the lockdown, resulting in the alleviation of the reduction in PM2.5 emission-related concentrations. The increase in emission-related O3 concentrations could be associated with the decreased NO and PM2.5 concentrations during the lockdown period. The elevated O3 partially offset the improved air quality brought by the reduced NO2and PM2.5 concentrations. After the lockdown, ρ(O3) related with meteorology at the suburban and urban background sites increased by 16.2 μg·m-3 and 16.1 μg·m-3, respectively, which could be attributed to the increased ambient temperature and decreased relative humidity. The decrease in PM2.5 and increase in O3 concentrations caused by reduced traffic and industrial emissions at the third ring road traffic and central urban regions can provide reference for the current coordinated and precise control of PM2.5 and O3 in subregions.