Variation Of Ozone Concentration Research Articles

Near-surface ozone concentration variability analysis in the Karadag Nature Reserve

The article presents the results of a study of a near-surface ozone concentration variability in the Crimea at the background environmental monitoring station in the Karadag Nature Reserve for 2012–2021 years with a more detailed analysis of the last six years from 2016 to 2021. A significantly high level air pollution of а near-surface ozone was revealed in the observation region, despite the absence of nearby sources of industrial emissions. The relationship of near-surface ozone concentration and meteorological parameters was investigated. Wind directions leading to increased levels of near-surface ozone pollution are established. Intra-annual variations of near-surface ozone concentration are analyzed, the factors causing the local summer minimum of surface ozone concentration in some years are established. By using the NOAA HYSPLIT model and ERA5 reanalysis, a spatial analysis of the atmospheric circulation pattern in the region was carried out. The recurrence of episodes of exceeding the ozone concentration 100 micrograms/m3 during 8 or more hours (WHO recommendation) was estimated. Possible causes of these episodes were determined. The mechanisms of long-range transport and its contribution to the near-surface ozone regime in the area of the station have been established. Annual trends of near-surface ozone concentration in the period 2012–2021 years are estimated as statistically insignificant.

Observation-Based Ozone Formation Rules by Gradient Boosting Decision Trees Model in Typical Chemical Industrial Parks

Ozone pollution in chemical industrial parks is severe and complicated and is significantly influenced by pollutant emissions and meteorological parameters. In this study, we innovatively investigated the formation rules of ozone by using observation-based analyses and a gradient-boosting decision tree (GBDT) model, focusing on a typical chemical industrial park located in the Yangtze River Delta of China. The results revealed that ozone concentration was positively correlated with temperature while negatively correlated with NO2 concentration and relative humidity (RH). Ozone pollution was predominantly observed from April to October (M4–10). The optimized GBDT model was subsequently utilized to establish a specific and quantifiable relationship between each single dominant impact factor (RH, NO2, temperature, and PM2.5) and ozone within a complex and uncertain multi-factor context during M4–10. Detailed discussions were conducted on the reaction rate of ozone-related to different levels of RH and temperature. The accumulation of ozone was favored by high temperature and low RH, with the maximum ozone concentration observed at the RH of 50% and the temperature of 35 °C. The NO2-O3 change curve exhibited distinct phases, including a period of stability, gradual increase, rapid increase, and equilibrium. During the second and third periods, the ratio of ozone production to NO2 consumption was 0.10 and 2.73, respectively. Furthermore, there was a non-monotonic relationship between variations in ozone concentration and PM2.5 concentration. Hence, it is imperative to implement fine control strategies in the park, such as adopting seasonal production strategies, implementing targeted measures for controlling NOx and active VOCs, and employing special control methods during periods of high temperature. This study provides aid in achieving effective management of localized ozone pollution and ensuring compliance with air quality standards.

Spatiotemporal ozone concentration variations and their influencing factors in Zhejiang, China, 2018–2022: A geographically and temporally weighted regression

In this study, we applied an extended geographically and temporally weighted regression model (GTWR) to explore the spatiotemporal non-stationarity of ground-level ozone (O3) concentrations and the influencing factors in four geographic zones of Zhejiang Province from 2018 to 2022. Our results showed that meteorological conditions and emissions were both important for O3 concentration during this period, and their influences showed significant spatial heterogeneity. Relative humidity (Rh) had negative associations with O3 concentration in all the 90 counties in our study period, and there was a strong positive correlation between maximum air temperature (MaxT) and O3 concentration. The impacts of precipitation on O3 were partly positive in Hangzhou Bay and mostly negative in the other three zones due to the precipitation time. The MaxT had the most significant influence in Hangzhou Bay due to the lowest NO2/VOCs ratio, and NO2 positively influenced O3 except in Hangzhou Bay. The high ground-level O3 occurrence in 2022 resulted from meteorological factors and emissions combined.

Spatiotemporal Changes in and Forces Driving Ozone Concentration in the Beijing–Tianjin–Hebei (Jing–Jin–Ji) Region from 2015 to 2022

This study aims to explore the factors affecting variation in ozone concentration, using ozone concentration monitoring data from the Beijing–Tianjin–Hebei (Jing–Jin–Ji) region from 2015 to 2022. The characteristics of the spatial and temporal variation in ozone concentration, as well as the impact of meteorological and land-use types on this variation, were investigated. Seasonally, ozone concentrations in the Jing–Jin–Ji region were highest in summer, followed by spring and autumn, and were lowest in winter. Spatially, a characteristic trend of lower concentrations in urban centers and higher concentrations in suburban areas was found. From the results of correlation analysis and a stepwise regression model, temperature and solar radiation were found to be the primary factors affecting seasonal variations in ozone concentration, explaining 67.9% of the variance. Additionally, the results of geographically weighted regression models confirmed the spatial heterogeneity of the impact of temperature and solar radiation on ozone concentration. In urban centers, such as Beijing and Tianjin, temperature and solar radiation more strongly influenced ozone concentration. To determine the factors driving this spatial heterogeneity, a geodetector was employed to analyze the impact of various geographical factors on the spatial distribution of ozone concentration. In winter, the impact of various factors of urbanization, such as lighting index, population density, and construction land, as well as air quality factors (PM2.5 and NO2), on ozone concentration was stronger, with q values greater than 0.2. However, in autumn, the influence of these factors on ozone concentration weakened. Furthermore, the results of the two-factor interaction detection analysis showed that the q values for the interaction between urban and air quality factors ranged between 0.40 and 0.47, which were substantially higher than those for other interactions. This further confirmed the strong influence of urban activities on ozone concentration.

The fingerprint of tropospheric ozone on broadleaved forest vegetation in Europe

Tropospheric ozone (O3) increased globally in the 20th century, contributes to climate change and can have adverse effects on terrestrial ecosystems. The response of forest vegetation to ozone is modulated by species- and site-specific factors and visible foliar symptoms (VFS) are the only direct evidence of ozone effects on vegetation. VFS have been observed and reproduced under (semi-) controlled conditions and their field assessment has been largely harmonized in Europe. We analyzed ozone concentration and VFS data as measured at (respectively) 118 and 91 intensive monitoring sites of the International Co-Operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) spanning over five European biogeographic regions from 2005 to 2018. Average values for VFS were calculated accounting for the number of species present and their observed frequency. Spatial and temporal variation of ozone concentrations, VFS, and their relationships across Europe were then investigated by applying Generalized Linear Mixed Models (GLMMs) and combined GLMMs. Ozone concentrations exceeded 40 ppb on 37.3 % of the sites and were significantly higher (p < 0.05) in the Alpine and the Mediterranean regions. Over the 2005–2018 period there was a substantial stagnation of ozone concentrations with a tendency towards decreasing values in the Alpine-Boreal sites and increasing values in the Atlantic sites. Ozone left a “fingerprint” in terms of VFS on 38 % of the observed broadleaved woody species across Europe, with no significant difference among biogeographic regions. Overall, and again with the exception of an increase at the Atlantic sites, the frequency of VFS remained unchanged or has been slightly declining over the investigated period. We found positive relationship between ozone concentrations and VFS across Europe (p < 0.05), while their temporal trends (both insignificant) were not related. The species with the highest frequency of VFS were those classified as sensitive species under controlled/semi-controlled experimental conditions. Frequency of VFS tends to be modulated by vegetation traits such as specific leaf area and leaf thickness (p < 0.10). Our results showed that, although ozone levels suggested a North-to-South gradient of increasing potential risk to vegetation with hot spots in the Alps and in the Mediterranean, VFS observed on the actual species assemblage at the sites modifies this picture. According to frequency of VFS, ozone risk for vegetation may be higher in parts of the Alpine and Continental Europe than in the Mediterranean region.

Research on the spatial and temporal patterns of ozone concentration and population health effects in the Central Plains Urban Agglomeration from 2017 to 2020.

Fine particulate matter (PM2.5) and near-surface ozone (O3) are the main atmospheric pollutants in China. Long-term exposure to high ozone concentrations adversely affects human health. It is of great significance to systematically analyze the spatiotemporal evolution mechanism and health effects of ozone pollution. Based on the ozone data of 91 monitoring stations in the Central Plains Urban Agglomeration from 2017 to 2020, the research used Kriging method and spatial autocorrelation analysis to investigate the spatiotemporal variations of ozone concentration. Additionally, the study assessed the health effects of ozone on the population using the population exposure risk model and exposure-response relationship model. The results indicated that: (1) The number of premature deaths caused by ozone pollution in the warm season were 37,053 at 95% confidence interval (95% CI: 28,190-45,930) in 2017, 37,685 (95% CI: 28,669-46,713) in 2018, and 37,655 (95% CI: 28,647-46,676) in 2019. (2) The ozone concentration of the Central Plains urban agglomeration showed a decreasing trend throughout the year and during the warm season from 2017 to 2020, there are two peaks monthly, one is June, and the other is September. (3) In the warm season, the high-risk areas of population exposure to ozone in the Central Plains Urban Agglomeration were mainly concentrated in urban areas. In general, the population exposure risk of the south is lower than that of the north. The number of premature deaths attributed to ozone concentration during the warm season has decreased, but some southern cities such as Xinyang and Zhumadian have also seen an increase in premature deaths. China has achieved significant results in air pollution control, but in areas with high ozone concentrations and high population density, the health burden caused by air pollution remains heavy, and stricter air pollution control policies need to be implemented.

The Responses of Ozone to the Solar Eclipse on the 21st of June 2020 in the Mesosphere and Upper Stratosphere

Microwave Limb Sounder (MLS) observations showed an obvious variation of ozone concentration during the annular solar eclipse on 21 June 2020 in the mesosphere and upper stratosphere. Ozone concentration slightly reduced near 40 km in the regions of 24°N–36°N, and increased in low latitudes at 40 km. In the heights of 45–60 km, the increase in ozone concentration in most of the regions was obvious. The ozone increases and decreases were more obvious between 60–65 km, where enhancement took the leading role. The nighttime ozone variation was weaker than the daytime in most of the heights of 30–65 km. The variation of HO2 and CO is investigated to study the photochemical and dynamical causes of ozone variation. As HO2 decreased at 1 hPa and increased at 60–65 km, ozone variation shows a mostly reversed relationship to HO2 variation. CO increased at 32–39 km and decreased at 52–60 km, which was related to the upwelling at these heights. The dynamic processes also contributed to the decrease in ozone concentration at 40 km and increase at 50–60 km.

Variations in Ozone Concentration in Seven Regions Under Different Temperature and Humidity Conditions

In recent years, the situation of ozone pollution in China has become increasingly severe, with PM2.5 being the main pollutant in the atmospheric environment of several cities. Meteorological conditions, particularly temperature and humidity, have a great influence on ozone formation. Therefore, understanding and quantifying the impact of the variation in temperature and humidity on ozone level can effectively provide the theoretical basis for the government to prevent and control ozone pollution. By analyzing the relationship among the daily maximum temperature (Tmax), relative humidity (RH), and the maximum 8-h running average ozone (O3-8h) measured from January 1, 2015 to July 31, 2022, a linear positive correlation between O3-8h and Tmax was observed in the seven regions with serious ozone pollution, and the temperature penalty factor ranged from 2.1-6.0 μg·(m3·℃)-1; a nonlinear correlation between O3-8h and RH was also observed, and O3-8h was the highest when RH was 55%. The sensitivity of different regions to Tmax and RH was slightly different; generally, the most suitable meteorological conditions for ozone formation were 29℃ ≤ Tmax< 38℃ and 40% ≤ RH<70%. In the Yangtze River Delta, Jiangsu-Anhui-Shandong-Henan, and the middle reaches of the Yangtze River, under extreme high temperature conditions (Tmax ≥ 35℃), O3-8h stopped increasing with the increase in temperature and even dropped; simultaneously, it was often accompanied with a small increase in particulate matter. It may be related to the heterogeneous reaction of some precursors with higher water vapor content and the increase in ozone heterogeneous sink.

Source apportionment of VOCs and ozone formation potential and transport in Chengdu, China

The meteorological conditions and monitoring data of ozone and its precursors (NOx, VOCs) in Chengdu during summer (June–August 2019) were analyzed. The average concentration of VOCs in Chengdu during the summer is 36.63 ± 12.92 ppbv. The daily time series variation of ozone concentration presented unimodal type distribution, reaching the peak in the afternoon (4:00–6:00 p.m.). The heavy ozone pollution was basically accompanied by meteorological conditions of high temperature, low humidity, and low wind speed. The source apportionment of VOCs was conducted by using the positive matrix factorization (PMF) receptor model. The industrial sources, vehicle emission and fuel evaporation were identified as the main sources of VOCs, contributing 26.43%, 21.97% and 17.63% to VOCs respectively. The ozone formation potential (OFP) values indicated that the key active species were aromatics and alkenes represented by m/p-xylene and ethylene. The ridge line of the EKMA curve showed that the VOCs/NOx ratio was about 9.5, hence the emission reduction ratio of VOCs should be much higher than that of NOx in the summer aiming for ozone prevention and control work. The results of backward trajectory clustering, potential source contribution function (PSCF), and concentration-weighted trajectory (CWT) showed that ozone in Chengdu was affected by both local and regional sources in the summer. The potential source regions of ozone are mainly come from local urban areas and eastern and southern Sichuan cities.

Impact of lake-atmosphere exchange on summertime ozone in the Lake Taihu region

Local circulation generated by thermal differences between a lake and the surrounding land impacts dispersion and chemical transformation of ozone, whereas it is challenging to resolve these processes in models. In this study, we use the WRF-Chem model coupled with the modified CLM4.5 lake model to reproduce the local circulation and ozone concentrations during a high-temperature and high-ozone-concentration event in the Lake Taihu (TH) region, and we explore how lake–atmosphere exchange affects ozone pollution. We find that the variations of ozone concentrations around Lake Taihu is regulated by lake breezes and the thermal inner boundary layer (TIBL). Due to the formation of the TIBL, ozone concentrations in the lakeshore area within 5 km from TH are significantly higher than those in other areas by approximately 20 ppb. Lake–atmosphere exchange positively contributes to the ozone pollution event for about 35 ppb at approximately noon, due to the accumulation of ozone and its precursors in the lake area caused by the land breeze at night and the favorable conditions for the photochemical reaction of ozone on the lake surface during the day. This research produces theoretical innovations on the frontier issues related to the interactions of shallow lake–atmosphere–ozone pollution and provides a new understanding of the interactions among the three.

Elucidating the Factors Governing the Interannual Variability of Ozone Concentrations During Fall 2015-2019 in Sanya, China

While ozone pollution has been a major air pollution concern in metropolitans in China, the characteristics and governing factors of ozone concentrations in Sanya remains unclear. In this study, we first analyze the interannual variability of ozone based on observational data in Sanya, and identify it is in general characterized by a peak ozone season in fall and minimal ozone season in summer. Meanwhile, the substantial ozone enhancement in 2019 compared to the previous three to four years over Sanya clearly stands out. To elucidate the possible governing factors, we design a few numerical experiments based on regional air quality model, and find that the modulation of meteorology is key to steering the interannual variability of ozone in fall in Sanya. The spatial evolution further indicates that the transport from upwind regions like Pearl River Delta region is crucial in stimulating the ozone accumulation in Sanya. In addition, ship emissions play important roles in further enhancing their ozone concentration, ranging from 7% -10% during 2015-2019. The findings in this study imply that whereas an overall low ozone concentration in Sanya, ozone exceedance may still occur in particular under unfavorable meteorological conditions together with the concomitant transport from other regions facing ozone pollution. It stresses the importance of regional emission control, including anthropogenic emissions and ship emissions, on improving air quality in Sanya.

Ozone concentration at various heights near the surface layer in Shenyang, Northeast China

Ozone pollution has been growing in the recent decade, becoming a critical urban environmental issue in China. However, Shenyang’s near-surface ozone concentration (ground to 100 m altitude) is of grave concern. To obtain the ozone concentration and the corresponding meteorological conditions from 1 October 2018, to 30 September 2019, we built observatory sites at low (≈15 m) and high (≈90 m) layers in Shenyang, northeast China. Then, we analyzed the temporal variation of ozone concentration and discussed the relationship between ozone concentration and the meteorological factors at low and high layers. Compared with the high layer, the ozone concentration in the low layer is chartered with an earlier peak and higher diurnal range. The ozone concentration in all seasons first peaked at a low-layer height, in early in winter (15:00) and late summer (17:00). Then, the peak at high-layer height lagged for behind 1–2 h, generally occurring between 17:00 and 19:00. The variations in ozone concentration at low and high layers differed in daytime and nighttime. The diurnal ranges of ozone concentrations at the low layer were higher than those at the high layer in the four seasons examined. Overall, ozone concentration correlated significantly with air temperature, having correlation coefficients of 0.814 (p &amp;lt; 0.05) in the low layer and 0.731 (p &amp;lt; 0.05) in the high layer. Inversely, it showed a weak correlation with relative humidity: 0.310 (p &amp;lt; 0.05) and 0.351 (p &amp;lt; 0.05) in the low and high layers, respectively. Also, the correlation coefficients with wind speed were meager at both layers. Generally, ozone pollution is not severe in Shenyang.

Potassium ferrate enhances ozone treatment of pharmaceutical wastewaters: Oxidation and catalysis

This work verified the ability of potassium ferrate to catalyse ozone-driven oxidation of refractory organic matter in a secondary effluent of a wastewater treatment plant at a local pharmaceutical factory. The effect of potassium ferrate reduction products on ozone decomposition was investigated by evaluating the variation in ozone concentration in water. The mechanisms were revealed by separating residual potassium ferrate reduction products from solutions and analysing the dissolved organic carbon (DOC), ultraviolet absorption (UV254), and fluorescent organics. The results showed that (1) the decomposition rate of aqueous ozone was higher with the potassium ferrate reduction products than without them, implying the catalytic ability on ozone decomposition. (2) The reduction products of potassium ferrate during ozone treatment resulted in an increase of 14.9 % in the DOC removal rate and 29.03 % in the UV254 removal rate compared with the results of treatment with potassium ferrate alone. These results show that the reduction products of potassium ferrate can promote ozone oxidation of refractory organic compounds in secondary effluents, and a multistage oxidation system in which potassium ferrate combined with ozone is technically feasible. To the best of our knowledge, this is the first study to verify that the synergistic effect of ferrate (VI)–ozone is due to the catalytic effect of reduction products on ozone oxidation.

Tropical ozone depletion and its correlation with relative sunspot numbers

The paper presents yearly and seasonal variations of ozone concentrations from 1988 to 2005 at Srinagar (34004/ N, 74029/E ), which is an Tropical station. Yearly mean ozone values decreased gradually with different rates from 1988 to 2005. Oscillatory nature of variation ozone concentration with relative sunspot numbers denotes similar contribution of solar parameter and chemical processes in depletion of ozone.

The Impact of Meteorology and Emissions on Surface Ozone in Shandong Province, China, during Summer 2014-2019.

China has been experiencing severe ozone pollution problems in recent years. While a number of studies have focused on the ozone-pollution-prone regions such as the North China Plain, Yangtze River Delta, and Pearl River Delta regions, few studies have investigated the mechanisms modulating the interannual variability of ozone concentrations in Shandong Province, where a large population is located and is often subject to ozone pollution. By utilizing both the reanalysis dataset and regional numerical model (WRF-CMAQ), we delve into the potential governing mechanisms of ozone pollution in Shandong Province—especially over the major port city of Qingdao—during summer 2014–2019. During this period, ozone pollution in Qingdao exceeded the tier II standard of the Chinese National Ambient Air Quality (GB 3095-2012) for 75 days. From the perspective of meteorology, the high-pressure ridge over Baikal Lake and to its northeast, which leads to a relatively low humidity and sufficient sunlight, is the most critical weather system inducing high-ozone events in Qingdao. In terms of emissions, biogenic emissions contribute to ozone enhancement close to 10 ppb in the west and north of Shandong Province. Numerical experiments show that the local impact of biogenic emissions on ozone production in Shandong Province is relatively small, whereas biogenic emissions on the southern flank of Shandong Province enhance ozone production and further transport northeastward, resulting in an increase in ozone concentrations over Shandong Province. For the port city of Qingdao, ship emissions increase ozone concentrations when sea breezes (easterlies) prevail over Qingdao, with the 95th percentile reaching 8.7 ppb. The findings in this study have important implications for future ozone pollution in Shandong Province, as well as the northern and coastal areas in China.

Seasonal Variation of Ozone Concentration and Night-time High Concentration in Sado Island, Japan

Seasonal Variation of Ozone Concentration and Night-time High Concentration in Sado Island, Japan

Spatiotemporal Variations and Influent Factors of Tropospheric Ozone Concentration over China Based on OMI Data

Spatiotemporal distribution of atmospheric pollutants has been a hot topic in the field of environmental science in recent years. Rapid economic development, urbanization, and industrialization have resulted in a significant increase in ozone emissions, and China is facing the issue of air pollution with high ozone concentrations in the ambient air. The Aura ozone monitoring instrument (OMI), can provide long-term and large-scale dynamic monitoring of tropospheric column ozone (TCO). The TCO concentrations over China were extracted and analyzed from 2005 to 2019. The results showed that the spatiotemporal distribution of ozone concentration decreased from northeast to southeast in China. The seasonal variations of ozone concentration were spring’s concentration is greater than winter’s concentration, winter’s concentration is greater than summer’s concentration, summer’s concentration is greater than autumn’s concentration. The monthly variation showed a cyclical trend, with low values observed from June to November and high values from December to May. In addition, the spatiotemporal distribution of ozone concentration was affected by natural factors and anthropogenic factors. Regarding natural factors, TCO concentration was positively correlated with temperature, wind field, and vegetation coverage, but negatively with precipitation in southwest China. Regarding anthropogenic factors, TCO concentration showed a significant positive correlation with regional GDP, oxides of nitrogen (NOx), and volatile organic compounds (VOCs) in most areas of China. Clustering of backward air trajectories revealed that northwest and southeast airflows led to a higher TCO concentration in northeast China. These further indicate that the variation of TCO concentration was affected by many factors, but temperature, wind field, and the emissions of NOx and VOCs were the key factors.

Seasonal and Interannual Variations of Ozone Concentration Across China and the Potential Connection to Enso

Seasonal and Interannual Variations of Ozone Concentration Across China and the Potential Connection to Enso

Spatiotemporal variability and driving factors of ground-level summertime ozone pollution over eastern China

Ground-level ozone is detrimental to human and vegetation health. The ground-level ozone concentration in China has been increasing in recent years. It is necessary to take actions to mitigate ozone pollution. Identifying the variability of ozone concentration is significant for developing an optimal mitigation strategy. In this study, complex empirical orthogonal functions (CEOFs) are used to analyze the spatial-temporal variability in the summertime surface ozone distribution over eastern China during 2015–2017. The first CEOF explains 50% of the ozone variance and represents anthropogenic sources over the North China, with an O3 exceedance period of 7–9 days (maximum daily 8-h average (MDA8) O3 more than 160 μg m−3). The second CEOF indicates a north-south dipole pattern of ozone fluctuation over eastern China. This pattern is mainly subject to the north-south movement of the subtropical high over eastern China and is ultimately modulated by the Philippine Sea Anticyclone Anomaly (PSAA). Our results suggest that policymakers should take weather patterns and interactions between different regions into consideration when designing a strategy to meet O3 air quality objectives.

The diurnal cycle of summer tropospheric ozone concentrations across Chinese cities: Spatial patterns and main drivers

China is experiencing severe tropospheric ozone pollution, especially during the summer period in cities. Previous studies have assessed the role of meteorological conditions and anthropogenic precursors in shaping the diurnal variation of ozone concentration in some Chinese cities or the spatial patterns of daytime ozone concentration, but less is known about the spatial variation and main regulators of the diurnal cycle of summer ozone concentrations in Chinese cities. Using monitoring data from 367 cities, we analyzed the spatial patterns and main regulators of daytime maximum, nighttime minimum and diurnal difference of summer (June–August) ozone concentration during 2015–2019. National mean values and standard deviations of daytime maximum and nighttime minimum of summer surface ozone concentration were 124.1 ± 27.5 and 33.4 ± 13.0 μg m−3, resulting in a diurnal difference of 90.7 ± 25.2 μg m−3. High values of daytime maximum, nighttime minimum, and diurnal difference of summer ozone concentration occurred in cities in northern China, especially in the North China Plain, and several city agglomerations in southern China. Daytime maximum ozone concentration was higher in cities with higher daytime PM2.5 and NO2 concentrations, lower daytime precipitation and lower elevation. Nighttime minimum ozone concentration increased with lower nighttime precipitation, lower NO2 concentration and CO concentration, higher nighttime maximum PM2.5 concentration and higher elevation. Diurnal difference of ozone concentration increased with lower elevation, lower daytime precipitation, and higher diurnal difference of CO and NO2 concentrations. Our findings highlight different regulators for daytime and nighttime ozone and imply the need of joint regulation of PM2.5 and NO2 emissions to control ozone pollution.