The sensitivity analysis of ozone generation in key ozone-polluted regions and cities is an important basis for the prevention and control of near-surface ozone （O3） pollution. Based on the five-year data of ozone, VOCs, and NOx from three typical stations in Shanghai, namely Dianshan Lake Station （suburban area）, Pudong Station （urban area）, and Xinlian Station （industrial area） from 2016 to 2020, the nonlinear relationship between ozone and precursors （VOCs and NOx） during the high-ozone season in the five years was quantitatively analyzed using an observation model. The results showed that the peak months of near-surface ozone in Shanghai were from April to September during 2016 to 2020, with the highest values appearing from June to August. The volume fraction of VOCs and NO2 concentration had a strong indicative significance for the O3 concentration at Pudong Station. The O3 concentration at Dianshan Lake Station was mainly influenced by regional environment, meteorological factors, and cross-regional transmission. The ozone concentration at Xinlian Station was a combination of environmental background concentration and industrial area photochemical pollution. Pudong Station and Dianshan Lake Station were in the VOCs control zone. Xinlian Station was gradually closer to the NOx control zone from 2016 to 2019, transitioning to the VOCs control zone since 2020. The L·OH of Pudong Station, Dianshan Lake Station, and Xinlian Station were： NOx control area>collaborative control area>VOCs control area.

Based on the one-year observational data of volatile organic compounds （VOCs） in an urban area of Yuncheng in 2021， the concentration， composition， sources， and ozone-sensitive species of VOCs in four seasons were analyzed. The results showed that the average annual concentration of VOCs was （32.1 ±24.2）×10-9， i.e.， at the national middle level. The seasonal concentrations of VOCs were in the order of： winter （46.3×10-9）> autumn （35.5×10-9）> spring （25.6×10-9）> summer （21.2×10-9）. Alkanes and OVOCs were the most dominant VOCs compounds， accounting for 69.0%-80.4% of TVOCs in Yuncheng. Affected by changes in source emissions， the proportion of OVOCs was higher in spring and summer （41%-43%）， whereas the proportion of alkanes was higher in autumn and winter （42%-43%）. Vehicle exhaust， LPG/NG， industrial production， and combustion sources were identified as the main sources of VOCs in Yuncheng. The largest contributors in the four seasons were vehicle exhaust （28.5% in spring）， secondary + combustion sources （29.0% in summer）， LPG/NG sources （30.4% in autumn）， and coal combustion （27.3% in winter）. The ozone formation was located in the transitional regime in summer and in the VOC-limited regime in other seasons. Ozone production was more sensitive to alkenes （isoprene， ethylene， and propene）， OVOCs （acetaldehyde and propanal）， and aromatics （xylene， toluene， and benzene）. Winter was more sensitive to ethylene， and the other seasons were more sensitive to isoprene. The primary emission sources related to these sensitive species should be reduced to achieve the goal of air quality improvement.

Tropospheric ozone (O3) is mainly produced through a series of photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). The reaction process presents complex non-linear relationships. In this work, datasets of atmospheric ozone and volatile organic compounds (VOCs) observed during the summer of 2018 in Nanjing were used. Combining with the framework for 0-D atmospheric model-master chemical mechanism (F0AM-MCM), the characteristics of photochemical reactions for ozone (O3) formation in Nanjing during the O3 episode days and non-episode days were investigated. The results showed that φ(O3) and φ(TVOCs) in the O3 episode days were 47.8×10-9 and 49.0×10-9, respectively, exceeding those in the non-episode days by factors of 1.8 and 1.6. Furthermore, F0AM, the empirical kinetic modeling approach (EKMA), and relative incremental reactivity (RIR) were utilized for the calculation of ozone chemical sensitivity. It was found that O3 formation in Nanjing was attributed to both VOCs and NOx limitation. In addition, the modeled ·OH and HO2 concentrations in the O3 episode days were 1.3 and 1.8 times higher than those in the non-episode days. The higher formation and loss rates of ·OH and HO2 were also found during O3 episode days. These findings reflected that the enhancements of atmospheric oxidation capacity resulted in increased production rates of O3, providing an explanation for the enhancements of O3 concentrations in Nanjing during the O3 episode days. The findings also improved the understanding of the O3 photochemical characteristics over Nanjing in the summer during the O3 episode days.

High tropospheric ozone (O3) concentrations prevent the improvement of the air quality in the Mexico City Metropolitan Area (MCMA). Although the problem has improved considerably since the 1990s, a rebound in O3 levels in recent years has raised concerns about the deteriorating air quality. The nonlinear relationship between O3 formation and the emissions of its main precursors, i.e., volatile organic compounds (VOCs) and nitrogen oxides (NOx), is a challenge when measures are enacted for effective mitigation of the O3 problem. This study evaluated the reduction in precursors, VOCs and NOx, using an up-to-date regional air quality model (HERMES-Mex-WRF-CMAQ). For evaluating realizable scenarios, the decline in VOC achieved in Japan after policy implementation was the targeted VOC reduction (40 % from area sources), and the NOx reduction observed in the MCMA during the COVID-19 pandemic was the targeted NOx reduction (40 % from mobile sources). The analysis evaluated the O3 responses to changes in a single precursor and a combination of both during a period of high O3 concentrations (April 2019). The results showed that 40 % reduction in VOC emissions would decrease the O3 8-h maximum concentrations by 16 %. However, 40 % reduction in NOx emissions would increase O3 by >15 %. The simultaneous reduction of both precursors did not significantly affect O3 levels. The diagnosis of ozone sensitivity using the H2O2/HNO3 ratios reinforced the simulation findings, indicating that VOC emissions limited ozone formation in most MCMA areas. As the simulated scenarios were based on factual case studies, our research offers insights into the realistic aims of MCMA policies to reduce O3 levels.

This study presents a comprehensive overview of the atmospheric pollutants including Sulfur dioxide (SO2), Nitrogen dioxide (NO2), Formaldehyde (HCHO), Particulate Matter PM; PM10: diameter ≤ 10 µm, and PM2.5: diameter ≤ 2.5 µm), and Ozone (O3), over Dongying (Shandong Province) from March-April 2018 and September-October 2019 by employing ground-based Multiple Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations along with the in-situ measurements attained by the national air quality monitoring platform. The concentrations of SO2 and NO2 were under the acceptable level, while both PM2.5, and PM10 were higher than the safe levels as prescribed by national and international air quality standards. The results depict that 21% of the total observation days were found to be complex polluted days (PM2.5 > 35 µg/m3 and O3 > 160 µg/m3). The secondary HCHO was used for accurate analysis of O3 sensitivity. A difference of 11.40% and 10% during March-April 2018 and September-October 2019 respectively in O3 sensitivity was found between HCHOtotal/NO2 and HCHOsec/NO2. The results indicate that primary HCHO have significant contribution in HCHO. O3 formation predominantly remained to be in VOC-limited and transitional regime during March-April 2018 and September-October 2019 in Dongying. These results imply that concurrent control of both NOx and VOCs would benefit in ozone reductions. Additionally, the criteria pollutants (PM, SO2, and NO2) depicted strong correlations with each other except for O3 for which weak correlation coefficient was obtained with all the species. This study will prove to be baseline for designing of air pollution control strategies.

Based on ambient air quality data, meteorological observation data, and satellite remote sensing data, the temporal and spatial variations in ozone (O3) pollution, the sensitivity of O3, and its relationship with meteorological factors in Hainan Island were analyzed in this study. The results showed that the maximum daily 8-h moving mean (O3-8h) in western and northern cities in Hainan Island was higher than that in the central, eastern, and southern cities. O3-8h was the highest in 2015, and O3-8h exceeding the standard proportion was the largest in 2019. In addition, O3-8h was positively correlated with average temperature (P<0.1), sunshine duration (P<0.01), total solar radiation (P<0.01), atmospheric pressure, and average wind speed and was negatively correlated with precipitation (P<0.05) and relative humidity. The satellite remote sensing data showed that the tropospheric NO2 column concentration (NO2-OMI) and HCHO column concentration (HCHO-OMI) displayed opposite trends in Hainan Island from 2015 to 2020. Compared with those in 2015, NO2-OMI increased by 7.74% and HCHO-OMI decreased by 10.2% in 2020. Moreover, Hainan Island belongs to the NOx control area, and the FNR value exhibited a fluctuating downward trend in the past 6 years, with a trend coefficient and climatic trend rate of -0.514 and -0.123 a-1, respectively. A strong correlation was observed between meteorological factors and the FNR value of Hainan Island.

Due to the nonlinear impacts of meteorology and precursors, the response of ozone (O3) trends to emission changes is very complex over different regions in megacity Beijing. Based on long-term in-situ observations at 35 air quality sites (four categories, i.e., urban, traffic, northern suburban and southern suburban sites) and satellite data, spatiotemporal variability of O3, gaseous precursors, and O3-VOCs-NOx sensitivity were explored through multiple metrics during the warm season from 2013 to 2020. Additionally, the contribution of meteorology and emissions to O3 was separated by a machine-learning-based de-weathered method. The annual averaged MDA8 O3 and O3 increased by 3.7 and 2.9 μg/m3/yr, respectively, with the highest at traffic sites and the lowest in northern suburb, and the rate of Ox (O3 + NO2) was 0.2 μg/m3/yr with the highest in southern suburb, although NO2 declined strongly and HCHO decreased slightly. However, the increment of O3 and Ox in the daytime exhibited decreasing trends to some extent. Additionally, NOx abatements weakened O3 loss through less NO titration, which drove narrowing differences in urban-suburban O3 and Ox. Due to larger decrease of NO2 in urban region and HCHO in northern suburb, the extent of VOCs-limited regime fluctuated over Beijing and northern suburb gradually shifted to transition or NOx-limited regime. Compared with the directly observed trends, the increasing rate of de-weathered O3 was lower, which was attributed to favorable meteorological conditions for O3 generation after 2017, especially in June (the most polluted month); whereas the de-weathered Ox declined except in southern suburb. Overall, clean air actions were effective in reducing the atmospheric oxidation capacity in urban and northern suburban regions, weakening local photochemical production over Beijing and suppressing O3 deterioration in northern suburb. Strengthening VOCs control and keeping NOx abatement, especially in June, will be vital to reverse O3 increase trend in Beijing.

Tropospheric ozone (O3) is a typical air pollutant with harmful effects on plants, whereas arbuscular mycorrhizal (AM) fungi are ubiquitous plant symbionts that enhance plant resistance to various abiotic stresses. However, whether AM symbiosis decreases plant O3 sensitivity and what the underlying mechanisms are remain unclear. In this study, O3-tolerant poplar clone 107 and O3-sensitive poplar clone 546 were used as test plants. An open-top chamber experiment was conducted to investigate the effects of AM inoculation on plant growth and physiological parameters under O3 enrichment. The results showed that O3 enrichment significantly decreased plant biomass and net photosynthetic rate and increased the leaf shedding rate and malondialdehyde concentration of clone 546. Generally, clone 107 was less responsive to O3 enrichment than clone 546 was. Differences in antioxidant enzyme activity, rather than in specific leaf weight or stomatal conductance, were responsible for the differences in O3 sensitivity between the two clones. AM inoculation significantly increased the biomass and decreased the leaf shedding rate and malondialdehyde concentration of clone 107 but had no significant effect on almost all the indexes of clone 546, suggesting a species-specific mycorrhizal effect on plant O3 sensitivity. Mechanistically, AM symbiosis did not significantly affect nutrient uptake, stomatal conductance, or specific leaf weight of poplar but did significantly increase antioxidant enzyme activity. Linear regression analysis of antioxidant enzyme activities and the effect of O3 on growth and physiological parameters showed that AM symbiosis mediated antioxidant enzyme activities to mitigate O3 injury to the two poplar clones. This study improved the understanding of the protective effects of AM fungi on plants against O3 pollution.

This study was based on the observation of volatile organic compounds (VOCs), conventional gaseous air pollutants, and meteorological parameters observed at the Xinxiang Municipal Party School site from June to August 2021. The ozone (O3) characteristics and sensitivity of O3 pollution days and the control strategy of its precursors were studied using an observation-based model (OBM). It was found that the meteorological conditions were characterized by high temperature, low humidity, and low pressure in O3-pollution days. The concentrations of O3 and its precursors all increased in the O3 pollution days. Oxygenated volatile organic compounds (OVOCs) and alkanes were the highest-concentration components of VOCs on O3 pollution days in Xinxiang, and OVOCs had the highest ozone formation potential (OFP) and hydroxyl (·OH) reactivity. According to the relative incremental reactivity (RIR) analysis, during the O3 pollution days in Xinxiang, O3sensitivity was in the VOCs-limited regime in June and in the transitional regime in July and August. Ozone production was more sensitive to alkenes and OVOCs. The RIR values of the precursors in June changed throughout the day, but O3 sensitivity remained the VOCs-limited regime. In July and August, O3 sensitivity was the VOCs-limited regime in the morning, transitional regime at noon, transitional and NOx-limited regime, respectively in the afternoon. By simulating different precursor-reduction scenarios, the results showed that the reduction of VOCs was always beneficial to the control of O3, whereas the reduction of NOx had little effect on the control of O3 and a risk of increasing O3.

Liucheng county, as a suburb of Liuzhou City in Guangxi province, has a prominent ozone (O3) pollution problem; however, there have been no relevant analyses of the cause of local O3 pollution reported. In order to investigate the causes of O3 pollution, an online observation of 116 VOCs with a time resolution of 1 h was carried out in Liucheng county from October 1st to 15th, and the sensitivity of ozone to the relative changes in the NOx and VOCs was analyzed. The results showed that the average value of φ［total volatile organic compounds (TVOCs)］ during the observation period was 27.52×10-9, and the average value of φ(TVOCs) during the polluting process (October 1st to 6th) was 32.15×10-9, which was 32.79% higher than that of the non-pollution process (October 8th to 15th). In terms of species concentration, oxygenated volatile organic compounds (OVOCs) contributed the highest, accounting for 43.70%, followed by alkanes (23.00%), aromatics (11.75%), and halocarbons (7.35%). In terms of ozone formation potential (OFP), OVOCs contributed the highest (41.96%) to OFP, followed by aromatics (32.60%) and alkenes (17.92%). During the observation period, VOCs mainly came from motor vehicle emissions (32.44%), biomass combustion sources (29.31%), solvent use sources (16.43%), plant sources (11.34%), and chemical industry emissions (10.49%). The contribution ratios of solvent use sources and plant sources in the pollution process increased by 28.58% and 28.53%, respectively. The EKMA curve shows that, during the observation period, Liucheng county was in a synergistic control area for VOCs and nitric oxide (NOx). Therefore, in the high ozone-occurrence autumn of Liucheng county, the key will be to reduce both VOCs and NOx emissions.