Photochemical ozone pollution in five Chinese megacities in summer 2018

Characteristics of surface ozone pollution and its correlations with meteorological factors: the case of Xiangtan

Based on ozone (O3) monitoring data for Xiangtan and meteorological observation data for 2020-2022, we examined ozone pollution characteristics and the effects of meteorological factors on daily maximum 8-h average ozone (O3-8h) concentrations in Xiangtan. Thus, we observed significant increases as well as notable seasonal variations in O3-8h concentrations in Xiangtan during the period considered. The ozone and temperature change response slope (KO3-T) indicated that local emissions had no significant effect on O3-8h generation. Further, average O3-8h concentration and maximum temperature (Tmax) values showed a polynomial distribution. Specifically, at Tmax < 27 °C, it increased almost linearly with increasing temperature, and at Tmax between 27 and 37 °C, it showed an upward curvilinear trend as temperature increased, but at a much lower rate. Then, at Tmax > 37 °C, it decreased with increasing temperature. With respect to relative humidity (RH), the average O3-8h concentration primarily exceeded the standard value when RH varied in the range of 45-65%, which is the key humidity range for O3 pollution, and the inflection point for the correlation curve between O3-8h concentration and RH appeared at ~55%. Furthermore, at wind speeds (WSs) below 1.5 m∙s-1, O3-8h concentration increased rapidly, and at WSs in the 1.5-2 m∙s-1 range, it increased at a much faster rate. However, at WSs > 2 m∙s-1, it decreased slowly with increasing WS. O3-8h concentration also showed the tendency to exceed the standard value when the dominant wind directions in Xiangtan were easterly or southeasterly.

Analyzing ozone formation sensitivity in a typical industrial city in China: Implications for effective source control in the chemical transition regime

Volatile organic compounds (VOCs) play a major role in O3 formation in urban environments. However, the complexity in the emissions of VOCs and nitrogen oxides (NOx) in industrial cities has made it challenging to identify the key factors influencing O3 formation. This study used observation-based-model (OBM) to analyze O3 sensitivities to VOCs and NOx during summer in a typical industrial city in China. The OBM model results were coupled with a receptor model to analyze the sources of O3. Higher concentrations of O3 precursors were observed during polluted periods indicating that precursor accumulation contributed to the higher maxima of the net ozone formation rate and HOx concentrations. Analyses of ROx· budgets and relative incremental reactivity (RIR) indicated that O3 production is in a chemical transition regime and was sensitive to both VOCs and NOx. Results from Positive Matrix Factorization (PMF) analysis indicated that gasoline vehicle emissions, industrial processes, and coal combustion were major sources of O3 precursors. The sensitivities of O3 production to these sources depend on if both VOC and NOx sensitivities are considered. If only VOCs sensitivity is considered, in contrast, the contribution of anthropogenic sources to O3 production was significantly underestimated. This study highlights the importance of accounting for both VOCs and NOx sensitivities when O3 chemistry is in a transition regime in O3 production attribution studies.

Research on ozone formation sensitivity based on observational methods: Development history, methodology, and application and prospects in China

Observation-based method for O3 formation sensitivity research is an important tool to analyze the causes of ground-level O3 pollution, which has broad application potentials in determining the O3 pollution formation mechanism and developing prevention and control strategies. This paper outlined the development history of research on O3 formation sensitivity based on observational methods, described the principle and applicability of the methodology, summarized the relative application results in China and provided recommendations on the prevention and control of O3 pollution in China based on relevant study results, and finally pointed out the shortcomings and future development prospects in this field in China. The overview study showed that the O3 formation sensitivity in some urban areas in China in recent years presented a gradual shifting tendency from the VOC-limited regime to the transition regime or the NOx-limited regime due to the implementation of the O3 precursors emission reduction policies; O3 pollution control strategies and precursor control countermeasures should be formulated based on local conditions and the dynamic control capability of O3 pollution control measures should be improved. There are still some current deficiencies in the study field in China. Therefore, it is recommended that a stereoscopic monitoring network for atmospheric photochemical components should be further constructed and improved; the atmospheric chemical mechanisms should be vigorously developed, and standardized methods for determining the O3 formation sensitivity should be established in China in the near future.

Sensitivities of ozone to its precursors during heavy ozone pollution events in the Yangtze River Delta using the adjoint method

Although the concentrations of five basic ambient air pollutants in the Yangtze River Delta (YRD) have been reduced since the implementation of the "Air Pollution Prevention and Control Action Plan" in 2013, the ozone concentrations still increase. In order to explore the causes of ozone pollution in YRD, we use the GEOS-Chem and its adjoint model to study the sensitivities of ozone to its precursor emissions from different source regions and emission sectors during heavy ozone pollution events under typical circulation patterns. The Multi-resolution Emission Inventory for China (MEIC) of Tsinghua University and 0.25° × 0.3125° nested grids are adopted in the model. By using the T-mode principal component analysis (T-PCA), the circulation patterns of heavy ozone pollution days (observed MDA8 O3 concentrations ≥160 μg m-3) in Nanjing located in the center area of YRD from 2013 to 2019 are divided into four types, with the main features of Siberian Low, Lake Balkhash High, Northeast China Low, Yellow Sea High, and southeast wind at the surface. The adjoint results show that the contributions of emissions emitted from Jiangsu and Zhejiang are the largest to heavy ozone pollution in Nanjing. The 10 % reduction of anthropogenic NOx and NMVOCs emissions in Jiangsu, Zhejiang and Shanghai could reduce the ozone concentrations in Nanjing by up to 3.40 μg m-3 and 0.96 μg m-3, respectively. However, the reduction of local NMVOCs emissions has little effect on ozone concentrations in Nanjing, and the reduction of local NOx emissions would even increase ozone pollution. For different emissions sectors, industry emissions account for 31 %-74 % of ozone pollution in Nanjing, followed by transportation emissions (18 %-49 %). This study could provide the scientific basis for forecasting ozone pollution events and formulating accurate strategies of emission reduction.

Severe photochemical pollution was found in large petrochemical complexes: A typical case study in North China

Large petrochemical complex (PC) widely exists in both developing and developed countries, and is expected to have a special photochemical pollution in local scale due to huge VOCs emissions. Here, a typical large-scale PC in North China was selected as the study case, to explore the character, formation and influence of local photochemical pollution regarding PCs based on an improved 0-D chemical model. In the study PC, VOCs-rich character was apparent with THCs level of 90.8 ± 28.0 ppb and THCs/NOx ratio of ∼26.2 mol/mol. Severe O3 pollution was found in warm months with monthly mean MDA1O3 of 67.3-96.0 ppb. Model simulations showed the heavy O3 pollution in this PC was attributed to high precursors rather than to unfavorable meteorology, and was more sensitive to NOx (with response of 1.42 g/g) than to THCs (with response of 0.12 g/g). The photochemical pollution formation potential of the emission plumes of this PC was very enormous, with production rate of 19.6 ppb h-1 for O3, 2.9 ppb h-1 for HCHO and 1.1 ppb h-1 for CH3CHO on daytime average, 1-5 greater than in normal urban areas. The higher production rates happened in morning hours, which explained the earlier peak time of observed O3 in PCs. And about 70% of photochemical pollution (represented by O3) would be transported to surroundings, leading to the significant photochemical-pollution hazard to the vicinity of PCs.

Atmospheric oxidation capacity and O3 formation in a coastal city of southeast China: Results from simulation based on four-season observation

The pollution of atmospheric ozone in China shows an obvious upward trend in the past decade. However, the studies on the atmospheric oxidation capacity and O3 formation in four seasons in the southeastern coastal region of China with the rapid urbanization remain limited. Here, a four-season field observation was carried out in a coastal city of southeast China, using an observation-based model combining with the Master Chemical Mechanism, to explore the atmospheric oxidation capacity (AOC), radical chemistry, O3 formation pathways and sensitivity. The results showed that the average net O3 production rate (14.55 ppbv/hr) in summer was the strongest, but the average O3 concentrations in autumn was higher. The AOC and ROx levels presented an obvious seasonal pattern with the maximum value in summer, while the OH reactivity in winter was the highest with an average value of 22.75 sec-1. The OH reactivity was dominated by oxygenated VOCs (OVOCs) (30.6%-42.8%), CO (23.2%-26.8%), NO2 (13.6%-22.0%), and alkenes (8.4%-12.5%) in different seasons. HONO photolysis dominated OH primary source on daytime in winter, while in other seasons, HONO photolysis in the morning and ozone photolysis in the afternoon contributed mostly. Sensitivity analysis indicated that O3 production was controlled by VOCs in spring, autumn and winter, but a VOC-limited and NOx-limited regime in summer, and alkene and aromatic species were the major controlling factors to O3 formation. Overall, the study characterized the atmospheric oxidation capacity and elucidated the controlling factors for O3 production in the coastal area with the rapid urbanization in China.

Evolution of Ozone Formation Sensitivity during a Persistent Regional Ozone Episode in Northeastern China and Its Implication for a Control Strategy

In recent years, the magnitude and frequency of regional ozone (O3) episodes have increased in China. We combined ground-based measurements, observation-based model (OBM), and the Weather Research and Forecasting and Community Multiscale Air Quality (WRF-CMAQ) model to analyze a typical persistent O3 episode that occurred across 88 cities in northeastern China during June 19-30, 2021. The meteorological conditions, particularly the wind convergence centers, played crucial roles in the evolution of O3 pollution. Daily analysis of the O3 formation sensitivity showed that O3 formation was in the volatile organic compound (VOC)-limited or transitional regime at the onset of the pollution episode in 92% of the cities. Conversely, it tended to be or eventually became a NOx-limited regime as the episode progressed in the most polluted cities. Based on the emission-reduction scenario simulations, mitigation of the regional O3 pollution was found to be most effective through a phased control strategy, namely, reduction of a high ratio of VOCs to NOx at the onset of the pollution and lower ratio during evolution of the O3 episode. This study presents a new possibility for regional O3 pollution abatement in China based on a reasonable combination of OBM and the WRF-CMAQ model.

Ambient ozone pollution impairs glucose homeostasis and contributes to renal function decline: Population-based evidence

Particulate matter pollution could increase the risk of kidney disease, while evidence for ozone exposure is less well-established. Here, we aimed to evaluate the effect of ozone pollution on renal function and explore mechanisms. We first conducted a cross-sectional study based on Wuhan Chronic Disease Cohort Study baseline information. We recruited 2699 eligible participants, estimated their residential ozone concentrations, collected fasting peripheral blood samples for biochemical analysis and calculated the estimated glomerular filtration rate (eGFR). The linear regression model was applied to evaluate the long-term association between ozone pollution and eGFR. Then, we recruited another 70 volunteers as a panel with 8 rounds follow-up visits. We calculated the eGFR and measured fasting blood glucose and lipid levels. The linear mixed-effect model along with mediation analysis were performed to confirm the short-term association and explore potential mechanisms, respectively. For the long-term association, a 10.95 μg/m3 increment of 3-year ozone exposure was associated with 2.96 mL/min/1.73 m2 decrease in eGFR (95%CI: -4.85, -1.06). Furthermore, the drinkers exhibited a pronounced declination of eGFR (-7.46 mL/min/1.73 m2, 95%CI: -11.84, -3.08) compared to non-drinkers in relation to ozone exposure. Additionally, a 19.02 μg/m3 increase in 3-day ozone concentrations was related to 2.51 mL/min/1.73 m2 decrease in eGFR (95%CI: -3.78, -1.26). Hyperglycemia and insulin resistance mediated 12.2% and 16.5% of the aforementioned association, respectively. Our findings indicated that higher ozone pollution could affect renal function, and the hyperglycemia and insulin resistance linked to ozone might be the underlying mechanisms.

Tibetan Plateau is vulnerable to aromatic-related photochemical pollution and health threats: A case study in Lhasa

Anthropogenic aromatics play a key role in photochemical pollution and pose a serious threat to human health. Current knowledge on source characteristics of aromatics in the urban region of the Tibetan Plateau (TP), the "Third Pole" and ecologically sensitive area, remains limited. In this study, an intensive observation of 17 aromatic hydrocarbons was conducted in Lhasa, the cultural and economic center of TP, during the second Tibetan Plateau Scientific Expedition and Research in summer 2020. The results showed that the average concentration of aromatics in Lhasa (7.6 ± 7.4 ppbv) was unexpectedly higher than those in megacities such as Beijing, Shanghai, and Guangzhou. Tripled concentrations and corresponding ozone formation potential during pollution episodes were recorded. Further source apportionment using positive matrix factorization revealed that solvent usage (60.0 %) was the dominant source, which may be due to the extremely low atmospheric pressure. Vehicle exhaust (15.4 %), industrial emissions (12.8 %), fuel evaporation (6.2 %), and burning emissions (5.7 %) were also important sources. The concentration weighted trajectory analysis revealed that the observed high levels of aromatics were mainly driven by local anthropogenic emissions, rather than the regional transport by the Indian summer monsoon. Long-term exposure to aromatics in Lhasa was assessed to pose carcinogenic risks to the population, with the risks of benzene and ethylbenzene 5 times the criteria. Our results suggest that, given the magnified emissions of aromatics in this extreme environment (low atmospheric pressure and strong solar radiation), the implementation of targeted pollution controls is urgently needed to mitigate the aromatic-related photochemical pollution and health threats in TP.

Photochemistry in the urban agglomeration along the coastline of southeastern China: Pollution mechanism and control implication

The concentrations of ground-level ozone (O3) in China have undergone a rapid increase in recent years, resulting in adverse impacts on the air quality and climate change. However, limited research has been conducted on the coastal urban agglomerations with increasingly serious O3 pollution. Therefore, in order to better understand in situ photochemistry, comprehensive field observations of O3 and its precursors, coupled with the model simulation, were conducted in autumn of 2019 at six sites in an urban agglomeration along the coastline of southeastern China. Results indicated that O3 pollution in the southern part of the urban agglomeration was more severe than that in the northern part, due to higher levels of O3 precursors and stronger atmospheric oxidation capacity (AOC) in the southern regions. Oxygenated volatile organic compounds (OVOCs), NO2, and CO dominated the total OH reactivity, and the site-average daytime Ox (O3 + NO2) increments correlated well (R2 = 0.94) with the total OH reactivity of CO and VOCs at these sites except for Quanzhou, where industrial emissions (35.1 %) and solvent usages (33.7 %) dominated the VOC sources. However, vehicle exhausts (31.1 %) were the most predominant contributors to the VOC sources at other sites. The results of model simulations showed that net O3 formation rates were larger at the southern sites. Furthermore, O3 production was mainly controlled by VOCs at most sites, but co-limited by VOCs and NOx at Quanzhou. The most significant VOC groups contributing to O3 formation were aromatics and alkenes, with m/p-xylene, toluene, propene, and ethene being the main contributors at these sites. This study offers a more comprehensive understanding of the characteristics and formation of photochemical pollutions on the scale of the urban areas, indicating the critical need to reduce VOC emissions as a means of mitigating their photochemical effects.

Long-term spatiotemporal evolution and coordinated control of air pollutants in a typical mega-mountain city of Cheng-Yu region under the "dual carbon" goal

Clarifying the spatiotemporal distribution and impact mechanism of pollution is the prerequisite for megacities to formulate relevant air pollution prevention and control measures and achieve carbon neutrality goals. Chongqing is one of the dual-core key megacities in Cheng-Yu region and as a typical mountain-city in China, environmental problems are complex and sensitive. This research aims to investigate the exceeding standard levels and spatio-temporal evolution of criteria pollutants between 2014 and 2020. The results indicated that PM10, PM2.5, CO and SO2 were decreased significantly by 45.91%, 52.86%, 38.89% and 66.67%, respectively. Conversely, the concentration of pollutant O3 present a fluctuating growth and found a "seesaw" phenomenon between it and PM. Furthermore, PM and O3 are highest in winter and summer, respectively. SO2, NO2, CO, and PM showed a "U-shaped", and O3 showed an inverted "U-shaped" seasonal variation. PM and O3 concentrations are still far behind the WHO, 2021AQGs standards. Significant spatial heterogeneity was observed in air pollution distribution. These results are of great significance for Chongqing to achieve "double control and double reduction" of PM2.5 and O3 pollution, and formulate a regional carbon peaking roadmap under climate coordination. Besides, it can provide an important platform for exploring air pollution in typical terrain around the world and provide references for related epidemiological research.Implications: Chongqing is one of the dual-core key megacities in Cheng-Yu region and as a typical mountain city, environmental problems are complex and sensitive. Under the background of the "14th Five-Year Plan", the construction of the "Cheng-Yu Dual-City Economic Circle" and the "Dual-Carbon" goal, this article comprehensively discussed the annual and seasonal excess levels and spatiotemporal evolution of pollutants under the multiple policy and the newest international standards (WHO,2021AQG) backgrounds from 2014 to 2020 in Chongqing. Furthermore, suggestions and measures related to the collaborative management of pollutants were discussed. Finally, limitations and recommendations were also put forward.Clarifying the spatiotemporal distribution and impact mechanism of pollution is the prerequisite for cities to formulate relevant air pollution control measures and achieve carbon neutrality goals. This study is of great significance for Chongqing to achieve "double control and double reduction" of PM2.5 and O3 pollution, study and formulate a regional carbon peaking roadmap under climate coordination and an action plan for sustained improvement of air quality.In addition, this research can advanced our understanding of air pollution in complex terrain. Furthermore, it also promote the construction of the China national strategic Cheng-Yu economic circle and build a beautiful west. Moreover, it provides scientific insights for local policymakers to guide smart urban planning, industrial layout, energy structure, and transportation planning to improve air quality throughout the Cheng-Yu region. Finally, this is also conducive to future scientific research in other regions of China, and even megacities with complex terrain in the world.

Spatiotemporal dynamics of near-surface ozone concentration and potential source areas in northern China during 2015-2020

Near-surface ozone (O3) pollution has become one of the main factors hampering urban air quality in northern China. However, on a spatiotemporal scale, dynamic transport paths and potential source areas of O3 in northern China are ambiguous. In addition, we suspect that the contribution of transportation activities to urban O3 concentrations developed in northern China may be underestimated. In this study, the HYSPLIT, PSCF, CWT and GTWR model were used to study the transmission paths, potential source areas and driving factors of urban O3 concentration on a spatiotemporal scale. The average annual concentration of surface O3 (the 90th percentile of MDA8) was 172 ± 29 μg/m3 in northern China from 2015 to 2020. In terms of inter-annual variation, the urban O3 concentration increased from 2015 to 2018, and decreased after 2018. On the spatial scale, the areas with high O3 concentration were mainly clustered in industrial cities (Tangshan, Baoding, Shijiazhuang, Xingtai and Handan). During the study period, the area with high O3 concentration in northern China shifted from northwest to southeast. From 2015 to 2020, the influence of long-distance air mass trajectories from Xinjiang and Siberi on airflow transport in Beijing city dominates (78.60%) The average percentage of short-distance transport trajectories from Shandong Peninsula region is about 21.40%. The core potential source areas of O3 pollution shifted from northwest to southeast, but the contribution to O3 pollution in Beijing gradually weakened during the same period. Temperature and relative humidity were the main meteorological driving factors affecting O3 concentration in the study area, while population density, the proportion of secondary industry in GDP, industrial smoke (dust) emissions, and passenger traffic were the main non-meteorological factors. During the period study, the influence of industrial and traffic emissions had a more significant impact on O3 concentration in northern China, which will require that more attention be paid to emission mitigation in the regional industrial and passenger transportation sector, as well as the joint prevention and control of O3 pollution in northern China in the future.

The contribution of industrial emissions to ozone pollution: identified using ozone formation path tracing approach

Wintertime meteorological conditions are usually unfavorable for ozone (O₃) formation due to weak solar irradiation and low temperature. Here, we observed a prominent wintertime O₃ pollution event in Shijiazhuang (SJZ) during the Chinese New Year (CNY) in 2021. Meteorological results found that the sudden change in the air pressure field, leading to the wind changing from northwest before CNY to southwest during CNY, promotes the accumulation of air pollutants from southwest neighbor areas of SJZ and greatly inhibits the diffusion and dilution of local pollutants. The photochemical regime of O₃ formation is limited by volatile organic compounds (VOCs), suggesting that VOCs play an important role in O₃ formation. With the developed O₃ formation path tracing (OFPT) approach for O₃ source apportionment, it has been found that highly reactive species, such as ethene, propene, toluene, and xylene, are key contributors to O₃ production, resulting in the mean O₃ production rate (P_O3) during CNY being 3.7 times higher than that before and after CNY. Industrial combustion has been identified as the largest source of the P_O3 (2.6 ± 2.2 ppbv h^-1), with the biggest increment (4.8 times) during CNY compared to the periods before and after CNY. Strict control measures in the industry should be implemented for O₃ pollution control in SJZ. Our results also demonstrate that the OFPT approach, which accounts for the dynamic variations of atmospheric composition and meteorological conditions, is effective for O₃ source apportionment and can also well capture the O₃ production capacity of different sources compared with the maximum incremental reactivity (MIR) method.

The contributions of non-methane hydrocarbon emissions by different fuel type on-road vehicles based on tests in a heavily trafficked urban tunnel

Automobile exhaust is a major source of volatile organic compounds (VOCs) in metropolitan areas, yet it is difficult to accurately determine the contributions of different types of on-road vehicles. Tunnel tests are an effective way to measure real-world vehicle emissions, and the data collected are also suitable for receptor modeling to analyze the contributions of non-methane hydrocarbons (NMHCs) from different types of vehicles, as the closed environment ensures good mixing and minimal aging. In this study, tunnel tests were conducted inside a heavily trafficked city tunnel in Guangzhou in south China, and the positive matrix factorization (PMF) model was applied to the inlet-outlet incremental NMHC data. The results revealed that gasoline vehicles (GVs), Liquefied Petroleum Gas vehicles (LPGVs), and diesel vehicles (DVs) were responsible for 39 %, 45 % and 16 % of NMHCs, and 52 %, 23 %, and 24 % of the ozone formation potentials, respectively. LPGVs were the largest contributor of (56 %) alkanes, and GVs were the largest contributor of aromatics (61 %) and C₂-C₄ alkenes (55 %). With the video-recorded traffic counts the emissions of different fuel types are further compared on a per-vehicle-per-kilometer basis, and the results reveal that LPGVs and GVs were comparable in the OFPs of NMHCs emitted per kilometer, while on average a DV emitted 2.0 times more NMHCs than a GV with 2.4 times more OFPs. This study highlights substantial contribution of reactive alkenes and aromatics by DVs and the benefits of strengthening diesel exhaust control in terms of preventing ozone pollution.

Air pollution exposure and heart failure: A systematic review and meta-analysis

While the literature strongly supports a positive association between particulate matter with diameter ≤ 2.5 μm (PM_2.5) exposure and heart failure (HF), there is uncertainty regarding the other pollutants and the dose and duration of exposure that triggers an adverse response. To comprehensively assess and quantify the association of air pollution exposure with HF incidence and mortality, we performed separate meta-analyses according to pollutant types [PM_2.5, PM₁₀, sulfur dioxide (SO₂), nitrogen dioxide (NO₂), carbon monoxide (CO), ozone (O₃)], and exposure duration (short- and long-term). We systematically searched PubMed, EMBASE, and Web of Science for relevant articles with publication dates up to July 12, 2022, identifying 35 eligible studies. Random-effects models were used to summarize the pooled odds ratios (ORs) and 95 % confidence intervals (95 % CIs). For long-term exposure, the growing risk of HF was significantly associated with each 10 μg/m³ increase in PM_2.5 (OR = 1.196, 95 % CI: 1.079-1.326; I² = 76.8 %), PM₁₀ (1.190, 1.045-1.356; I² = 76.2 %), and NO₂ (1.072, 1.028-1.118; I² = 78.3 %). For short-term exposure, PM_2.5, PM₁₀, NO₂, and O₃ (per 10 μg/m³ increment) increased the risk of HF, with estimated ORs of 1.019 (1.008-1.030; I² = 39.9 %), 1.012 (1.007-1.017; I² = 28.3 %), 1.016 (1.005-1.026; I² = 53.7 %), and 1.006 (1.002-1.010; I² = 0.0 %), respectively. No significant effects of SO₂ and CO exposure on the risk of HF were observed. In summary, our study powerfully highlights the deleterious impact of PM_2.5, PM₁₀, and NO₂ exposure (either short- or long-term) on HF risk. Serious efforts should be made to improve air quality through legislation and interdisciplinary cooperation.

Integrating ambient carbonyl compounds provides insight into the constrained ozone formation chemistry in Zibo city of the North China Plain

Quantifying the impact of carbonyl compounds (carbonyls) on ozone (O₃) photochemical formation is crucial to formulating targeted O₃ mitigation strategies. To investigate the emission source of ambient carbonyls and their integrated observational constraint on the impact of O₃ formation chemistry, a field campaign was conducted in an industrial city (Zibo) of the North China Plain from August to September 2020. The site-to-site variations of OH reactivity for carbonyls were in accordance with the sequence of Beijiao (BJ, urban, 4.4 s^-1) > Xindian (XD, suburban, 4.2 s^-1) > Tianzhen (TZ, suburban, 1.6 s^-1). A 0-D box model (MCMv3.3.1) was applied to assess the O₃-precursor relationship influenced by measured carbonyls. It was found that without carbonyls constraint, the O₃ photochemical production of the three sites was underestimated to varying degrees, and the biases of overestimating the VOC-limited degree were also identified through a sensitivity test to NO_x emission changes, which may be associated with the reactivity of carbonyls. In addition, the results of the positive matrix factorization (PMF) model indicated that the main source of aldehydes and ketones was secondary formation and background (81.6% for aldehydes, 76.8% for ketones), followed by traffic emission (11.0% for aldehydes, 14.0% for ketones). Incorporated with the box model, we found that biogenic emission contributed the most to the O₃ production at the three sites, followed by traffic emission as well as industry and solvent usage. Meanwhile, the relative incremental reactivity (RIR) values of O₃ precursor groups from diverse VOC emission sources featured consistencies and differences at the three sites, which further highlights the importance of the synergetic mitigation of target O₃ precursors at regional and local scales. This study will help to provide targeted policy-guiding O₃ control strategies for other regions.

The spatiotemporal variation of PM2.5-O3 association and its influencing factors across China: Dynamic Simil-Hu lines

In recent years, PM_2.5 and O₃ composite airborne pollution has become one of the most severe environment issues in China. To get a better understanding and tackle these problems, we employed multi-year data to explore the spatiotemporal variation of the PM_2.5-O₃ relationship in China and investigated its major driving factors. Firstly, interesting patterns were found that named dynamic Simil-Hu lines, which presented a combined effect of natural and anthropogenic influences, were closely related to the spatial patterns of PM_2.5-O₃ association across seasons. Furthermore, regions with lower altitudes, higher humidity, higher atmospheric pressure, higher temperature, fewer sunshine hours, more accumulated precipitation, denser population and higher GDP often show positive PM_2.5-O₃ associations, regardless of seasonal variations. Amongst these factors, humidity, temperature and precipitation were dominant factors. This research suggests that the collaborative governance of composite atmospheric pollution should be implemented dynamically, in consideration of geographical locations, meteorological conditions and socioeconomic conditions.

Diagnosis of photochemical O3 production of urban plumes in summer via developing the real-field IRs of VOCs: A case study in Beijing of China

This study mainly developed an estimate method for photochemical ozone (O₃) production from urban plumes in hot season, through simulating O₃ evolution from precursors locally emitted and determining the real-field O₃ increment reactivity (IR) of volatile organic compounds (VOCs) based on the box chemical model. Our simulation on June-2019 indicated that Beijing local emissions produced O₃ at the rate of 0.7-9.2 ppb/h and led to an O₃ increase of 48.9 ppb during 05:00-18:00, accounting for 68.3% of the observed O₃ increase. The maximum level and production rate of simulated O₃ showed a linear response to VOCs, therefore we can use VOCs levels in urban plumes to quantify O₃ formation in summer. The IR (g O₃ formed per g VOCs) was calculated on the actual precursor and meteorology condition of this megacity, 0.12-4.90 g/g for individual VOCs and 1.49 g/g for comprehensive TVOCs. The weighted average of individual IRs agreed well with that of TVOCs, but these IRs were 34.5% of MIR values that were widely used in references. It's noteworthy that these IRs had greater sensitivity to precursor levels, and broadly remained stable under the fixed VOCs:NOx. Considering the synchronous reductions of precursors in Beijing, we applied these IRs to quantify chemical O₃ evolution from Beijing local emissions in summer of recent years, declining from 63.5 ppb in 2016 to 44.0 ppb in 2020 for June. The contributions of the diagnosed chemical O₃ to Beijing O₃ better matched with the atmospheric transport paths on daily basis, higher than 100% when the transport paths starting from the clean neighbor cities, but lower to 45%-66% when the transport paths originating from the highly-polluted neighbor cities. This consistence indicated the reliability of our IR calculation method for quickly estimating chemical O₃ production of urban plumes in summer.

A review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species. Since the first tropospheric chemical mechanism was proposed in the 1960s, various mechanisms including Master Chemical Mechanism (MCM), Carbon Bond Mechanism (CBM), Statewide Air Pollution Research Center (SAPRC) and Regional Atmospheric Chemistry Mechanism (RACM) have been developed for different research purposes. This work summarizes the development and applications of these mechanisms, introduces their compositions and lumping methods, and compares the ways the mechanisms treat radicals with box model simulations. CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM, whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM. The photolysis rates of some oxygenated compounds are low in SAPRC07, which may result in underestimation of radical levels. As an explicit chemical mechanism, MCM describes the chemical processes of primary pollutants and their oxidation products in detail. MCM can be used to investigate certain chemical processes; however, due to its large size, it is rarely used in regional model simulations. A box model case study showed that the chemical behavior of OH and HO₂ radicals and the production of ozone were well described by all mechanisms. CBM and SAPRC underestimated the radical levels for different chemical treatments, leading to low ozone production values in both cases. MCM and RACM are widely used in box model studies, while CBM and SAPRC are often selected in regional simulations.

Vertical profiles of O3, NO2 and PM in a major fine chemical industry park in the Yangtze River Delta of China detected by a sensor package on an unmanned aerial vehicle

The vertical profiles and diurnal variations of air pollutants at different heights in the fine chemical industry park (FCIP) were systematically studied in this study. Air pollutants in a major FCIP in the Yangtze River Delta of China within 500 m above ground level (AGL) detected by a sensor package on an unmanned aerial vehicle (UAV). The air pollutants including ozone (O₃), nitrogen dioxide (NO₂), particulate matter (PM), total volatile organic compounds (TVOCs) and carbon monoxide (CO), respectively, had been measured through more than one hundred times of vertical flights from Aug. 2020 to Jul. 2021. The concentrations of NO₂ and CO generally decreased with the height while the concentrations of O₃ increased with the height within 500 m AGL. The photochemical reaction resulted in a strong inverse relationship between the vertical profiles of O₃ and that of NO₂. The concentrations of PM_2.5 and TVOCs generally decreased with the height below 100 m AGL and were fully mixed above 100 m AGL. The vertical profiles of different particle sizes were well consistent with the R² value of 0.97 between PM₁ and PM_2.5 and 0.93 between PM_2.5 and PM₁₀. The NO₂ and PM_2.5 concentrations sometimes increased with height maybe due to the influence of temperature inversion layer or long-distance transportation from northern China. The diurnal variations of NO₂, O₃, TVOCs and CO concentrations at different heights within 500 m AGL were basically consistent. The diurnal variations range of PM_2.5 concentrations below 100 m AGL was large and different from other heights, which should be greatly influenced by the local emissions. The unstable atmospheric stability was accompanied by strong photochemical reactions and convective activities, resulting in low concentrations of NO₂ and PM_2.5, while high concentrations of O₃.

Species profiles, in-situ photochemistry and health risk of volatile organic compounds in the gasoline service station in China

Accompanying with increases in vehicle population and gasoline consumption, gasoline evaporation accounted for an enlarged portion of total volatile organic compound (VOC) emissions in China, raising increasing environmental concerns especially in megacities. In this study, an intensive sampling campaign was performed in a gasoline service station, to reveal emission characteristics, environmental and health impacts of VOCs. It was strikingly found that 24 % of air samples exceeded the national standard of 4 mg/m³ for non-methane hydrocarbons (NMHCs) on the boundary of the station, with the equipment of Stage I and II controls. VOC groups and species profiles showed that alkanes dominated total VOCs. As typical markers of evaporative loss of gasoline, C4-5 species (i-pentane, n-pentane and n-butane) as well as methyl tert-butyl ether (MTBE) accounted for 49.6 % of VOCs. Species profile and diagnostic ratios indicated the prominent contribution of gasoline evaporative losses from refueling or breathing processes, as well as the interference of vehicle exhaust in the ambient air at the site. Intensive O₃ production was reproduced by the photochemical box model, demonstrating that O₃ formation was co-limited by both VOCs (especially trans-2-butene) and NO_x. Inhalation health risk assessment proved that exposure to hazardous VOCs caused non-cancer risk (HQ = 3.08) and definitely posed cancer risks at a probability of 1.3 × 10^-4 to workers. Remarkable health risks were mainly imposed by halocarbons, aromatics and alkenes, in which 1,2-dichloropropane caused the highest non-cancer risk (HQ = 1.3) and acted as the primary carcinogen (ICR = 5.1 × 10^-5). This study elucidated the high unqualified rate in gasoline service stations after the implementation of latest standards in China, where new regulations targeted halocarbons and updates in existing vapor recovery systems were suggested for VOC mitigation.

Chemical reactivity of volatile organic compounds and their effects on ozone formation in a petrochemical industrial area of Lanzhou, Western China

Measurements of ozone (O₃) and its precursors were performed in the summer of 2019 in Lanzhou, a petrochemical industrial city, to better understand the reactivity of volatile organic compounds (VOCs) and their effects on O₃ production. During the campaign, the daily maximum 8-hour average (MDA8) O₃, NO₂, and total VOC (TVOC) concentrations reached 72.2 ± 19.9 ppb, 24.9 ± 10.8 ppb, and 50.8 ± 46.1 ppb, respectively. Alkanes, alkenes, halocarbons, aromatics, and alkynes contributed 45.3%, 24.0%, 16.5%, 10.0%, and 4.2% to TVOCs, respectively. The OH reactivity and relative incremental reactivity (RIR) of VOCs at different times were calculated. The results indicated that alkenes played a predominant role, accounting for an average of 68.5% of the initial VOC reactivity. Compared to other regions, alkenes are relatively more important for O₃ formation in the petrochemical industry area of Lanzhou, while aromatics are relatively less important. Generally, O₃ formation occurred in a VOC-limited regime in the morning and in a transitional regime in the afternoon. The response surface methodology (RSM) combined with a chemical box model was applied to obtain relationships between O₃ and its precursors and determine the most effective way to reduce the O₃ concentration. Reduction in the non-alkene concentration slightly affected the O₃ concentration. In contrast, the effect of nitrogen oxides (NO_x) was closely related to the alkene concentration, and NO_x concentration reduction could lead to an increase in the O₃ concentration when alkenes were abated to less than 80% of the present concentration. To mitigate O₃ pollution near the petrochemical industrial area of Lanzhou, reducing the alkene concentration, especially the C4 alkene concentration (1,3-butadiene, cis-2-butene, and trans-2-butene), was the fastest and most effective control strategy. The results of this study serve as a reference for O₃ pollution control in petrochemical industrial areas.

Impact of NOx reduction on long-term surface ozone pollution in roadside and suburban Hong Kong: Field measurements and model simulations

Continuous measurements of ozone (O₃) and nitrogen oxides (NO_x = NO + NO₂) were conducted from 2007 to 2019 in Hong Kong in order to evaluate the effectiveness of control strategies for NO_x emission from diesel commercial vehicles (DCV). DCV control programs were periodically applied in three phases starting from 2007, 2010 and 2014. It was found that NO and NO₂ levels decreased during the study period but more dramatically after the implementation of DCV Phase III than pre-DCV Phase III. Source apportionment analysis confirmed that the ambient NO and NO₂ in Hong Kong attributed to the regulated DCV emissions in Phase III reduced at rates of 5.1-14.4 ppbv/yr in roadside environment and 1.6-3.1 ppbv/yr in suburban area. Despite overall NO_x reduction, increased NO₂/NO_x ratios were recorded during the study period possibly due to the application of diesel particulate filter (DPF) in DCVs. However, after introducing DCV Phase III, observed O₃ values experienced more dramatic increasing trends in most areas of Hong Kong than pre-DCV Phase III. Model simulations revealed that O₃ production rate kept increasing and turned to be less sensitive to NO_x from 2014 to 2019. On the roadside, net O₃ production rate was more than doubled during 2014-2019 owing to NO_x reduction. Moreover, the levels of oxidants (OH, HO₂ and RO₂) were 1.5-5 times those before 2014. In suburban environment, NO_x reduction also facilitated O₃ production and radical cycling, but made smaller contributions than those on the roadside during 2014-2019. This study unraveled that NO_x reductions benefited from DCV regulations caused increase in surface O₃ and fueled O₃ photochemistry in various environments. More stringent control measures on emissions of VOCs, especially those with high OH reactivity, might help to better mitigate O₃ pollution.

Spatiotemporal characteristics of PM2.5 and ozone concentrations in Chinese urban clusters

Despite China's public commitment to emphasise air pollution investigation and control, trends in PM_2.5 and ozone concentrations in Chinese urban clusters remain unclear. This study quantifies the spatiotemporal variations in PM_2.5 and surface ozone at the scale of Chinese urban clusters by using a long-term integrated dataset from 2015 to 2020. Nonlinear Granger causality testing was used to explore the spatial association patterns of PM_2.5 and ozone pollution in five megacity cluster regions. The results show a significant downward trend in annual mean PM_2.5 concentrations from 2015 to 2020, with a decline rate of 2.8 μg m^-3 yr^-1. By contrast, surface ozone concentrations increased at a rate of 2.1 μg m^-3 yr^-1 over the 6 years. The annual mean PM_2.5 concentrations in urban clusters show significant spatial clustering characteristics, mainly in Beijing-Tianjin-Hebei (BTH), Fenwei Plain (FWP), Northern slope of Tianshan Mountains urban cluster (NSTM), Sichuan Basin urban cluster (SCB), and Yangtze River Delta (YRD). Surface ozone shows severe summertime pollution and distributional variability, with increased ozone pollution in major urban clusters. The highest increases were observed in BTH, Yangtze River midstream urban cluster (YRMR), YRD, and Pearl River Delta (PRD). Nonlinear Granger causality tests showed that PM_2.5 was a nonlinear Granger cause of ozone, further supporting the literature's findings that PM_2.5 reduction promoted photochemical reaction rates and stimulated ozone production. The nonlinear test statistic passed the significance test in magnitude and statistical significance. FWP was an exception, with no significant long-term nonlinear causal link between PM_2.5 and ozone. This study highlights the challenges of compounded air pollution caused primarily by ozone and secondary PM_2.5. These results have implications for the design of synergistic pollution abatement policies for coupled urban clusters.