Seasonal variations in meteorological simulations has been evaluated using four PBL schemes, showing that MYNN excels in summer and MYJ performs better in winter, highlighting the importance of scheme selection for accurate simulations.

The relationship between O3 and its precursors during urban polluted episodes remains unclear. In this study, the si-multaneous source apportionment of VOCs, NOx, and O3 over the Yangtze River Delta (YRD) region during the O3 pol-luted episode on July 24-30, 2018, was performed based on the Integrated Source Apportionment Method (ISAM) embedded in the Community Multiscale Air Quality Modeling System (CMAQ). The results of the ISAM were com-pared with those of the Brute Force Method (BFM) and Positive Matrix Factorization (PMF). Furthermore, the differ-ences between the transport contributions of VOCs and NOx, and their impacts on O3 were analyzed. The results indicate that observations of VOCs species can be well captured by simulated VOCs, and the ISAM has a significant ad-vantage in the source apportionment of VOCs, especially for sources emitting highly reactive species. In the clean and polluted periods, the local contribution percentages of VOCs in urban sites ranged from 60 % to 77 %, much higher than those of NOx (31 %-43 %) and O3 (16 %-33 %). NOx and O3 have strong transport abilities with high and close contribution percentages, which are highly correlated, mainly because oxygen atoms produced by the photolysis of NO2 in the aged air mass combined rapidly with O2 to form O3 during transport. The VOCs chemical loss caused by the oxidation of OH radicals during transport makes the ability of VOCs for long-distance transport much weaker than that of NOx. Furthermore, owing to the sufficient aging of VOCs, those contributed by long-distance transport have lit-tle effect on O3. To a certain extent, controlling one's NOx emissions can help other cities more, while controlling one's VOCs emissions can help itself more. Therefore, it is recommended to attach enough importance to joint prevention and control of NOx among cities and even long-distance areas to alleviate regional O3 pollution.

A novel integrated machine learning method to analyze O3 changes is proposed. ● Various factors affecting long-term changes of O3 in Shanghai are quantified. ● Meteorological, photochemical, and regional background O3 are well separated. Surface ozone (O3) is influenced by regional background and local photochemical formation under favorable meteorological conditions. Understanding the contribution of these factors to changes in O3 is crucial to address the issue of O3 pollution. In this study, we propose a novel integrated method that combines random forest, principal component analysis, and Shapley additive explanations to distinguish observed O3 into meteorologically affected ozone (O3_MET), chemically formed from local emissions (O3_LC), and regional background ozone (O3_RBG). Applied to three typical stations in Shanghai during the warm season from 2013 to 2021, the results indicate that O3_RBG in Shanghai was 48.8 ± 0.3 ppb, accounting for 79.6%–89.4% at different sites, with an overall declining trend of 0.018 ppb/yr. O3_LC at urban and regional sites ranged from 5.9–9.0 ppb and 8.9–14.6 ppb, respectively, which were significantly higher than the contributions of 2.5–7.4 ppb at an upwind background site. O3_MET can be categorized into those affecting O3 photochemical generation and those changing O3 dispersion conditions, with absolute contributions to O3 ranging from 13.4–19.0 ppb and 13.1–13.7 ppb, respectively. We found that the O3 rebound in 2017, compared to 2013, was primarily influenced by unfavorable O3 dispersion conditions and unbalanced emission reductions; while the O3 decline in 2021, compared to 2017, was primarily influenced by overall favorable meteorological conditions and further emissions reduction. These findings highlight the challenge of understanding O3 change due to meteorology and regional background, emphasizing the need for systematic interpretation of the different components of O3.

Elevated ozone (O3) concentration during the COVID-19 lockdown is a matter of great concern, but the changes of its sensitivities to key precursors remains unclear. This study utilized the Community Multiscale Air Quality (CMAQ) model coupled with the Higher-order Decoupled Direct Method in Three Dimensions (HDDM-3D) and the process analysis (PA) modules to reveal in detail the changes in O3-precursors sensitivities and the contri-bution of major chemical and physical processes to O3 formation/loss during the month-long COVID-19 lock -down in early 2020 over the Yangtze River Delta (YRD) region of China. The results indicate that the contributions (absolute value) of gas-phase chemistry to O3 and Ox (i.e., O3+NO2) were reduced by 35-50% and 8-24%, respectively, under lockdown-specific (LCD) scenario compared with the business-as-usual (BAU) sce-nario in the highly urbanized areas of eastern and central YRD. Under the BAU (LCD) scenario, the first-order and second-order O3-NOx sensitivities averaged about-25 (-36) mu g/m3 and about 8 (28) mu g/m3, respectively. These 1st- and 2nd-order sensitivities of O3 to NOx were both intensified due to COVID-19 lockdown, which potentially contributed to O3 increases of between 5 and 10 mu g/m3. In other words, the concentration of O3 and its rate of increase were both amplified due to COVID-19 lockdown. Overall, this study highlighted a significant wintertime "NOx reduction disbenefit" phenomenon over YRD because of both strong 1st-order (negative) and 2nd-order (positive) O3 sensitivities to NOx emissions, which were further reinforced due to COVID-19 lockdown.

In recent years, ground‐level ozone (O3) has been one of the main pollutants hindering air quality compliance in China's large city‐clusters including the Yangtze River Delta (YRD) region. In this work, we utilized the process analysis (PA) and the higher‐order decoupled direct method (HDDM‐3D) tools embedded in the Community Multiscale Air Quality model (CMAQ) to characterize O3 formation and sensitivities to precursors during a typical O3 pollution episode over the YRD region in July 2018. Results indicate that gas‐phase chemistry contributed dominantly to the ground‐level O3 although a significant proportion was chemically produced at the middle and upper boundary layer before reaching the surface via diffusion process. Further analysis of the chemical pathways of O3 and Ox formation provided deep insights into the sensitivities of O3 to its precursors that were consistent with the HDDM results. The first‐order sensitivities of O3 to anthropogenic volatile organic compounds (AVOC) were mainly positive but small, and temporal variations were negligible compared with those to NOx. During the peak O3 time in the afternoon, the first‐ and second‐order sensitivities of O3 to NOx were significantly positive and negative, respectively, suggesting a convex response of O3 to NOx over most areas including Shanghai, Hangzhou, Nanjing and Hefei. These findings further highlighted an accelerated decrease in ground‐level O3 in the afternoon corresponding to continuous decrease of NOx emissions in the afternoon. Therefore, over the YRD region including its metropolises, NOx emission reductions will be more important in reducing the afternoon peak O3 concentration compared with the effect of VOC emission control alone.

This study investigates the transport of air pollutants around the Yangtze River Delta with an aim to identify if there would be a relationship towards health effects during the COVID-19 lockdown period. It is well-known that due to lockdown, the number of socio-economical activities are reduced and hence there is an observable reduction in air pollution. We would like to investigate if this consequential reduction of air pollution would lead to improvement in health amongst its population. A number of integrated methodologies are utilized, including collection and correlation of statistical data and numerical modeling to correlate the mortality rates difference with and without COVID-19 lockdown. In particular air quality changes during the COVID-19 lockdown period are compared with similar periods of the previous years using Brute Force Method. It is found that in general there is significant reduction in air-pollution related mortality, like stroke, ischemic cardio diseases, obstructive pulmonary disease, lung cancer and acute lower respiratory infection are all reduced as a result of relative improvement in PM2.5 level during the lockdown period. Further investigation of the trajectories suggests that these PM2.5 originate from afar with multiple sources, and do not suggest COVID-19 are transported to the region via long-range transport. Our results demonstrate the need for more stringent policy measure to tackle air pollution as it has strong evidence that it increases mortality rate.

Biogenic volatile organic compounds (BVOCs) play an important role in atmospheric chemistry due to their large quantities and high reactivity. In this study, the impacts of BVOC emissions on ozone formation were investigated based on MEGANv3.1 in the Yangtze River Delta (YRD) region, which has increasingly suffered from ozone pollution in recent years. The sensitivities of BVOC emissions to different drought stress configurations and the quality of emission factors were evaluated. Furthermore, BVOC contribution to ozone formation was simulated by integrated meteorology and air quality model system and the impacts of different BVOC emission scenarios on ozone concentration were discussed. Annual BVOC emissions estimated with the default drought stress configuration (i.e., base case) was 6.8 × 10 5 tons. The drought stress algorithm implemented in MEGANv3.1 could suppress BVOC emissions by 58% and this algorithm was sensitive to the choice of wilting point values. The BVOC contribution to the average of daily maximum 8 h ozone concentration without drought stress effect in July 2016 was 104% higher than that in the base scenario when the drought stress effect is activated. Using an alternative set of wilting point led to BVOC contribution being 48% higher than that in the base scenario. High contributions of BVOCs to simulated ozone concentration were found in northern Zhejiang, especially in Hangzhou and its surrounding areas.

To continuously improve air quality, after implementation of the “Clean Air Action Plan, 2013–2017” (CAAP), the “Three-year Action Plan to Fight Air Pollution” (TYP) was further conducted from 2018 to 2020. However, the effectiveness of the TYP remains unclear in one of the major city-clusters of China, the Sichuan Basin. In this study, the bottom-up method was used to quantify the emission reduction during TYP based on the emissions inventory in Sichuan Basin in 2017 and the air pollution control measures adopted from 2018 to 2020 in each city. The reduction of PM2.5 concentration and the avoided premature deaths due to implementation of air pollution control measures were assessed by using an integrated meteorology and air quality modeling system and a concentration-response algorithm. Emissions of SO2, NOx, PM2.5, and VOCs in the Sichuan Basin have been reduced by 42.6, 105.2, 40.2, and 136.6 Gg, respectively. The control of non-electricity industry contributed significantly to the emission reduction of all pollutants, accounting for 26–49%. In addition, the control of mobile sources contributes the most to NOx reductions, accounting for 57%. The results illustrate that the focus of air pollution control in Sichuan Basin is still industrial sources. We also found that the emission reduction of NOx, PM2.5, and VOCs in Chengdu is significantly higher than that of other cities, which were about 3.4~15.4 times, 2.2~40.1 times, and 4.3~24.4 times that of other cities, respectively. In Sichuan Basin, the average reduction rate of PM2.5 concentration due to air pollution control measures was 5% on average, with the highest contributions from industry, mobile source, and dust emission control. The decrease rate in each city ranges between 1~10%, and the decreasing ratios in Dazhou (10%), Chengdu (8%), and Zigong (7%) are relatively higher. The number of premature deaths avoided due to air pollution control measures in Sichuan Basin is estimated to be 22,934. Chengdu and Dazhou have benefitted most from the air pollution control measures, with 6043 and 2713 premature deaths avoided, respectively. Our results indicate that the implementation of TYP has achieved remarkable environmental and health benefits.

Air pollution has a significant impact on tourism; however, research in this area is still limited. In this study, we applied grey relational analysis to panel data from 31 provinces in China and evaluated the relationship between air quality and inbound tourist arrivals. The study focused on provincial-level disparities for the different key air quality evaluation standards during 2009–2012 and 2013–2019. For instance, we considered PM10, SO2, NO2 and the excellent and good ratings of Air Pollution Index (API) during 2009–2012 and the additional PM2.5, CO, O3 and the excellent and good ratings of Air Quality Index (AQI) from 2013 to 2019. Results indicate that: (1) Inbound tourist arrivals are significantly and positively affected by ambient air quality, and the impact from 2013 to 2019 was greater than that from 2009 to 2012; (2) there is regional diversity in inbound tourist arrivals, and the impact of the different air quality indicators varies; (3) inbound tourists showed greater sensitivity to air pollution under the AQI standard; (4) the impact of air quality indicators on the inbound tourist arrivals shows grey relational order, and the concentration of PM2.5, PM10 and SO2 have less impact than NO2, CO and O3 on changes in tourism numbers; (5) consistency in the air quality impact on foreign tourists and compatriot tourists from HK, MO and TW varies by air quality indicators. This study highlights the need for appropriate measures to improve air quality for high-quality and sustainable development of inbound tourism.

In recent years, ozone pollution has become one of the most severe environmental problems in China. Evidence from observations have showed increased frequency of high O3 levels in suburban areas of the Yangtze River Delta (YRD) region. To better understand the formation mechanism of local O3 pollution and investigate the potential role of isoprene chemistry in the budgets of ROx (OH+HO2+RO2) radicals, synchronous observations of volatile organic compounds (VOCs), formaldehyde (HCHO), and meteorological parameters were conducted at a suburban site of the YRD region in 2018. Five episodes with elevated O3 concentrations under stagnant meteorological conditions were identified; an observation-based model (OBM) with the Master Chemical Mechanism was applied to analyze the photochemical processes during these high O3 episodes. The high levels of O3, nitrogen oxides (NOx), and VOCs facilitated strong production and recycling of ROx radicals with the photolysis of oxygenated VOCs (OVOCs) being the primary source. Our results suggest that local biogenic isoprene is important in suburban photochemical processes. Removing isoprene could drastically slow down the efficiency of ROx recycling and reduce the concentrations of ROx. In addition, the absence of isoprene chemistry could further lead to a decrease in the daily average concentrations of O3 and HCHO by 34 % and 36 %, respectively. Therefore, this study emphasizes the importance of isoprene chemistry in the suburban atmosphere, particularly with the participation of anthropogenic NOx. Moreover, our results provide insights into the radical chemistry that essentially drives the formation of secondary pollutants (e.g., O3 and HCHO) in suburban areas of the YRD region.

In the recent decade, the North China Plain (NCP) has been among the region’s most heavily polluted by PM2.5 in China. For the nonattainment cities in the NCP, joint pollution control with related cities is highly needed in addition to the emission controls in their own cities. However, as the basis of decision-making, the spatial characteristics of PM2.5 among these cities are still insufficiently revealed. In this work, the spatial characteristics among all nonattainment cities in the northern part of the North China Plain (NNCP) region were revealed based on data mining technologies including clustering, coefficient of divergence (COD), network correlation model, and terrain and meteorology analysis. The results indicate that PM2.5 pollution of cities with a distance of less than 180 km exhibits homogeneity in the NCP region. Especially, the sub-region, composed of Xinxiang, Hebi, Kaifeng, Zhengzhou, and Jiaozuo, was strongly homogeneous and a strong correlation exists among them. Compared with spring and summer, much stronger correlations of PM2.5 between cities were found in autumn and winter, indicating a strong need for joint prevention and control during these periods. All nonattainment cities in this region were divided into city-clusters, depending on the seasons and pollution levels to further helping to reduce their PM2.5 concentrations effectively. Air stagnation index (ASI) analysis indicates that the strong correlations between cities in autumn were more attributed to the transport impacts than those in winter, even though there were higher PM2.5 concentrations in winter. These results provided an insight into joint prevention and control of pollution in the NCP region.

Fine particulate matter (PM2.5)-bound nonpolar organic compounds (NPOCs), including polycyclic aromatic hydrocarbons (PAHs) and alkanes, are commonly used as typical molecular markers for detailed source identification. Online thermal desorption aerosol gas chromatography-mass spectrometry (TAG) system can obtain ambient data with hourly resolution, which is of great importance for investigating the diurnal characteristics and refined source identification of NPOCs. From June to October 2020, hourly ambient aerosol samples were collected and analyzed to investigate the characteristics and sources of 14 PAHs and 15 alkanes (C21-C35) in PM2.5 using TAG at a suburban site of Baoshan district in Shanghai, China. The average concentration of summed PAHs and alkanes during the sampling period was 1.27 ± 1.4 ng/m3 and 8.87 ± 3.46 ng/m3, respectively, in which Benzo[b]fluoranthene (BbF), Benzo[ghi]perylene (BghiP) and Indeno[1,2,3-cd]pyrene (IcdP) are the dominant PAHs species, with n-Heptacosane (C27), n-Nonacosane (C29) and n-Hentriacontane (C31) being the most abundant n-alkane species. Carbon preference index (CPI) and carbon maximum (Cmax) number indicated that the sources of alkanes shifted from biogenic-oriented (such as plant wax) in the summer to anthropogenic-dominated (such as fossil fuels) in the autumn. Results from trajectory cluster analysis and potential source contribution function (PSCF) modeling showed that alkanes were mainly from the middle and lower reaches of the Yangtze River Plain including Anhui, Jiangxi, and Zhejiang provinces, while PAHs were mainly from northeastern China. Positive Matrix Factorization (PMF) model results indicated that gasoline (41.48%) and diesel (21.82%) were the two major sources of PM2.5-bound PAHs in summer and fall of 2020 in Shanghai, followed by coal consumption or catering (19.96%) and biomass burning (16.74%). Diurnal variation of PAHs sources resolved by PMF showed characteristic features consistent with the corresponding anthropogenic activities. For example, gasoline vehicle exhaust showed higher concentrations during traffic rush hours; while coal consumption or catering presented higher concentrations during lunch times from 10:00 to 12:00. In addition, the TAG data coupling with PMF also can be capable for source appointment of short-duration episodes. Health risk assessment showed that adult women were at greater lifetime cancer risk (ILCR) than people in other age groups, and people may subject to higher health risks at morning and night time. This work demonstrates that hourly NPOCs measured by TAG are uniquely specific on refined source identification and investigation into the characteristics of diurnal variations.

The city clusters in eastern coastal China have suffered from serious air pollution in the past decades, which is partially related to the complex local topography and meteorological conditions. The planetary boundary layer (PBL) scheme is a critical parameter for accurate meteorology simulations and air quality predictions. In this study, we analyze the impact of four typical PBL schemes, namely, Yonsei University (YSU), Mellor-Yamada-Nakanishi-Niino Level 2.5 (MYNN), Asymmetric Convective Model version 2 (ACM2), and Mellor-Yamada-Janjic (MYJ) PBL, within the Weather Research and Forecasting (WRF) model to assess their impacts on the simulations of air pollutant concentrations based on the Comprehensive Air Quality Model with Extensions (CAMx) for the Yangtze River Delta (YRD) region, one of the most developed city clusters in eastern China. The results indicate that the MYNN scheme performs best in terms of fine particulate matter (PM2.5) and nitrogen dioxide (NO2) simulations, with mean bias of 4.8 mu g m(-3) and 9.3 mu g m(-3) in summer and 11.7 mu g m(-3) and 5.4 mu g m(-3) in winter, respectively. The YSU scheme performs best for ozone (O-3) prediction, with better simulation results in summer than in winter. Notably, some discrepancies among different PBL schemes in the prediction of air pollution are directly associated with the complex topography. For the prediction of PM2.5, the MYJ and YSU schemes tend to overestimate the concentrations in the plains of Jiangsu and northern Anhui while underestimating PM2.5 in the hilly areas compared with the MYNN scheme. Meanwhile, the performance of the ACM2 scheme is opposite to that of the MYJ and YSU schemes. For O-3 predictions, MYJ overestimates O-3 in the eastern coastal area and underestimates O-3 in the inland areas in summer compared with the YSU scheme, while the ACM2 scheme is the opposite of the MYJ scheme, and the MYNN scheme consistently overestimates O-3.

Although many efforts have been done to reduce PM2.5 pollution in recent years, there are still many nonattainment cities in East China for national standards. How to collaborate with surrounding cities to reduce PM2.5 has been a critical issue for all nonattainment cities. In this work, a framework of developing a practical scheme system of joint prevention and control of PM2.5 pollution based on data mining technologies was presented. Moreover, one specific scheme system for Yangtze River Delta region was developed and evaluated by five indicators. The results indicate that the scheme system was divided into three grades depending on pollution levels except for summer with two grades. The heavier the PM2.5 pollution was, the higher the grade was recommended to be implemented along with low grades. Moreover, more city-clusters were involved in each grade in winter than any other season. For Grade one in winter, the city-cluster with Xuzhou and Suqian was the most urgent to control emission jointly; the city-cluster with Suzhou and Shanghai was linked to the biggest impact on health, but for integrated indicator, the city-cluster with Nanjing and Changzhou ranked first. These results not only enable an enriched understanding of the significance of PM2.5 control in city-clusters, but also accelerate PM2.5 reduction for all nonattainment cities by collaboration with their most correlated cities in this region.