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. 2022 Dec 28;57(1):109–117. doi: 10.1021/acs.est.2c08205

Evolution of Ozone Pollution in China: What Track Will It Follow?

Jia Guo †,‡, Xiaoshan Zhang †,‡,*, Yi Gao ‡,∥, Zhangwei Wang †,‡, Meigen Zhang ‡,∥, Wenbo Xue ⊥, Hartmut Herrmann #, Guy Pierre Brasseur ∇,○,◆, Tao Wang ∇, Zhe Wang §,*
PMCID: PMC9835882  PMID: 36577015

Abstract

graphic file with name es2c08205_0009.jpg

Increasing surface ozone (O3) concentrations has emerged as a key air pollution problem in many urban regions worldwide in the last decade. A longstanding major issue in tackling ozone pollution is the identification of the O3 formation regime and its sensitivity to precursor emissions. In this work, we propose a new transformed empirical kinetic modeling approach (EKMA) to diagnose the O3 formation regime using regulatory O3 and NO2 observation datasets, which are easily accessible. We demonstrate that mapping of monitored O3 and NO2 data on the modeled regional O3–NO2 relationship diagram can illustrate the ozone formation regime and historical evolution of O3 precursors of the region. By applying this new approach, we show that for most urban regions of China, the O3 formation is currently associated with a volatile organic compound (VOC)-limited regime, which is located within the zone of daytime-produced O3 (DPO3) to an 8h-NO2 concentration ratio below 8.3 ([DPO3]/[8h-NO2] ≤ 8.3). The ozone production and controlling effects of VOCs and NOx in different cities of China were compared according to their historical O3–NO2 evolution routes. The approach developed herein may have broad application potential for evaluating the efficiency of precursor controls and further mitigating O3 pollution, in particular, for regions where comprehensive photochemical studies are unavailable.

Keywords: ozone pollution, diagnosis approach, ozone formation regime, ozone−precursor relationship, air pollution mitigation

Short abstract

An approach for diagnosing ozone pollution evolution, chemical formation regimes, and responses to precursor controls is developed.

Introduction

Ozone (O3) has been regarded as a principal component of photochemical pollution in urban regions worldwide and has received continuous attention from both the scientific and regulatory communities due to its adverse impacts on human health, air quality, the climate, and the natural environment.1,2 Tropospheric O3 is produced from the sunlight-initiated photochemical processing of volatile organic compounds (VOCs), nitrogen oxides (NOx), and, in a condition-dependent manner, carbon monoxide (CO), emitted from a vast variety of sources.3,4 Although extensive efforts have been made to regulate O3 precursor emissions worldwide,5,6 O3 concentrations reached very high levels, for example, in North America, before responding to control strategies developed and implemented over several decades.7,8 Moreover, O3 pollution continues to increase markedly in East Asia.9−13 The nonlinear responses of O3 formation to precursor emissions represent a major issue regarding O3 pollution control, thus posing challenges to the formulation of a universal and efficient O3 control strategy in regions with various chemical environments and regimes.14,15

Several approaches have been developed and utilized to identify O3 formation regimes and their relationships with precursor emissions. These methods include onsite observations of indicator ratios,16 emission-based air quality models,13,17 relative incremental reactivity (RIR) assessments performed with observation-constrained models,18 and the remote sensing of formaldehyde-to-NOx ratios.13,19 Most of these methods require sophisticated measurements, remote sensing data, accurate emission inventories, or detailed speciation information of emitted VOCs, while the O3 and NO2 data provided by regulatory monitoring networks are typically used to validate modeling results. In this work, we try to explore the utility of easily accessible NO2 and O3 monitoring datasets and develop an alternative approach analogous to the empirical kinetic modeling approach (EKMA) to obtain a classification scheme for diagnosing O3 formation regimes in different regions. By visualizing the site-to-site variations and evolving routes of O3–NO2 relationships, we shed some light on the efficiency of precursor controls in different regions of China and the development of more cost-effective emission control strategies in the future.

Materials and Methods

Continuous and standardized NO2 and O3 monitoring has been carried out at over 1200 sites in China by the National Environmental Monitoring Center (CNEMC) since 2013. This monitoring network provides long-term NO2 and O3 data at urban and suburban sites, covering different climatic regions of China. Hourly O3 and NO2 data recorded at national monitoring sites in China from January 2015 to December 2020 were obtained from the monitoring network website (http://106.37.208.233:20035). Historical O3 and NO2 monitoring data from Hong Kong and the United States were also incorporated into the analysis of this work. O3 and NO2 data recorded at the Air Quality Monitoring Stations (AQMS) in Hong Kong from 2010 to 2020 were obtained from the Hong Kong Environmental Protection Department (HKEPD) website. O3 and NO2 data collected by the United States Air Quality System (AQS) network from 1980 to 2020 were obtained from the Environmental Protection Agency (EPA) website. More detailed information on the sources and selection criteria of O3 and NO2 monitored data are described in the Supporting Information. In this work, we examined the relationship of 8h-NO2 with the daytime-produced O3 value (DPO3 = MDA8O3-O3 (6:00LT)). MDA8O3 refers to the maximum daily 8 h average ozone. 8h-NO2 is the average NO2 in the same 8 h period of MDA8O3. Daytime-produced O3 value is defined as the difference between the MDA8O3 value and the pre-sunrise O3 measured at 6:00 am local time in the day. The metric DPO3 (MDA8O3-O3) (6:00LT) is used instead of MDA8O3 for better conforming to the definition of the O3 formation. As shown in Figure S1, the O3 diurnal variations indicate small O3 production at clean regions, such as the two background sites in Wyoming, US, whereas the MDA8O3 levels cannot reflect the small O3 local formation at these sites.

A zero-dimensional (0D) photochemical box model based on the Regional Atmospheric Chemistry Modeling (RACM) mechanism20 was utilized to simulate the photochemical relationship between DPO3 and 8h-NO2 in a similar manner to the EKMA application in different regions and cities. We defined a default setting as a typical condition representing the average of meteorological and environmental situations. The default case was run under a moderate-condition setting in China, with assumptions of national average latitude 34 °N, temperature 290 K, relative humidity (RH) 50%, mixing layer height (MLH) varying from 200 to 1000 m, and on date of September 23rd (average solar radiation of a year). The speciation of anthropogenic VOCs (AVOCs) was derived from the Multiresolution Emission Inventory for China (MEIC) 2017 inventory,21 which provides the emissions of the top-30 AVOC species with the highest ozone formation potential (OFP). The biogenic VOC (BVOC) emissions were classified into categories of d-limonene and other monoterpenes with two double bonds (LIM), monoterpenes with one double bond (API), and isoprene (ISO) categories, according to a previously published speciation scheme.22 More detailed information on the model configuration and scenario settings are described in the Supporting Information.

Results and Discussion

DPO3 and 8h-NO2 Trends in China

Based on the monitoring dataset of China, we examined the changes in the annual DPO3 and corresponding 8h-NO2 at a total of 1281 selected CNEMC monitoring sites from 2015 to 2020 (Figure 1). The linearly regressed, nationally averaged increasing rate of DPO3 was 1.12 ppb·y–1, and the decreasing rate of 8h-NO2 was −0.35 ppb·y–1 during 2015–2020 (Figure 1A). These results are consistent with previous observations obtained from individual photochemistry projects and those recorded at long-term background monitoring stations, where elevated ground O3 levels over China have been widely reported.9,10,23

Figure 1.

Figure 1

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DPO3 and 8h-NO2 trends in China from 2015 to 2020. (A) Trends and linear regression of the nationally averaged DPO3 and 8h-NO2 concentrations. Spatial distribution of the annual increase rates of (B) DPO3 and (D) 8h-NO2 at the national monitoring sites (N = 1281) in China for the 2015–2020 period. (C) Quadrant distributions of the DPO3 and 8h-NO2 change rates in different provincial capital cities in China. The cities were marked using acronyms. The full names of the provincial capital cities and their locations on a map are also provided in Figure S2 in the Supporting Information.

As shown in Figure 1, different cities have reflected different change directions and degrees in their DPO3 and 8h-NO2 levels. The increase in DPO3 and decrease in 8h-NO2 were both more noticeable in the cities in the North China Plain (NCP) and Eastern China than elsewhere in the country (Figure 1B,D). Some cities (e.g., Tianjin (TJ)) have experienced a slight 8h-NO2 decrease but a large DPO3 increase over the past six years, whereas cities such as Chengdu (CD) and Guiyang (GY) have shown minor decreases in DPO3 relative to their substantial 8h-NO2 reductions (Figure 1C). The O3–NO2 diagram approach, which was subsequently described, was utilized to further interpret the different O3–NO2 relationships of these Chinese cities.

O3–NO2 Diagram Approach

The “O3–NO2 diagram approach”, like a transformed EKMA, traces the relationship between the modeled DPO3, 8h-NO2, and the precursor conditions on the O3–NO2 diagram modeled from the gas-phase 0D box model. The default case was performed under the typical condition of China. In addition to the default case, different scenario tests were conducted at various latitudes, temperatures, seasons, RH, MLH, and VOC speciation (see the Supporting Information).

By representing the calculated DPO3 concentrations as the Y-values and 8h-NO2 as the X-values in Figure 2A, the resulting VOC emission isopleth (color lines) depicts how photochemically produced O3 and NO2 respond to changes in NOx emissions under fixed VOC emission conditions. The VOC emission isoline reflects the nonlinear response of O3 formation to NOx emissions and reveals a distinct transition of the O3 formation sensitivity at turning points (B), at which the DPO3 concentration increases (decreases) as NO2 increases on the left (right) (Figure 2A). The role of these B points as sensitivity thresholds was also confirmed by their locations on the DPO3 isopleth diagram modeled using the traditional EKMA and shown in Figure 2B. NOx-limited regimes are represented by the left sides of the B points (Figure 2A), where the VOC isolines are densely arranged but do not overlap or cross (Figure S4). These closely spaced paths are consistent with the known insensitivity of O3 formation to VOCs under NOx-limited regimes. In contrast, VOC-limited regimes are represented by the right sides of the B points, with the low-to-high VOC emission isolines arranged from the bottom-left to the top-right (Figure 2A). The slopes of the C-to-B segments of the VOC isolines in Figure 2A represent the extent to which O3 increases in response to NOx mitigation under fixed VOC conditions in a VOC-limited regime.

Figure 2.

Figure 2

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Relationship between DPO3 (= MDA8O3-O3 6:00) and 8h-NO2. (A, left panel) The modeled relationship between DPO3 and 8h-NO2 follows the A-to-D path along the VOC emission isolines (solid color isolines) with increasing NOx emissions. Ten VOC emission settings (from 0.3 × 10–13 to 3.0 × 10–13 mol·cm–2·s–1 in ten equal intervals) were prescribed and marked with the normalized ratios (0.1–1.0) on the isolines. The corresponding reactivity of VOC emissions, as represented by the equation ∑ Emission-VOCi × MIRi, ranged from 0.146 × 10–10 to 1.46 × 10–10 gram O3 cm–2·s–1. The NOx emissions (addressed as NO emissions in the model) were prescribed from 0.05 × 10–12 to 2.4 × 10–12 mol cm–2 s–1 with 24 different values. Some representative NOx emission isolines (gray dashed isolines) marked by the normalized ratios (0.1–2.4) are shown in the figure. The modeled data of default case to produce the diagram is listed in Table S3. (B, right panel) The modeled EKMA (empirical kinetic modeling approach) DPO3 diagram of the default case, with the corresponding locations of the B points marked on the EKMA diagram. The red dashed line aligns with points A–B–C–D in panel B representing the VOC emission isoline shown in panel A. The O3 formation regimes identified as NOx-limited or VOC-limited are displayed in different background colors in panels A and B.

We also examined the DPO3–8h-NO2 diagrams modeled under scenarios of different seasons, latitudes, temperatures, relative humidity as well as the VOC speciation (see Figure S5). The VOC isolines tend to be steeper during summer, during high temperatures, and at low latitude, and will be flatter or more inclined if all VOCs are alkenes or aromatics, respectively. In rare cases, the VOC isolines may be bent at the high NOx emission ends (Figure S5); this condition is dependent on the time course of the NO+NO+O2 = 2NO2 reaction complementing the suppressed photochemical processes (Figure S6).

The positions of the division points (B) are important for distinguishing among different O3 formation regimes. The influence of environmental factors (seasons, latitudes, temperatures, RH, VOC speciation, etc.) on the division point (B) locations was examined. The division points (B) of all of the examined Chinese scenarios were located in a quite narrow area when considering the possible factor ranges characterizing Chinese cities (Figure 3), thus revealing the possibility of identifying the O3 formation regime by directly referring to these B point locations. In the figure, the linear equation of [DPO3] = 8.3 × [8h-NO2] represents the “safe” boundary, indicating a VOC-limited regime with regard to the annual mean O3–NO2 relationship for most Chinese cities. For more accurate analysis, it is recommended that a localized DPO3–8h-NO2 diagram should be produced for a specific region or city, with the specialized meteorological condition and VOC speciation of this region/city.

Figure 3.

Figure 3

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Sensitivity of the locations of the B points to the temperature, RH, season, latitude, MLH, and VOC speciation conditions. The black dashed line, represented by [DPO3] = 8.3 × [8h-NO2], indicates a safe boundary for VOC-limited regimes under various scenarios in China. The inset shows the expansion of the regime-transition region. The purple dashed lines represent the upper and lower bounds of the regime-transition region with regards to the annually averaged DPO3 and 8h-NO2 data characterizing Chinese cities, which were modeled under the southmost conditions (20 °N, 303 K, MLH 100–700 m) and northmost conditions (50 °N, 273 K, MLH 400–1000 m), respectively.

O3 Formation Regimes and Historical Routes Response to Precursor Controls

We mapped the annually averaged DPO3 and 8h-NO2 data of the Chinese monitoring sites on the modeled O3–NO2 relationship diagram (Figure 4A). As shown in the figure, most of the measurement data were located to the right of the safe regime-transition boundary line [DPO3] = 8.3 × [8h-NO2], indicating VOC-limited O3 formation regime. We also mapped the seasonal averaged DPO3 and 8h-NO2 data on modeled seasonal O3–NO2 relationship diagrams (Figure 5), which show similar results to the yearly averages. The patterns of the monitoring scatters and the predicted seasonal diagrams consistently depicted the varying characteristics of temperature and solar radiation across the seasons. Both the isolines and scatter distribution were steeper in the summer and flatter in the winter.

Figure 4.

Figure 4

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Annual DPO3–8h-NO2 data and evolving trends in China and USA. Locations of the annually averaged DPO3–8h-NO2 data recorded at (A) 1281 sites in China from 2015 to 2020 and at (B) monitoring sites in the USA from 1980 to 2020, superposed on the VOC emission isolines (0.1–1.0) derived under the default modeling conditions shown in Figure 2. The dots represent the annual average values at monitoring sites in China or the USA, and the triangles represent the national annual mean values at all sites in different years. The color scales indicate the year in which the data were measured.

Figure 5.

Figure 5

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Graphs illustrating the locations of seasonally averaged DPO3–8h-NO2 data recorded at 1281 sites in China from 2015 to 2020 on modeled seasonal DPO3–8h-NO2 diagrams.

Previous measurements and modeling studies have generally suggested that ozone production is under VOC-limited regimes in urban and industrial regions but under NOx-limited regimes in most rural areas in China.9,13,24,25 The present work tends to suggest the dominance of VOC-sensitive O3 formation regimes in the regions represented by the national monitoring stations. The DPO3–8h-NO2 scatterplot exhibits an evolving trend toward the upper-left direction with annually increasing O3 values during the 2015–2020 period (Figure 4A). This evolution direction is consistent with the NOx emission control efforts enacted in the country over the past five years. But referring to the location of the modeled safe boundary line [DPO3] = 8.3 × [8h-NO2], most of the Chinese sites still have a long way to go to reach NOx-limited regimes though the significant NOx emission control.

The United States (US) has suffered from high O3 pollution for a long time, and the successful emission reductions in recent decades can provide insights into the potential evolution and control of O3 pollution.7,26 The annual average O3 production and NO2 data recorded by the US EPA monitoring network from 1980 to 2020 are depicted in Figure 4B, which shows the overall historical route of the USA’s DPO3–8h-NO2 in the past few decades. The data clearly show an evolution toward the lower DPO3 and NO2 region moving toward the NOx-limited regime. More sites have passed the transition point in recent years. Notably, concurrent decreases in O3 and NO2 were observed when the 8h-NO2 value reached approximately 10 ppbv after 2000, but this should not be interpreted as the turning point to the NOx-limited regime. Instead, these decreases are the result of the simultaneous successful control of NOx and VOC emissions. By comparing the O3 evolution route with the modeling results shown in Figure 4B, the USA data exhibit a shift across approximately five VOC isolines since 1980. An estimated 65% reduction in VOCs and a 70% reduction in NOx can be inferred based on the locations at which the VOC and NOx isolines cross. According to the US EPA, national emissions (excluding biogenic and wildfires) of VOCs and NOx were reduced by 60 and 70%, respectively, from 1980 to 2020 (https://www.epa.gov/air-trends/air-quality-national-summary). Though this semiquantitative estimation is associated with many uncertainties, including VOC speciation and meteorological condition differences between the countries, the trends and changing degrees estimated from the diagram generally match the emission inventory well.

In another direct and useful application of the diagram, we visualized the site-to-site variations and evolution routes of different cities/regions on the DPO3–8h-NO2 diagram (Figure 6). The locations of the Chinese provincial capitals on the diagram (Figure 6A) are consistent with current knowledge regarding the spatial distributions of NOx and VOC pollution in China.27,28 The highly industrialized and populated cities (e.g., cities in NCP, shown with a yellow background) are located in the upper-right quadrant of Figure 6A, suggesting that this highly polluted region experiences concurrently high NOx and VOC emissions. In contrast, the relatively less-industrialized cities in southwestern China (green background) are generally located in the bottom-left quadrant of the diagram.

Figure 6.

Figure 6

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Annual DPO3–8h-NO2 locations and evolving trends in different regions and cities of China. (A) Annually averaged DPO3–8h-NO2 data in all provincial capital cities in China in 2020 superposed on the VOC emission isolines established under the default case. The cities are marked using acronyms and grouped in different colors, according to their locations in different regions of China, as in Figures 2 and S2. (B–D) Evolution of DPO3–8h-NO2 over the past decade in cities located in the major regions of China, superposed on the VOC emission isolines obtained from the average condition of each region. The coverage of the NCP, PRD, and YRD regions is described in Figure S7 in the Supporting Information. The marker size represents the annual data values obtained in different years.

The evolving routes of different cities/regions shown on the diagram shed light on the historical precursor control strategies enacted in these cities/regions. As an example, we compared the evolving routes of the major cities in three most-developed regions in China shown in Figure 6B–D. The NCP, PRD, and YRD cities mostly showed steep DPO3–8h-NO2 tracks moving along the VOC emission isoline direction. Beijing and Hong Kong presented a relatively flat trace, crossing more VOC isolines and shifting leftward with a large 8h-NO2 decrease but a small O3 increase. This shift of Beijing toward the lower VOC isolines on the diagram suggests a reduction in VOC emissions of approximately 24% from 2015 to 2020. The O3 pollution in Hong Kong is shown to be primarily VOC-limited, exhibiting an O3 increasing trend overall throughout the last 20 years.5 As shown in Figure 6D, the annual DPO3–8h-NO2 data recorded in Hong Kong from 2010 to 2020 moved leftward, exhibiting a slight increase in O3 production but crossing two VOC emission isolines. This trend implies an approximate VOC reduction of 22% and a NOx reduction of 24% from 2010 to 2018 in Hong Kong. This estimation agrees well with the emission inventory from HKEPD, in which 26 and 23% reductions during this period were, respectively, reported in VOCs and NOx emissions,29 though the estimation was made based on the assumption of default VOC speciation of China. We also compared the estimated precursor emissions at the 1281 sites from the DPO3–8h-NO2 diagram with the emission rates obtained from the MEIC for the same region (Figure S8). The general trend of the inferred emission conditions was consistent with the bottom-up emission inventory, thus imparting confidence in the capability of the DPO3–8h-NO2 approach for diagnosing O3 formation and precursor controls in different regions.

It should be noted that this diagram approach would perform better on long-term historical analysis. This is because the utilized long-term observation dataset to some extent can overcome the short-time fluctuations and provide an overall diagnosis of the O3–precursor relationship. For example, interannual variations of meteorology would impact the evolving trace of the monitoring data. In addition, the fast reduction in PM2.5 concentrations in China in recent years could cause the near-surface radiation increase and reduce the heterogeneous aerosol sink of HO2 radicals,30,31 thus contributing to O3 concentration increases and upward shifting of the DPO3–8h-NO2 data on the diagram. These factors cannot be distinguished from the modeled DPO3–8h-NO2 diagram and would bias the diagram-estimated emission changes over the years. In estimating the trend in precursors, long-term datasets have an advantage due to their ability to overcome fluctuations between years.

Future O3 Pollution Mitigation in China

In view of the severe O3 pollution in China, the government has implemented and planned aggressive control measures regarding the emissions of pollutant species. Thus, we evaluated the impacts of future precursor controls on O3 pollution using the diagram estimation approach (Figure 7). The approach shows that a minimum reduction ratio (0.75:1.0) for VOC/NOx is required to achieve nonincreasing O3 production from the current annual levels. China′s 14th five-year plan calls for reductions of more than 10% in both VOC and NOx emissions by 2025 and emphasizes that the VOC reduction ratio should be no less than that of NOx in polluted regions. This synergetic 10% reduction in VOC and NO emissions is estimated to decrease the photochemically produced annual O3 concentration by approximately 2.0 ppbv (Figure 7A) and the summer O3 production by 1.5 ppbv on the national average (Figure S9). Based on the locations of the present DPO3–8h-NO2 and the localized baseline for the NCP, YRD, and PRD regions, regional reduction effects were further investigated (Figure 7B–D). With a synergetic 10% reduction in both VOC and NOx emissions, the anticipated decreases in DPO3 would be 2.5, 2.6, and 0.5 ppbv for the NCP, YRD, and PRD regions, respectively. On the contrary, DPO3 in Hong Kong may increase by 0.6 ppb under the same reduction scenario and will reduce only when a higher VOC reduction percentage can be achieved. The estimated DPO3 levels will fall by 10.4, 10.5, and 9.2 ppbv in NCP, YRD, and PRD, if the NOx and VOC emissions can be reduced by 10 and 20%, respectively. For the cases with only VOCs decreased by 20%, the predicted DPO3–8h-NO2 points would move downward along the NOx isolines (gray dash lines in Figure 7), with the DPO3 drops of 16.8, 16.3, 14.5, and 6.1 ppbv in NCP, YRD, PRD, and Hong Kong, respectively; meanwhile, the 8h-NO2 level would likely rise.

Figure 7.

Figure 7

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Prediction of DPO3–8h-NO2 changes under different VOC and NOx emission control scenarios in China after 2020. (A) Estimated future changes in the national annual average DPO3–8h-NO2 level in China and (B–D) regions of NCP, YRD, and PRD in China under different emission–reduction scenarios in VOC and NO emissions. The regionally averaged data are superimposed on the VOC emission isolines (solid color lines) and NO emissions isolines (gray dashed lines) derived under the corresponding average condition of each region. The predicted locations and evolving traces of the annual DPO3–8h-NO2 levels under different VOC/NOx control strategies or in different regions are shown in open markers and dashed arrows on the diagrams.

Despite the fact that the O3–NOx–VOC sensitivity could vary across regions and seasons, the majority of recent studies suggest that VOC-targeted management is a more workable solution in China. For example, based on the WRF-CMAQ modeling, Wang et al. proposed that O3 pollution mitigation in NCP, YRD, and PRD would be effective when the VOCs/NOx reduction ratio is more than 2:1.32 Another modeling study in PRD showed that a reduction ratio of VOC/NOx more than 1:1 was necessary to accomplish synergetic control, and the best O3 reduction was found for a VOC-only control scenario.33 A recent study based on satellite retrievals also suggested that the ozone concentration in Beijing, Chengdu, and Guangzhou would be significantly lowered if the reduction ratio of VOCs/NOx is between 2:1 and 4:1.34 These previous investigations, together with the present work, all highlighted that the basis for O3 pollution management is an approximately 1:1 synergetic reduction of VOC and NOx and that a ratio greater than 2:1 could contribute to significantly reduced O3 levels.

In summary, the good performance on estimating the historical precursors controlling in the US and Hong Kong provides supporting evidence for the applicability of the DPO3–8h-NO2 diagram when addressing O3 pollution and evolution in different regions. This robust and rapid classification approach, in which only the continuous NO2 and O3 measurement data were utilized, may have broad application potential in evaluating the precursor control effects and assisting in developing O3 pollution mitigation strategies, in particular, for regions where comprehensive photochemical studies are unavailable. The historical evolution of air pollution in the US indicates that successfully controlling O3 pollution is possible. Synergetic VOC and NOx reduction and increasingly strict anthropogenic VOC control should be the primary focus at the present stage for controlling O3 pollution in China.

Acknowledgments

The authors would like to acknowledge the China Ministry of Ecology and Environment, Hong Kong Environmental Protection Department, and the United States Environmental Protection Agency for the O3 and NO2 measurement data. They thank the Leibniz Institute for Tropospheric Research (TROPOS) for providing the source code (http://projects.tropos.de/capram) for the photochemical mechanism modeling. This work was supported by the National Key R&D Program of China (2016YFC0203200), the National Natural Science Foundation of China (NSFC) project (42122062 and 41605093), the Research Grants Council of Hong Kong Special Administrative Region (grant nos. T24/504/17-N and 16209022), and the Hong Kong Environment and Conservation Fund (project 125/2020).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.2c08205.

  • Including monitoring data sources, modeling settings, the data for default case isolines plotting (Table S3), other supporting figures (Figures S1–S9), and tables (Tables S1 and S2) (PDF)

Author Present Address

The Hong Kong University of Science and Technology, Kowloon 999077, Hong Kong, China

Author Contributions

J.G., X.Z., and Z.W. designed the study; J.G. performed the modeling simulation and data analysis; J.G. and Z.W. led the manuscript writing with specific comments and edits from all other co-authors.

The authors declare no competing financial interest.

Notes

All data are available in the main text or Supporting Information. Correspondence and requests for further materials should be addressed to X.Z. (zhangxsh@rcees.ac.cn) and Z.W. (z.wang@ust.hk).

Supplementary Material

es2c08205_si_001.pdf (1.2MB, pdf)

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