*Article* **Characteristics of Ozone Pollution in Tai'an and Topographic Effects of Mount Tai**

**Yanzhen Ge <sup>1</sup> , Chunqiang Chen <sup>2</sup> , Yaping Jiang <sup>3</sup> , Tongsuo Yang <sup>2</sup> , Hongyang Kang <sup>4</sup> , Jingzhong Li <sup>1</sup> , Xinran Zhao <sup>1</sup> , Yibo Zhang <sup>3</sup> , Mengying Li <sup>3</sup> , Tangyan Hou <sup>3</sup> , Jianmin Chen <sup>5</sup> and Shaocai Yu 3,\***


**Abstract:** Through the analyses of the observed concentrations of ozone (O<sup>3</sup> ) in Tai'an from 2016 to 2021, the results show that O<sup>3</sup> pollution was relatively serious, with the mean concentrations of the 90th percentile of daily maximum 8-h O<sup>3</sup> (O<sup>3</sup> -8h-90per) above 180 µg/m<sup>3</sup> . O<sup>3</sup> pollution in Tai'an mainly occurred from May to September, accounting for 69%~100% of the total O<sup>3</sup> pollution days, of which the most serious pollution occurred in June. Combined with the analyses of temperature, humidity, and wind speeds, the probability of O<sup>3</sup> exceedances in Tai'an increased significantly under the conditions with the temperatures higher than 30 ◦C, the relative humidity of 20%~40%, and the wind speeds of 1~3 m/s. The dominant wind directions on O<sup>3</sup> pollution days in Tai'an were southerly winds, based on the analyses of wind directions and their clustering trajectories. Based on the results at three monitoring stations at different distances from Mount Tai from May to September in 2021, the average O<sup>3</sup> concentrations at the Renkou School station near the mountain was about 13~15 µg/m<sup>3</sup> higher than those at the other two stations (Dianli College and Shandong First Medical University) which are far away from the mountain, indicating that the Renkou School site was more affected by the obstruction of the mountain. In addition, the WRF-CMAQ model was used to simulate ten O<sup>3</sup> pollution events in 2021, showing that the average O<sup>3</sup> concentrations in Tai'an were reduced by 1.7~7.5 µg/m<sup>3</sup> after changing the topographic height of Mount Tai.

**Keywords:** ozone; pollution characteristics; terrain; mount tai; WRF-CMAQ

## **1. Introduction**

In recent years, ozone (O3) has gradually become the primary pollutant affecting air quality in China, and it has shown a rapid growth and deterioration trend in vast areas of China [1–5]. Ground-level O<sup>3</sup> is generated due to complex photochemical reactions of volatile organic compounds (VOCs) and nitrogen oxides (NOx) under sunlight [6–9]. The surface O<sup>3</sup> can stimulate the respiratory tract and cause changes in lung function and respiratory inflammation and asthma, seriously threatening human health [10–13]. In order to improve the regional air quality and reduce the serious harm of O<sup>3</sup> pollution to the human body, effectively reducing the concentrations of surface O<sup>3</sup> has become an arduous task for local environmental protection departments.

Since 2016, O<sup>3</sup> pollution in Tai'an has been serious. In 2016, the 90% percentile of annual O<sup>3</sup> concentrations ranked 168th among 168 key cities specially selected by the Ministry of Ecology and Environment for the national urban air quality ranking in China.

**Citation:** Ge, Y.; Chen, C.; Jiang, Y.; Yang, T.; Kang, H.; Li, J.; Zhao, X.; Zhang, Y.; Li, M.; Hou, T.; et al. Characteristics of Ozone Pollution in Tai'an and Topographic Effects of Mount Tai. *Atmosphere* **2022**, *13*, 1299. https://doi.org/10.3390/ atmos13081299

Academic Editors: Elena Hristova, Manousos Ioannis Manousakas, Anikó Angyal and Maria Gini

Received: 6 July 2022 Accepted: 12 August 2022 Published: 16 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In 2021, it ranked 166th among 168 cities, showing that O<sup>3</sup> has become the primary pollutant affecting air quality in Tai'an [14,15]. Tai'an city has a special geographical environment, with the terrain high in the north and low in the south. It is located at the foot of Mount Tai, with the Taishan Mountains in the north and the Culai Mountain in the east, being favorable for pollutants to accumulate at the foot of the mountain and the downtown area [15]. VOCs and NOx are transported northward by the southerly winds, providing precursors for the photochemical reactions of O3. In addition, Mount Tai is rich in vegetation, thus VOCs emitted by vegetation can provide certain precursors to O<sup>3</sup> generations in Tai'an. This study analyzed the characteristics of O<sup>3</sup> pollution in Tai'an from 2016 to 2021, and explored the effects of Mount Tai on O<sup>3</sup> concentrations. It may provide scientific basis and technical support for the causes of O<sup>3</sup> pollution and air pollution prevention and control measures in Tai'an.

#### **2. Materials and Methods**

#### *2.1. Observation Data*

The observation data of O3, CO, SO2, NO2, PM2.5, and PM<sup>10</sup> at six air quality stations of Tai'an, as shown in Figure 1, from 2016 to 2021 were obtained from the Tai'an ambient air quality automatic monitoring system. The daily data were obtained from the average of observations at these six air quality observation stations, and the monthly and annual average O<sup>3</sup> concentrations were taken as the 90th percentile of the maximum 8 h average concentration of O<sup>3</sup> (O3-8h-90per). The O<sup>3</sup> observation data of other cities were from China National Environmental Monitoring Centre. The observational data of temperatures, relative humidity, and wind speeds in Tai'an were provided by the Tai'an ambient air quality automatic monitoring system, and the wind direction data were obtained from the weather post report website (http://tianqihoubao.com/weather/top/taian.html) (accessed on 1 January 2022).

**Figure 1.** Map of Tai'an and distributions of six air quality monitoring stations in Tai'an.Map data © 2022, Gaode Map.

#### *2.2. Analytical Method*

According to the relevant regulations of the "Ambient Air Quality Standards" in China (GB 3095-2012), this study defined an O<sup>3</sup> pollution day when the maximum 8 h average concentration of O<sup>3</sup> (O3-8h) exceeded 160 µg/m<sup>3</sup> , and then conducted monthly and annual compliance evaluations based on the "Ambient Air Quality Assessment Technical Specifications (Trial)" (HJ 663-2013). According to the relevant requirements of the "Ambient Air Quality Index (AQI) Technical Regulations (Trial)" (HJ 633-2012), the air quality was divided into five levels depending on the concentrations of O3-8h: excellent (0–100 µg/m<sup>3</sup> ), good (101–160 µg/m<sup>3</sup> ), mild (161–215 µg/m<sup>3</sup> ), moderate (216–265 µg/m<sup>3</sup> ), and heavy pollutions (266–800 µg/m<sup>3</sup> ).

## *2.3. Model Settings*

The Community Multiscale Air Quality model (CMAQ) was applied to simulate the O<sup>3</sup> concentrations in Tai'an (https://www.epa.gov/cmaq) (accessed on 9 August 2022). The model meteorological field was provided by the Weather Research and Forecasting Model (WRF) (https://www.mmm.ucar.edu/weather-research-and-forecasting-model) (accessed on 9 August 2022). The input data for the WRF model were obtained from the Meteorological Reanalysis Data (FNL) of the US National Center of Environmental Prediction (NCEP). The CMAQ model adopted the CB05 carbon bond chemistry mechanism and the AERO6 aerosol chemistry mechanism [16,17]. The model had a horizontal resolution of 36 km and covered central and eastern China, as shown in Figure 2. Figure 2b,c show the terrain heights in the baseline simulation and the sensitivity simulation scenario, respectively, and the difference in the height of Mount Tai, as can be seen in Figure 2d. The anthropogenic emission sources were provided by the Multi-resolution Emission Inventory for China (MEIC) developed by Tsinghua University, and the biological sources were calculated by the online Biogenic Emission Inventory System version 3.14 (BEISv3.14).

**Figure 2.** Map of (**a**) the model domain; (**b**) default terrain height (m); (**c**) modified terrain height (m); and (**d**) the terrain height difference between (**b**) and (**c**) in the model.

#### **3. Results and Discussion**

#### *3.1. Overview of Ozone Pollutions in Tai'an*

The annual average concentrations of O3-8h in Tai'an from 2016 to 2021 were selected to analyze the interannual variation characteristics of O<sup>3</sup> pollution. As shown in Table 1, the annual average concentrations of O3-8h-90per in Tai'an from 2016 to 2021 were above 180 µg/m<sup>3</sup> (except in 2018), suggesting that the O<sup>3</sup> pollution in Tai'an has been very serious in recent years. Among them, the highest annual average concentration of O3-8h occurred in 2019, reaching 195 µg/m<sup>3</sup> , while the lowest concentration was in 2018. From 2019 to 2020, the O<sup>3</sup> concentrations in Tai'an showed a decreasing trend with the reduction of 12 µg/m<sup>3</sup> , while O<sup>3</sup> concentrations in 2021 were similar to those in 2020, indicating that the O<sup>3</sup> pollution situation in Tai'an was still relatively severe.


**Table 1.** Annual average concentrations of pollutants from 2016 to 2021 and O<sup>3</sup> pollution days for different wind directions in Tai'an during May–September from 2016 to 2021. The quantities in parenthesis represent the proportion of polluted days in a certain wind direction to the total polluted days.

Ground-level O<sup>3</sup> is mainly produced due to complex photochemical reactions of volatile organic compounds (VOCs) and nitrogen oxides (NOx) under the conditions of strong light, high temperature, and low humidity [18–21]. In view of the strong solar radiation, high temperature, and low humidity from May to September in Tai'an (Figure S1), these meteorological factors are favorable for generation of near-surface O3. Therefore, this study selected the period of May–September to conduct a statistical analysis of the O<sup>3</sup> pollution in Tai'an from 2016 to 2021 (Figure 3a).

**Figure 3.** The number of days of O<sup>3</sup> pollution in different levels in Tai'an for (**a**) the annual values from 2016 to 2021 and (**b**) monthly values during May–September from 2016 to 2021, scatter plots between hourly O<sup>3</sup> concentrations and temperature (**c**); relative humidity (**d**); and wind speeds (**e**) in Tai'an during May–September from 2016 to 2021.

As seen from Figure 3a, the number of O<sup>3</sup> pollution days in Tai'an from May to September was in the range of 50~79 days from 2016 to 2021 for the annual values, accounting for 32.7% to 51.6% of the total days from May to September every year. Among them, the number of pollution days in 2019 was the most (79 days), and in 2021 it was 65 days, indicating that the O<sup>3</sup> pollution in Tai'an was still relatively severe. From the perspective of pollution extent, O<sup>3</sup> pollution in Tai'an was mainly mild pollution, accounting for 69.0%~94.0% of the total pollution days from 2016 to 2021. The number of days with moderate and above O<sup>3</sup> pollution gradually decreased from 16 days in 2019 to 14 days in 2020, and then to 10 days in 2021 (Table 2). The O3-8h-90per concentrations also decreased year by year in the recent three years. This shows that Tai'an achieved certain improvements in O<sup>3</sup> controls in recent years.

**Table 2.** Monthly mean concentrations of maximum 8 h O<sup>3</sup> and NO<sup>2</sup> (µg/m<sup>3</sup> ) in Tai'an during May–September from 2016 to 2021.


Table 2 shows the statistical results of O3-8h-90per in Tai'an during May–September from 2016 to 2021. The concentrations of O3-8h-90per in May from 2020 to 2021 were both above 200 µg/m<sup>3</sup> , indicating that the occurrence times of O<sup>3</sup> pollution in Tai'an had a tendency to advance. June was the most polluted month, with the most days of pollution exceedance, the concentrations of O3-8h-90per ranging from 208–244 µg/m<sup>3</sup> and a total of 130 pollution days from 2016 to 2021, accounting for 34.7% of the total pollution days (Figure 3b). In addition, the moderate and above pollution in Tai'an occurred most frequently in June, with 43 days of O<sup>3</sup> moderate pollution, accounting for 61.4% of the total days of O<sup>3</sup> moderate pollution from 2016 to 2021. Notably, the four heavy pollution processes from 2016 to 2021 all occurred in June, which may be attributed to the relatively strong solar radiation and high temperature in June, resulting in strong photochemical reactions of O<sup>3</sup> in the atmosphere and high concentrations of ground-level O<sup>3</sup> [17].

#### *3.2. Analyses of Ozone Precursors and Meteorology*

Based on the analyses of NO<sup>2</sup> concentrations in Tai'an from 2016 to 2021, it was found that the overall NO<sup>2</sup> concentrations showed a downward trend, as shown in Table 2 and Figure S2. The average NO<sup>2</sup> concentration during May–September in 2021 was 17 µg/m<sup>3</sup> , while the NO<sup>2</sup> concentration in 2018 was relatively low, which might be caused by meteorological conditions such as typhoons. The NO<sup>2</sup> concentrations were relatively high from May to June in 2017, being consistent with the high levels of O<sup>3</sup> concentrations. From 2018 to 2020, the NO<sup>2</sup> concentrations were the highest in September, followed by May, while the highest NO<sup>2</sup> concentration occurred in June (20 µg/m<sup>3</sup> ) and May (19 µg/m<sup>3</sup> ) in 2021, being consistent with the months with high O<sup>3</sup> levels.

According to the analysis of temperature, humidity, and wind speed during the high-O<sup>3</sup> period (May–September) in Tai'an from 2016 to 2021, the probability of O<sup>3</sup> exceedances rose sharply under the conditions with the temperature higher than 30 ◦C, relative humidity of 20–40%, and wind speeds of 1–3 m/s, as shown in Figure 3c–e.

According to the statistical analysis of the wind directions during the O<sup>3</sup> pollution periods in Tai'an from 2016 to 2021, the dominant wind directions leading to O<sup>3</sup> pollution days were southerly (southwest, south, and southeast), accounting for 65%~100% of the total number of pollution days each year (see Table 1). The pollution days dominated by the northerly winds accounted for 17%~24%, and those dominated by easterly winds accounted for 0%~8%. In 2021, number of the O<sup>3</sup> pollution days in Tai'an dominated by southerly winds were 52 days, accounting for 80% of the total pollution days from May to September.

In order to study the effects of air flow trajectory differences on O<sup>3</sup> concentrations in Tai'an, cluster analysis was performed on the hourly backward trajectory during May–September from 2020 to 2021. Results show that the summer winds were dominated by southwesterly, northeasterly, and southeasterly winds, and the proportion of trajectory sources varied slightly between different years (see Table 3). Moreover, combining the hourly O<sup>3</sup> observation values in Tai'an with the backward trajectories, the trajectory pollution characteristics were explored. Referring to the secondary standard of O3-8h (160 µg/m<sup>3</sup> ) in "Ambient Air Quality Standards" (GB 3095-2012), the backward trajectories were divided into clean and polluted trajectories: the polluted trajectory was defined as the one with the corresponding O<sup>3</sup> concentrations greater than 160 µg/m<sup>3</sup> , otherwise as the clean trajectory [22]. The statistical results of various trajectories are shown in Figure 4 and Table 3. Results show that there were significant differences in the O<sup>3</sup> concentrations corresponding to different airflow trajectories and the proportions of pollution trajectories. In summer, the pollution trajectories in the southwesterly wind direction accounted for the highest proportion. The above analysis shows that due to the effects of southern transport and local topography, the proportion of O<sup>3</sup> pollution in Tai'an increased significantly under the southerly winds. The transports of ozone and its precursors were considered in the all simulation cases. Since Tai'an is located in the south of Mount Tai, O<sup>3</sup> concentrations in the other eight cases decreased after reducing the height of Mount Tai, indicating that the existence of Mount Tai led to the accumulation of pollutants blown from the south and east in Tai'an. A typical O<sup>3</sup> pollution event on 21 June 2021 was selected to compare the O<sup>3</sup> concentrations in Tai'an, Jinan, and Jining, and analyze the O<sup>3</sup> and NO<sup>2</sup> data in Tai'an. On 21 June 2021, the maximum concentration of O3-8h in Tai'an was 254 µg/m<sup>3</sup> (corresponding to AQI of 189), which reached a moderate pollution level, significantly higher than that those in Jining (219 µg/m<sup>3</sup> ) and Jinan (230 µg/m<sup>3</sup> ) (Figure 5a). For the hourly variations of O<sup>3</sup> concentrations in Tai'an (Figure 5b), the O<sup>3</sup> concentrations gradually decreased from 0:00 to 7:00 (stage 1) due to the titration of nitrogen oxides (NOx) on O<sup>3</sup> [23,24]. From 7:00 to 13:00, the O<sup>3</sup> concentrations gradually increased and reached a peak at 13:00, and O<sup>3</sup> and NO<sup>2</sup> showed a significant negative correlation during the hours of 7:00–15:00 (stage 2), indicating that the near-surface O<sup>3</sup> during this period was mainly generated from the photochemical reactions [22]. From 13:00 to 20:00, the NO<sup>2</sup> concentrations gradually increased, which may be affected by the upwind transports and local effects. From 13:00 to 18:00, the O<sup>3</sup> concentrations remained above 250 µg/m<sup>3</sup> for 6 consecutive hours, significantly higher than that those in Jining and Jinan. Considering that the dominant wind direction in Tai'an was southerly with the wind power of 2~3 during this period, the persistent high O<sup>3</sup> in Tai'an was caused by the mountain blocking effects, and then the O<sup>3</sup> concentrations gradually decreased due to the titration of NOx.


**Table 3.** Statistics of occurrence frequencies of O<sup>3</sup> trajectories and their corresponding mean O<sup>3</sup> concentrations in Tai'an during May–September in 2020 and 2021.

**Figure 4.** Clusters of average trajectory pathways of O<sup>3</sup> in Tai'an during May–September in 2020 (**a**) and 2021 (**b**).

**Figure 5.** Comparisons of (**a**) diurnal variations of O<sup>3</sup> among Tai'an and surrounding cities and (**b**) diurnal variations of hourly O<sup>3</sup> and NO<sup>2</sup> concentrations in Tai'an on 21 June 2021; wind rise of observed O<sup>3</sup> concentrations in Tai'an during (**c**) 3–9 July and (**d**) 7–12 August 2021.

## *3.3. The Effects of Mount Tai on O<sup>3</sup>*

Three air quality stations in Tai'an, namely, Renkou School, Dianli College, and Shandong First Medical University, were selected to analyze the effects of the mountain on O<sup>3</sup> (see Table 4). The Renkou School, Dianli College, and the Shandong First Medical University stations are about 0.5, 3.5, and 8 km away from the mountain, respectively. The specific distributions of the three stations are shown in Figure 1. Based on the comparisons of monthly average concentrations of O<sup>3</sup> at the three stations from May to October 2021, the average O<sup>3</sup> concentration at the Renkou School station closer to the mountain was 118 µg/m<sup>3</sup> , significantly higher than that at the Dianli College and Shandong First Medical University stations, which were relatively far away from the mountain. For different months from May to October 2021, the O<sup>3</sup> concentrations at the Renkou School station were 8~19 µg/m<sup>3</sup> higher than those at the Dianli College station, and 10~20 µg/m<sup>3</sup> higher than those at the Shandong First Medical University station. In summary, the Renkou School station was relatively more affected by the blocking of the mountain due to being closer to the mountain, leading to the significantly higher O<sup>3</sup> concentrations relative to those of other stations.

**Table 4.** Comparison of mean O<sup>3</sup> -8h-90per concentrations (µg/m<sup>3</sup> ) at three monitoring stations in Tai'an from May to September 2021.


By setting the baseline simulations and sensitivity simulation scenarios of reducing the Mount Tai terrain heights to 150 m (Tai'an terrain height) (see Figure 2), the WRF-CMAQ air quality model was applied to simulate the changes of O<sup>3</sup> concentrations in Tai'an through the adjustment of topographic heights of Mount Tai. Note that biogenic VOCs in Mount Tai were not removed in the sensitivity simulations. A total of 10 O<sup>3</sup> pollution events that

occurred in Tai'an from May to September 2021 were selected to compare the changes of O<sup>3</sup> concentrations between the baseline and sensitivity simulations. Results show that after changing the terrain height of Mount Tai in the 10 selected pollution events, the simulated O<sup>3</sup> concentrations in eight pollution events decreased by 1.7~7.5 µg/m<sup>3</sup> relative to the baseline simulations, suggesting the positive contributions of the topography of Mount Tai to the occurrences of ozone pollutions in Tai'an (see Table 5). The simulated O<sup>3</sup> concentrations after reducing the height of Mount Tai in another two pollution events increased relative to the base simulations. Among the three monitoring stations, the reductions of O<sup>3</sup> concentrations caused by reducing terrain height were 2.4~8.8, 2.4~8.8, and 0.3~6.3 µg/m<sup>3</sup> for the Renkou School, Dianli College, and Shandong First Medical University stations, respectively. In the simulations, Renkou School and Dianli College were on the same grid, so the reductions of O<sup>3</sup> concentrations were the same. Through the analyses of the wind directions in the two pollution events in which the O<sup>3</sup> concentrations increased (3–9 July 2021 and 7–12 August 2021), it was found that the prevailing wind directions in Tai'an were northeasterly during the two pollution events (see Figure 5c,d). This might affect the transports of ozone and precursors from the upwind areas and result in an increase in O<sup>3</sup> concentrations after reducing the terrain heights of Mount Tai.

**Table 5.** Comparisons of O<sup>3</sup> concentrations between the baseline and scenario simulations by reducing the Mount Tai terrain heights for ten O<sup>3</sup> pollution events in 2021 in Tai'an (µg/m<sup>3</sup> ).


#### **4. Conclusions**

In recent years, Tai'an has suffered from serious O<sup>3</sup> pollution, with the annual average concentrations of O3-8h-90per above 180 µg/m<sup>3</sup> from 2016 to 2021. In this study, we analyzed the O<sup>3</sup> pollution situation in Tai'an from May to September, and found that the pollution during this period was mainly light pollution, accounting for 69% to 100% of the total polluted days, with June being the most polluted month. High temperature (>30 ◦C), low relative humidity (20%~40%), and low wind speed (1~3 m/s) provided favorable meteorological conditions for the generation of O3, especially with prevailing southerly wind, resulting in light and above pollution in Tai'an. The results of backward trajectory analysis showed that in the summer pollution trajectories, the pollution trajectories in the southwest direction accounted for the highest proportion of all kinds of trajectories. When the southerly winds dominated, the accumulated O<sup>3</sup> concentrations were significantly higher than those of the surrounding cities due to the blocking of the northern mountains. Comparing the three monitoring stations in Tai'an, the average O<sup>3</sup> concentrations from May to September 2021 at the Renkou School station closer to the mountain were 13~15 µg/m<sup>3</sup> higher than those at the Dianli College and the Shandong First Medical University stations, which are relatively far away from the mountains, indicating that the Renkou School station was more affected by the obstructions of the mountains. The WRF-CMAQ was applied in this study to quantify the blocking effect of mountains, and results indicated that the average O<sup>3</sup> concentrations in Tai'an decreased about 1.7~7.5 µg/m<sup>3</sup> after reducing the terrain height of Mount Tai in the model. Our results are of great significance for guiding

Tai'an to control O<sup>3</sup> in the future, and measures can be formulated in advance based on our conclusions.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/atmos13081299/s1. Figure S1: The variations of average monthly concentrations of temperature, relative humidity, precipitation, and wind speed in Tai'an from 2016 to 2021; Figure S2: The monthly mean concentrations of maximum 8-h O<sup>3</sup> (a) and NO<sup>2</sup> (b) in Tai'an during May-September from 2016 to 2021.

**Author Contributions:** S.Y. and Y.G. designed this research; C.C., T.Y., H.K., J.L. and X.Z. performed data analyses; Y.Z., M.L., T.H. and Y.J. performed model simulations; J.C. made contributions to discussion; S.Y., Y.G., C.C., Y.Z., Y.J. and M.L. wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Nos. 42175084, 21577126, and 41561144004), Department of Science and Technology of China (Nos. 2018YFC0213506 and 2018YFC0213503), and National Research Program for Key Issues in Air Pollution Control in China (No. DQGG0107).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The observation data and model results are available upon request.

**Acknowledgments:** The authors would like to thank Qiang Zhang of Tsinghua University for providing the MEIC emission inventory in East Asia for this study. This study was guided and assisted by the tracking team of "One City, One Policy".

**Conflicts of Interest:** The authors declare no conflict of interest.

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