3.1.3. O<sup>3</sup>

Figure 4 shows the change in O<sup>3</sup> in the CCEC from 2017–2020. In 2017, the concentration of O<sup>3</sup> for the whole CCEC was 139.6 µg/m<sup>3</sup> (the range was 60.0 µg/m<sup>3</sup> , and standard deviation was 15.6 µg/m<sup>3</sup> ), and 11.0% of the areas failed to reach Grade II of AAQS. The area with the highest concentration of O<sup>3</sup> was the Jiangjing district in Chongqing (164 µg/m<sup>3</sup> ), which was 9.0% higher than Grade II of AAQS, and the area with the lowest concentration of O<sup>3</sup> was Mianyang city in Sichuan province (114 µg/m<sup>3</sup> ). The mean values of O<sup>3</sup> were slightly increased at 144.8 µg/m<sup>3</sup> in 2018 and 140.6 µg/m<sup>3</sup> in 2019. The distributions mainly located in Chengdu–Meishan, Chongqing, and Zigong were consistent with three air pollution transmission channels. Based on our results, the pollution of O<sup>3</sup> was severe in the area of Chengdu. Yang et al. found that the gasoline vehicle exhaust and the use of solvents was the main reason [35]. Due to the low atmospheric pressure, small pressure gradient, and the stable weather, the condition of horizontal diffusion was bad for the diffusion of pollutants [36]. The photochemical reactions were promoted in Chongqing due to the increase in oxidation after 2016, and the exogenous input of VOCs in nearby areas led to the increase in O<sup>3</sup> concentration [37]. The concentration of O<sup>3</sup> for the whole CCEC decreased to 133.8 µg/m<sup>3</sup> (the range was 65.0 µg/m<sup>3</sup> , and standard deviation was 16.4 µg/m<sup>3</sup> ) in 2020. The area with the highest concentration of O<sup>3</sup> was Chengdu city in Sichuan province (169 µg/m<sup>3</sup> ), which was 6.0% higher than Grade II of AAQS, and the area with the lowest concentration of O<sup>3</sup> was Qianjiang district in Chongqing (104 µg/m<sup>3</sup> ). In total, 97.2% of the areas met the Grade II of AAQS.

Long-term exposure to high concentrations of O<sup>3</sup> could cause chronic damage to the human body. The concentrations of O<sup>3</sup> for each area in the CCEC were higher than the air quality guidelines (100 µg/m<sup>3</sup> ). The number of areas exceeding interim target 1 (160 µg/m<sup>3</sup> ) and interim target 2 (120 µg/m<sup>3</sup> ) were 2.8% and 72.2%, respectively. If mortality in a population exposed to ozone at the air quality guideline level was arbitrarily set at 100, then it would be 103 and 101 in the populations exposed to ozone at the interim target 1 and 2 levels. The results suggest that more strict measures are needed to be implemented for O<sup>3</sup> control in the future.

**Figure 4.** Spatial distribution of O<sup>3</sup> in CCEC during 2017 to 2020.
