3.1.2. PM2.5

Figure 3 shows the change in PM2.5 in the CCEC from 2017–2020. The average annual concentration of PM<sup>10</sup> for the whole CCEC was 48.6 µg/m<sup>3</sup> (the range was 31.0 µg/m<sup>3</sup> , and standard deviation was 7.2 µg/m<sup>3</sup> ) in 2017. In total, 97.0% of the cities failed to reach Grade II of AAQS. The area with the highest concentration was Zigong city in Sichuan province (66 µg/m<sup>3</sup> , which was 89.0% higher than Grade II of AAQS), while the area with the lowest values was Yunyang county in Chongqing (35 µg/m<sup>3</sup> ). The heavily polluted areas were mainly located in Chengdu–Deyang and Leshan–Zigong, and the PM2.5 distributions were consistent with air pollution transmission channels 1 and 2, which were consistent with the results of Luo et al. [9] and Liao et al. [30]. Some studies showed that air stagnation always happened in the Sichuan–Chongqing region in winter, which caused the high concentration of PM2.5 [30]. Furthermore, as one of the most important agricultural regions in China, the combustion of biomass has also caused the severe pollution in Sichuan Basin [31]. The average annual concentration of PM2.5 in CCEC decreased to 31.9 µg/m<sup>3</sup> (the range was 22.9 µg/m<sup>3</sup> , and standard deviation was 5.0 µg/m<sup>3</sup> ) in 2020, and 75.0% of the cities met the Grade II of AAQS. The area with highest concentration of PM2.5 was Rongchang district in Chongqing (44 µg/m<sup>3</sup> , which was 25.7% higher than the Grade II of AAQS), while the area with lowest value was Ya'an city in Sichuan province (21 µg/m<sup>3</sup> ). Based on the mean values, ranges and standard deviations of PM<sup>10</sup> and PM2.5, the control of particulate matter had achieved remarkable improvement in 2020.

**Figure 3.** Spatial distribution of PM2.5 in CCEC during 2017 to 2020.

It was said by QAGQ that priority should be given to the air quality guidelines of PM2.5 when considering the impact of particulate matter. The average annual concentrations of PM2.5 for each area in the CCEC were higher than the air quality guidelines (5 µg/m<sup>3</sup> ) in 2020. The number of areas exceeding the interim target 1 (35 µg/m<sup>3</sup> ), interim target 2 (25 µg/m<sup>3</sup> ) and interim target 3 (15 µg/m<sup>3</sup> ) were 55.6%, 69.4% and 2.8%, respectively. If mortality in a population exposed to PM2.5 at the air quality guideline level was arbitrarily set to 100, then it would be 124, 116 and 108, respectively, in populations exposed to PM2.5 at interim target 1, 2 and 3 levels. The results showed that the concentration level of PM2.5 causes a high risk to public health and should be given priority to control.

It was found that the higher the ratio value of PM2.5/PM10, the heavier the influence on the atmospheric environment [32]. The mean value of the PM2.5/PM<sup>10</sup> ratio in Beijing was 0.72, which ranged from 0.31~0.96 [32]. The PM2.5/PM<sup>10</sup> ratio values of Kaohsiung and Hong Kong were around 0.62 and 0.63 [33,34], respectively. The ratio values of PM2.5/PM<sup>10</sup>

in the whole CCEC were 0.68, 0.66, 0.66 and 0.64 from 2017 to 2020, respectively. It was shown that the larger the proportion of PM2.5 in particulate matter, the higher the risk to public health. The range of PM2.5/PM<sup>10</sup> ratio in cities of Sichuan province in the CCEC were between 0.63–0.64. The ratio of PM2.5/PM<sup>10</sup> showed a small decrease from 0.7 to 0.64 in the districts (counties) of Chongqing in the CCEC. The results showed that the pollution of particulate matter in Chongqing was much heavier than that in Sichuan province.
