3.1.4. SO<sup>2</sup>

The average annual concentration of SO<sup>2</sup> for the whole CCEC met Grade II of AAQS in 2017 (Figure 5). The mean value, range, and standard deviation were 15.4 µg/m<sup>3</sup> , 26.0 µg/m<sup>3</sup> , and 5.9 µg/m<sup>3</sup> , respectively. The area with the highest annual average SO<sup>2</sup> concentration was Nanchuan district in Chongqing (34 µg/m<sup>3</sup> ), while the areas with the lowest annual average SO<sup>2</sup> concentration were the counties of Yunyang and Zhong in Chongqing (8 µg/m<sup>3</sup> ). The average annual concentration of SO<sup>2</sup> for the whole CCEC decreased to 10.3µg/m<sup>3</sup> in 2020; the range and standard deviation were 10.0 µg/m<sup>3</sup> and 2.8 µg/m<sup>3</sup> , respectively. In recent years, the average annual concentration of SO<sup>2</sup> was far below Grade II of AAQS. The Sichuan Basin has suffered from pollution of SO<sup>2</sup> for the past decades, because 80% SO<sup>2</sup> concentrations in some areas of the Sichuan Basin was from industries such as chemical, textile, electronics, etc. [38]. Since the implementation of "the Division Plan of Acid rain and Sulfur dioxide Pollution Control Areas", the control of SO<sup>2</sup> achieved obvious improvement, basically removing the long-term problems of acid rain and sulfur dioxide pollution. The mean concentration of SO<sup>2</sup> in areas of Sichuan province in CCEC were reduced from 13.3 µg/m<sup>3</sup> in 2017 to 8.2 µg/m<sup>3</sup> in 2020, and the mean concentration of SO<sup>2</sup> in areas of Chongqing in the CCEC were reduced from 17.0 µg/m<sup>3</sup> in 2017 to 12.2 µg/m<sup>3</sup> in 2020. The concentration of SO<sup>2</sup> in the air depends on the consumption of coal [39]. The proportion of coal consumption in Sichuan province was 32.0% in 2019, while that in Chongqing was 53.0%. Therefore, the control of SO<sup>2</sup> in Sichuan province was better.

**Figure 5.** Spatial distribution of SO<sup>2</sup> in CCEC during 2017 to 2020.
