**2. Materials and Methods**

## *2.1. Air Quality Monitoring Sites*

The air quality was monitored at 127 stations spread over 22 cities across Sichuan Basin, covering an area of over 260,000 square kilometers. Located in the central and southern part of the Asian continent, with a total population of more than 100 million, Sichuan Basin is one of the most densely populated areas in China and the world. Completely surrounded by high mountains and plateaus, it is a vast subtropical low hills and plains. The west is surrounded by the high-altitude Qinghai-Tibet Plateau, the south is the Yunnan-Guizhou Plateau, the east is Wushan, and the north is Dabashan. Due to low wind speed and high relative humidity, it was one of the four traditional areas with acidic rain and frequent haze events [25]. Figure 1 showed the locations of the 22 cities that collected the air quality data used in this study.

Considering the completeness of the data, this study collected the socio-economic data of each city in Sichuan Basin during 2018 (source: http://tjj.cq.gov.cn/zwgk\_233/tjnj/2019 /zk/indexce.htm (accessed on 15 September 2021)). Table 1 listed the city's abbreviations, number of vehicles, population, and GDP (Gross Domestic Product). In 2018, Chongqing's total population was 31.01 million, the total number of vehicles was 6.31 million, and the GDP was 20363 billion yuan, ranking first among the cities. As another megacity in Sichuan Basin, Chengdu has a total population of 16.33 million, a total of 4.87 million vehicles, and a GDP of 1.5342 billion yuan, second only to Chongqing.

**Figure 1.** The map of Sichuan Basin and the locations of 22 city stations.


