*2.2. Air Quality Data*

The concentrations of six pollutants, SO2, NO2, CO, O3, PM2.5 and PM10, were monitored hourly over 22 cities across Sichuan Basin from January 2015 to December 2020. The data were made available by the China National Environmental Monitoring Center (http://www.cnemc.cn/ (accessed on 15 September 2021)).

The instruments for air quality monitoring were deployed according to the China Environmental Protection Standard HJ 664-2013. The equipment came from Shenzhen Aosen Purification Technology Co., Ltd., China. The gaseous pollutant and PM concentrations were measured following the Specifications and Test Procedures for Ambient Air Quality Continuous Automated Monitoring System HJ 654-2013 for SO2, NO2, O3 and CO, and HJ

653-2013 for PM2.5 and PM10, as stipulated in the National Environmental Protection Standards of the People's Republic of China. The air quality monitoring stations were located at least 50 m away from any notable stationary pollution sources, and the inlets for the instruments were placed at least 1 m higher than the roof of the building or wall [26]. Data quality assurance and quality control (QA/QC) were conducted following the technical guidelines on environmental monitoring quality management (HJ 630-2011) established in the National Environmental Protection Standards of the People's Republic of China. The validity of the data was checked following the national ambient air quality standards specified in the National Standards of the People's Republic of China (GB 3095-2012), as used in earlier studies [27,28]. The daily, monthly, and annual means of the data were calculated from the hourly concentrations (with ~80% of the available data to be considered as valid for calculating the mean).
