*3.8. Statistical Analysis*

The result of Pearson's correlation (Table S2) indicated that AQI was strongly positively correlated (R > 0.5) with PM2.5, PM10, SO2, NO2, and CO on an annual basis and strongly anti-correlated (R > −0.5) with O3 in the NWC (Figure S3a). The seasonal variation in the correlation between AQI and different pollutants was evident (Figure S3b–e). In spring, AQI was strongly correlated (R > −0.5) with PM2.5, PM10, SO2, and CO, moderately correlated (R > −0.3) with NO2, while strongly anti-correlated (R > −0.5) with O3 (Figure S3b). In summer, AQI was strongly correlated (R > 0.5) with PM2.5 PM10, and O3, weakly correlated (R > 0.2) with SO2, weakly anti-correlated (R > −0.1) with CO, and strongly anti-correlated (R > −0.5) with NO2 (Figure S3c). In autumn, AQI was strongly correlated (R > 0.5) with PM2.5, PM10, SO2, NO2, and CO), while strongly anti-correlated (R > −0.5) with O3 (Figure S3d). In winter, AQI was strongly correlated (R > 0.5) with PM2.5, PM10, SO2, and CO, moderately correlated (R > 0.3) with NO2, and moderately anti-correlated (R > −0.3) with O3 (Figure S3e). Throughout the study period, all the pollutants were positively correlated with each other except O3 (Table S2).
