*3.2. Seasonal Variation of Six Criteria Pollutants*

In terms of seasonality, all the pollutants (PM2.5, PM10, SO2, NO2, and CO) observed the highest concentration in winter and the lowest occurred in summer except O3 (vice versa) between 2015 and 2018 (Figure 3). PM2.5 exceeded CAAQS Grade II (75 μg/m3, daily mean) in SN, and XJ during winter (Figure 3a), while PM10, SO2, NO2, CO, and O3 complied with CAAQS Grade II (150 μg/m3, 150 μg/m3, 80 μg/m3, 4 mg/m3, and 160 μg/m3, daily mean) in SN, XJ, GS, NX, QH, and NWC during spring, summer, autumn, and winter (Figure 3b–f). The average concentration of PM2.5, PM10, SO2, NO2, and CO decreased in all seasons, e.g., spring, summer, autumn, and winter, while the average concentration of O3 increased in all seasons between 2015 and 2018. Figure S2 explains the spatial distribution of the criteria pollutants in 53 cities of NWC during different seasons. PM2.5 exceeded CAAQS Grade II standards (daily mean) in 1.89%, 5.56%, and 32.1% of the cities during spring, summer, and winter, respectively (Figure S2a). Similarly, PM10 exceeded the daily standard in 5.66%, 7.55%, 1.89%, and 26.4% of the cities in spring, summer, autumn, and winter, respectively (Figure S2b), while, SO2, NO2, CO, and O3 complied with CAAQS Grade II standards (daily mean) in all the cities of NWC during all seasons (Figure S2c–f).

Figure 4 illustrates the monthly variation in the criteria pollutants, PM2.5/PM10 ratio, and AQI in NWC between 2015 and 2018. PM2.5, PM10, NO2, SO2, and CO explicated "U" shaped curves, with the highest concentration in winter (October to January) due to increased coal combustion for civil heating and stagnant meteorological conditions, e.g., lower wind speed, low temperature, etc., while the lowest concentration occurred in summer (June to August) due to seasonal rains and favorable atmospheric conditions that help in pollution dispersion. In the case of PM10, higher pollution levels also occurred in spring (March to May) due to haze events. In contrast to other pollutants, the O3 levels were higher in June to August (summer) and lower in winter.
