*3.4. Air Quality Index (AQI)*

Air quality index (AQI) is a color-coded scale that simplifies different pollutants concentrations into a single numerical value that reflects overall air quality, health effects, sensitive groups, and required precautionary measures. During the study period (2015– 2018), the annual average AQI in NWC was 88.1 ± 24.1, 93.5 ± 36.3, and 82 ± 18.7, 69.2 ± 14.8, respectively, and improved by 21.3% (Figure 1h). The highest AQI occurred in XJ, followed by SN, NX, GS, and QH. The AQI improved in all the cities except a few cities in SN (Weinan, Xian), and XJ (Changji, Ili, Shihezi, Tacheng, Wujiaqu). In 2018, the average AQI was under the threshold value of 100 in all the cities except Shihezi and Wujiaqu in Xinjiang (Figure 6d).

In the case of seasonal variation, the highest AQI occurred in winter, followed by spring, summer, and autumn, respectively, and improved by 17.5%, 30.8%, 18.7%, 17.4%, respectively, in NWC during 2018 as compared with 2015. The seasonal variation was consistent throughout NWC, e.g., highest AQI in winter and lowest in autumn, except XJ (Figure 3h). Figure 6 illustrates the seasonal (spring (e–h), summer (i–l), autumn (m–p), and winter (q–t)) spatial distribution of AQI in NWC between 2015 and 2018. In different seasons, e.g., spring, summer, autumn, and winter, the number of cities exceeding the AQI threshold value of 100 decreased from 24.5% to 0% (Figure 6e–h), 7.55% to 0% (Figure 6i–l), 7.55% to 0% (Figure 6m–p), and 50.9% to 22.6% (Figure 6q–t), respectively, in NWC. In the case of the monthly variation, a "U" shaped curve was observed with the highest value in winter and the lowest value in summer Figure 4h.
