*3.5. Proportion of Six Air Quality Index (AQI) Classes*

Figure 7 explains the annual (a–d) and seasonal (spring (e–h), summer (i–l), autumn (m–p), and winter (q–t)) proportion of AQI classes in NWC during 2015 to 2018. During the study period, the average proportion of Class I, Class II, Class III, Class IV, Class V, and Class VI accounted for 17.2%, 63.1%, 13.1%, 3.01%, 2.14%, and 1.03% of the days, respectively. In NWC, the proportion of Class I, Class II, Class III, Class IV, Class V, and Class VI experienced an average change of 114.1%, −1.08%, −55.2%, −29.5%, −69.2%, and −58.3%, respectively, in 2018, with respect to 2015. The combined proportion of Class I and II increased by 18.5%, with the highest increase in spring (28.73%), followed by winter (27.2%), autumn (11.2%), and summer (10.9%), indicating significant improvement in air quality over the time span.
