*3.6. The Major Pollutants/Primary Pollutants*

During the study period, PM10 was a major pollutant, accounting for more than 32.9% of the days, followed by O3 (25.9%), PM2.5 (16.4%), NO2 (3.52%), CO (1.43%), and SO2 (1.01%) in NWC (Figure 8). In 2018, the number of days with PM10, PM2.5, SO2, and CO as major pollutants decreased by 35%, 38%, 52%, and 90%, respectively, and increased by 46% and 11% for O3 and NO2, respectively. PM10 was a major pollutant in autumn (41.3%), spring (39.1%), and a second major pollutant in winter (34.3%), while PM2.5 was a major pollutant in winter (42.1%), and O3 was a major pollutant (59.7%) in summer. The number of days with O3 as a major pollutant was higher in the hotter months March–September), and, for PM2.5, it was higher in the colder months (November–February). During the study period, the number of days with PM10 as a major pollutant decreased by 47.2%, 76.3%, and 19.7% in spring, summer, and autumn, respectively, and increased by 4.58% in winter 2018 as compared with 2015. Similarly, the number of days with PM2.5 as a major pollutant decreased by 62.3%, 90.8%, 42.9%, and 24.6% in spring, summer, autumn, and winter, respectively. The number of days with SO2 as a major pollutant decreased in all the seasons except summer, NO2 decreased in summer and winter, CO decreased in spring, summer, autumn, and winter, while O3 increased in all the seasons, e.g., in spring, summer, autumn, and winter 2018 as compared with 2015.

**Figure 6.** Annual (**a**–**d**) and seasonal (spring (**e**–**h**), summer (**i**–**l**), autumn (**m**–**p**), and winter (**q**–**t**)) spatial distribution of AQI between 2015 and 2018 in 53 cities of northwest China (NWC). Color represents the different classes of air quality index, e.g., green (0–50, good), yellow (51–100, moderate), orange (101–150, unhealthy for the sensitive group), red (151–200, unhealthy for all), purple (201–300, very unhealthy), and maroon (300+, hazardous).

**Figure 7.** The annual (1st row) and seasonal (spring (2nd row), summer (3rd row), autumn (4th row), and winter (5th row)) distribution of AQ class, e.g., Class I (0–50, good, green), Class II (51–100, moderate, yellow), Class III (101–150, unhealthy for a sensitive group, orange), Class IV (151–200, unhealthy for all, red), Class V (001-300, very unhealthy, purple), and Class VI (300+, hazardous, maroon) in northwest China (NWC) between 2015 and 2018.

**Figure 8.** The annual (**a**) and seasonal (spring (**b**), summer (**c**), autumn (**d**), winter (**e**)) percentage of days with different primary pollutants (PM2.5, PM10, SO2, NO2, CO, and O3) in five provinces (Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX), and Qinghai (QH)) of northwest China (NWC) between 2015 and 2018. Descriptions are as follows: light blue bar (Shaanxi), orange bar (Xinjiang), grey bar (Gansu), yellow bar (Ningxia), blue bar (Qinghai), and parrot bar (NWC). The abbreviations are as follows: PM2.5 (fine particulate matter), PM10 (coarse particulate matter), SO2 (sulfur dioxide), NO2 (nitrogen dioxide), CO (carbon monoxide), and O3 (ozone).
