*3.3. PM2.5/PM10 Ratio*

The PM2.5/PM10 ratio normally reflects the composition and quality of air, e.g., a higher PM2.5/PM10 ratio indicates the increased proportion of PM2.5 and a lower PM2.5/PM10 ratio indicates a higher concentration of PM10 in the atmosphere. The annual average PM2.5/PM10 ratio in NWC during 2015 to 2018 was 0.480 ± 0.08, 0.478 ± 0.07, 0.483 ± 0.08, and 0.478 ± 0.07, respectively, and experienced a reduction of 0.43% over time (Figure 1g). The highest PM2.5/PM10 ratio occurred in SN followed by QH, GS, XJ, and NX. In terms of seasonality, the highest PM2.5/PM10 ratio occurred in winter, followed by autumn, spring, and summer, and experienced an average change of −2.77%, −4.98%, 1.82%, and 5.31%, respectively (Figure 3g). In the case of monthly variation, a "U" shaped curve was observed with the highest value in winter and the lowest value in the summer Figure 4g. Figure 5 illustrates the annual (a–d) and seasonal (spring (e–h), summer (i–l), autumn (m–p), and winter (q–t)) spatial distribution of PM2.5/PM10 ratio in NWC during 2015 to 2018. In 2018, 49.1% of the cities of NWC experienced an increase in the PM2.5/PM10 ratio as compared with 2015. Similarly, 49.1%, 62.3%, 35.9%, and 39.6% of the cities of NWC experienced increased PM2.5/PM10 ratio in spring, summer, autumn, and winter, respectively, in 2018 against 2015. From 2015 to 2018, approximately 1.89%, 1.89%, 3.74%, and 26.4% of the cities experienced PM2.5/PM10 ratio higher than 0.60 in spring, summer, autumn, and winter, respectively, in NWC.

**Figure 3.** Seasonal variation of PM2.5 (**a**), PM10 (**b**), SO2 (**c**), NO2 (**d**), CO (**e**), O3 (**f**), PM2.5/PM10 ratio (**g**), and AQI (**h**) in five provinces (Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX), and Qinghai (QH)) of northwest China (NWC) and NWC as a whole between 2015 and 2018. Descriptions are as follows: light blue bar (spring), orange bar (summer), grey bar (autumn), yellow bar (winter), and blue line with dots (CAAQS Grade II standards, daily mean). The abbreviations are as follows: PM2.5 (fine particulate matter), PM10 (coarse particulate matter), SO2 (sulfur dioxide), NO2 (nitrogen dioxide), CO (carbon monoxide), O3 (ozone), PM2.5/PM10 (ratio of PM2.5 with PM10), and AQI (air quality index).

**Figure 4.** The monthly average concentration of PM2.5 (**a**), PM10 (**b**), SO2 (**c**), NO2 (**d**), CO (**e**), O3 (**f**), PM2.5/PM10 (**g**), and AQI (**h**) 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 line with dots (SN), orange line with dots (XJ), grey line with dots (GS), yellow line with dots (NX), blue line with dots (QH), and parrot line with dots (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).

**Figure 5.** Annual (**a**–**d**) and seasonal (spring (**e**–**h**), summer (**i**–**l**), autumn (**m**–**p**), and winter (**q**–**t**)) spatial distribution of PM2.5/PM10 in 53 cities of northwest China (NWC) between 2015 and 2018. Color represents the different pollution levels, e.g., green (good), yellow (moderate), orange (unhealthy for a sensitive group), red (unhealthy for all), purple (very unhealthy), and maroon (hazardous).
