*2.1. Site Selection*

In this study, we examined the ambient air quality in the northern and western parts of China, known as northwest China (NWC), from January 2015 to December 2018 to understand the spatio-temporal variation across NWC better. NWC is a mixture of agricultural areas, deserts, mountains, etc. with significant coal reserves, industrial activity, covering an area of 3.1 million sq. km area (32.4% of China), having a population of more than 96.65 million, and experiencing degraded air quality. NWC consists of five provinces, namely Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX), and Qinghai (QH) (Supplementary Material Figure S1). Table 1 gives detailed information about 5 provinces and 53 cities.


**Table 1.** Description of cities under observation in five provinces (Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX), and Qinghai (QH)) of northwest China (NWC).
