*2.2. Data Collection*

To meet the objectives of the study, we analyzed the ambient air quality in 53 cities located in five provinces (SN, XJ, GS, NX, and QH) of NWC (Table 1) for a period of four years (2015–2018). The hourly concentration of PM2.5, PM10, SO2, NO2, CO, and O3 was collected from China National Environmental Monitoring Center (CNEMC). The online data-sharing platform covers 367 cities of China, and dispatch/publish information according to the Technical Guideline on Environmental Monitoring Quality Management (HJ 630-2011) [39].
