*2.2. PM2.5 Data of Chinese Cities*

In recent years, with the acceleration of the Chinese urbanization process, PM2.5 has become one of the most important factors affecting the air quality and has caused frequent air pollution events in Chinese cities [49]. In recent years, hazy weather caused by multiple pollutants, especially represented by PM2.5 as the main pollutants, has affected large areas of China, lasting for a long time [50]. With the rapid economic development, China is suffering from serious air pollution, where PM2.5 has gradually become the primary pollutant, which has attracted widespread social concern [51,52]. The existing studies also show that PM2.5 is an important factor affecting China's population mobility. In China, with the development of society and the improvement of living conditions, people's demands on the living environment are gradually increasing.

Furthermore, interregional migration in China is no longer only determined by the levels of regional economic development and social employment. Having a favorable environment in any given region also significantly influences the location decisions of Chinese migrants, which can provide sustainable human capital for economic development and increase the external benefits of a favorable environment in the region [53]. Therefore, population mobility not only depends on the quality of the economic conditions but also on the living environment, which has become an increasingly important indicator for people to consider. In particular, air quality is gradually becoming an important indicator for people to judge the quality of a living environment [54].

The PM2.5 concentration can be calculated using satellite remote sensing data, which exhibits higher accuracy than air quality data from other sources. Therefore, this study used the annual average concentration of PM2.5 (µg/m<sup>3</sup> ) to measure the air quality in Chinese cities. Due to the inaccessibility of the 2017 PM2.5 dataset of Chinese cities from official channels, this study used the satellite-based grid data on the global PM2.5 concentrations released by the Atmospheric Composition Analysis Group at Dalhousie University and used ArcGIS 10.2 (Esri, Redlands, CA, USA) to calculate the annual data of the average PM2.5 concentrations of Chinese cities in the prefecture-level cities or above in 2017. The results showed that the average annual PM2.5 concentration in 2017 in the sample of Chinese cities was 44.80 µg/m<sup>3</sup> . Among the results, the highest annual average concentration of PM2.5 was 80.66 µg/m<sup>3</sup> in Hengshui city of Hebei province and the lowest annual average concentration of PM2.5 was 10.02 µg/m<sup>3</sup> in Hulunbuir city of Inner Mongolia.

The spatial distribution of the PM2.5 concentration in each Chinese city in 2017 in the sample exhibited significant differences. A city's PM2.5 concentration was highly correlated to its economic development level, industrial structure, and natural environment [55]. For example, almost all the cities in the developed provinces of China, including Shandong, Jiangsu, Zhejiang, Shanghai, and Beijing, had higher PM2.5 concentrations. Simultaneously, cities in provinces with higher ratios of secondary industries, including Hebei and Tianjin, also had higher PM2.5 concentrations. However, because of the humid and rainy climate, the PM2.5 concentrations in the cities of southern China were not so high.
