**1. Introduction**

With the advancement of industrialization and urbanization, many cities around the world are experiencing severe air pollution, especially particulate matter pollution. On a global scale, China, India, and South Asia have the most severe particulate matter pollution in the world [1]. In China, since 2011, Beijing–Tianjin–Hebei [2], the Yangtze River Delta [3], and other regions have experienced frequent smog, and air pollution has caused widespread concern. High concentrations of PM2.5 can, not only accelerate the formation of haze but also significantly affect people's health [4]. It has been proved that long-term exposure to high PM levels can easily cause a variety of diseases [5] and increase

**Citation:** Wang, J.; Li, R.; Xue, K.; Fang, C. Analysis of Spatio-Temporal Heterogeneity and Socioeconomic driving Factors of PM2.5 in Beijing–Tianjin–Hebei and Its Surrounding Areas. *Atmosphere* **2021**, *12*, 1324. https://doi.org/10.3390/ atmos12101324

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 15 September 2021 Accepted: 8 October 2021 Published: 10 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

the risk of death [6]. In 2017, the State Ministry of Environmental Protection issued the "Beijing–Tianjin–Hebei and Surrounding Area Air Pollution Prevention and Control Work Plan in 2017", which first proposed the concept of "2 + 26 cities" and implemented a large number of pollution control measures in these cities to alleviate air pollution in North China. Therefore, strengthening scientific understanding of the regulations of regional air particulate pollution will help to formulate urbanization policies and ensure that targeted air pollution control measures are properly implemented.

At present, research on PM2.5 pollutions mainly focuses on temporal and spatial distribution rules [7], influencing factor analyses [8], source analyses [9], and health risk assessments [10] along with other aspects. Among them, influencing factors mainly include meteorological factors and socio-economic factors. Meteorological factors affect PM2.5 concentration by changing its diffusion and chemical reaction conditions. Chen et al. [11] summarized the methods to quantify the impact of meteorological factors on PM2.5 and comprehensively reviewed their impact mechanisms. Xu et al. [12] conducted a study on the temporal and spatial distributions of the influence of meteorological conditions on PM2.5 concentration in China from 2000 to 2017, which showed an overall downward trend in PM2.5 concentration, and the influence of meteorology varied greatly between different provinces. The socio-economic factors that directly or indirectly affect PM2.5 concentration in the process of urbanization and economic development, include the national economy, industrial structure, population density, transportation, and other factors [13]. These factors mainly represent the impact of human activities on PM2.5. The average urban PM2.5 level is mainly affected by anthropogenic emissions of local air pollutants and the surrounding ecological level. Cheng et al. [14] used a dynamic spatial panel model to analyze the impact of foreign direct investment (FDI) on China's PM2.5 pollutions, and the results showed that FDI significantly aggravated China's urban PM2.5 pollutions. The study of Yan et al. [15] expressed that there was a heterogeneous relationship between PM2.5 concentration and economic growth, urbanization, industrialization, and FDI and that population density had the greatest positive impact on PM2.5 pollution. Zhang et al. [16] noted that PM2.5 pollution was positively correlated with urbanization and road density, and negatively correlated with the proportion of tertiary industries.

Although many studies have been conducted on the temporal and spatial distributions of PM2.5 and its influencing factors, the study areas of most studies mainly concentrate on the level of countries, urban agglomerations, and cities, while comparisons between regions are relatively rare. In addition, with rapid economic development, the North China region has been experiencing severe PM2.5 pollution. Shanxi Province is located in the central region and has a decreasing economic development. Therefore, this study selects Beijing City, Tianjin City, Hebei Province, Henan Province, and Shanxi Province as the study areas. There are significant distinctions of PM2.5 and economic development levels between the different cities, which provides advantages for studying the impact of socio-economic factors and spatial spillover effects on the PM2.5 level. The aims of this study are: (1) explore the temporal and spatial distribution characteristics of PM2.5 levels; (2) compare the spatial heterogeneity of PM2.5 distribution characteristics in different regions, and (3) determine the influence of socio-economic factors and spatial spillover effects on PM2.5 levels.
