The Relationship between Urban Expansion and Air Pollution Depends on Urban Spatial Morphology in the “2 + 26” Cities of Beijing-Tianjin-Hebei, China

Urban spatial morphology reflects the processes of external urban expansion and the internal spatial structure of the human economy, and exploring urban spatial morphology is important for regional management of PM2.5 pollutants in the air environment. Whether urban spatial morphology can influence air pollution is a question that has been explored and evaluated. Therefore, we constructed a spatial-temporal driving mechanism framework and explored the interactions among urban expansion and PM2.5 pollutants in the “2 + 26” cities of Beijing-Tianjin-Hebei (BTH). Based on 2010, 2015, and 2020 urban expansion factors and PM2.5 pollutants data from the “2 + 26” cities of BTH, the results of the Geo-Detector medal imply that: (1) The spatial distribution characteristics of areas with high PM2.5 pollutants in the “2 + 26” cities of BTH were affected by the PM2.5 concentration value, and the high-value phenomenon appeared to decrease with the improvement of PM2.5 pollutants. (2) The level of urbanization in the “2 + 26” cities of BTH had been increasing annually, with the most significant growth in construction land area located in the northeastern coastal cities. (3) The compactness index, was the most critical influencing urban expansion factor, explaining 0.63, 0.64, and 0.66 of PM2.5 pollutants in 2010, 2015, and 2020. (4) In factorial interactions, the compactness index and the centrality ratio explained spatial and temporal analysis characteristics of PM2.5 pollutants, and the interaction factor enhanced the explanation for the spatial-temporal characteristics. This study reveals the coupled spatial-temporal driving mechanism of air pollution in the “2 + 26” cities of BTH and also shows that reasonable urban planning has a positive impact on the regional ecological environment in the future.