Reconsidering the effects of urban form on PM concentrations: an urban shrinkage perspective.

The phenomenon of urban shrinkage is currently occurring worldwide; however, the "growth-oriented" planning paradigm is not suitable for these shrinking cities. Reconsidering the relationship between urban form and PM concentrations from the perspective of urban shrinkage can help provide a research reference for controlling air pollution and optimizing the spatial layout of shrinking cities. This study takes shrinking areas in China as the research subject, which are divided into four research groups according to their shrinkage degree. The empirical results indicate that the average PM concentrations decrease with the aggravation of urban shrinkage. In terms of the effect of urban form on PM concentrations, the urban size is always positively related to PM concentrations, while the impact of urban fragmentation on PM concentrations is negligible. Further, urban shape positively affects PM concentrations only in moderately and severely shrinking cities. Cities with sprawling urban forms have higher PM concentrations, except for those facing severe shrinking trends. This study suggests that governments in shrinking cities should reasonably adjust both the urban form and land use to improve air quality based on the degree of urban shrinkage.