How does the urban form-PM2.5 concentration relationship change seasonally in Chinese cities? A comparative analysis between national and urban agglomeration scales

Understanding how urban forms affect fine particulate (PM2.5) concentrations within different seasons is important for sustainable urban development. Thus, the purpose of this study was to quantify the relationships between urban forms and PM2.5 concentration change seasonally in 279 Chinese cities, with the explicit consideration of a comparative analysis between national and urban agglomeration scales. A comprehensive evaluation index system of urban forms was calculated based on six urban form metrics (total urban area (CA), number of patches (NP), landscape shape index (LSI), percentage of like adjacencies (PLADJ), patch cohesion index (COHESION), and aggregation index (AI)) by integrating three control variables (temperature, NDVI, and nighttime light). The spatial regression model was subsequently adopted to quantify the effects of urban forms on PM2.5 concentrations. The results revealed that, during the summer and autumn, only urban form compactness was significantly correlated with the PM2.5 concentrations, but more urban form metrics were significantly associated with the PM2.5 concentrations during the spring and winter in Chinese cities. The scattered urban form could effectively reduce the PM2.5 concentrations in the Yangtze River Delta, Pearl River Delta, and Chengdu-Chongqing but not in the Beijing-Tianjin-Hebei during the winter. The effects of urban forms on PM2.5 concentrations become increasingly noticeable from the national scale to the urban agglomeration scale with seasonal change. This study suggests that the urban form-PM2.5 concentration relationships are sensitive to seasonal variations across different regions. Thus, it is an important approach to solve PM2.5 pollution problem to construct an ideal urban form through flexible urban planning strategies.