Impacts of urban land morphology on PM2.5 concentration in the urban agglomerations of China

Accurate understanding of the relationship between urban land morphology and the concentration of PM2.5 is essential for achieving high-quality development of urban agglomerations. Based on a mechanism framework of "Internal-External driving force", 19 Chinese urban agglomerations at different development levels were analysed using the geographically weighted regression model to evaluate the impacts of urban land morphology on PM2.5 concentrations in years 2000-2017. The results show: (1) The PM2.5 average concentrations of all 19 urban agglomerations continue to increase from 30 mu g/m(3) in 2000 to 52 mu g/m(3) in 2007 but decreased to 34 mu g/m(3) in 2017. The changes in PM2.5 concentrations vary for urban agglomerations at different development levels. Spatial differences in PM2.5 concentrations are significant, forming a pattern that decreases from the centre to the periphery regions; (2) The urban land morphology of the entire urban agglomeration areas has undergone significant changes. The fractal dimension index (from 4.150 to 2.731) and the compactness (from 0.647 to 0.635) showed a downward trend, while the shape indices (from 1.421 to 1.606) demonstrated an increasing trend. National-level urban agglomerations are more compact and more complex in shape, while more fragmented are regional and local urban agglomerations; (3) Different parameters of urban land morphology have varying effects on PM2.5 concentration varies and at different development levels of urban agglomerations. The combination of urban land morphology, socio-economic factors, and natural elements has a complex effect on PM2.5 concentrations. It can contribute to understanding the linkage between urban land morphology and PM2.5, providing references for future studies.