Quantifying the Role of Urban Form in the Coupling Coordination of PM2.5 and Carbon Emissions: Evidence from 336 Cities in China

Urban form profoundly influences the effectiveness of coordinated management of carbon emissions and air pollution, which is of great significance for sustainable urban development. This study took 336 Chinese cities as research samples and used carbon emissions, PM2.5, and nighttime light datasets from 2000 to 2020 to investigate the contribution of urban form to the degree of coordination between urban carbon emissions and PM2.5. We constructed a multi-dimensional indicator system for urban form and adopted the coupling coordination degree (CCD) model to quantify the coordinated development of these two variables. A novel analytical framework was established using the stacking ensemble learning model. Combined with the SHAP method, this framework provided nonlinear associations of various urban morphological factors with the CCD. The results are as follows: (1) The stacking ensemble model performs best in capturing high-dimensional nonlinear relationships, with test set R2, RMSE, and RPD values of 0.75, 0.082, and 1.98, respectively. (2) Urban form exhibits nonlinear threshold effects on the carbon-pollution coordination degree. The top predictors contributing to the model output are the class area (CA), the landscape shape index (LSI), the percentage of like adjacencies (PLADJ), and the number of patches (NP). Among them, CA exhibits a monotonically increasing asymptotic saturation response, while the LSI presents an inverted U-shaped response. (3) The contribution of urban form to the CCD exhibits significant spatial heterogeneity. The developed regions of eastern China constitute the high-sensitivity zone of the CCD, and are mainly constrained by scale expansion, land fragmentation, and excessive agglomeration, while the central and western regions show the low-sensitivity zone of the CCD. These results offer a scientific reference for designing pollution control and carbon mitigation strategies.