Nonlinear impact and spatial spillover effect of new urbanization on PM2.5 from a multi-dimensional perspective

Under accelerating urbanization, urban air pollution has emerged as a critical research area. Fine particulate matter (PM2.5) poses significant health risks, yet research exploring the nonlinear impact of multi-dimensional urbanization (M-URB) on PM2.5 and its spatial spillover effect remains insufficient. Considering China's emphasis on new urbanization and ecological civilization construction, both of which are defined by sustainable development attributes, research on the effects of urbanization on PM2.5 becomes imperative. Therefore, we performed exploratory spatial data analysis (ESDA) and standard ellipse deviation (SED) analysis to investigate the dynamic spatiotemporal characteristics of PM2.5 in 285 cities in China from 2006 to 2020. In addition, we employed a spatial econometric model to examine the nonlinear impact and spatial spillover effect of M-URB on PM2.5. Our analysis revealed that between 2006 and 2020, China experienced a fluctuating and declining trend in PM2.5 levels, with a regional distribution characterized by east-west polarization. The PM2.5 showed a pronounced clustering characteristic, with its centroid demonstrating a discernible "south to north" movement pattern. In addition, M-URB displayed varying degrees of nonlinearity along with geographical spillover effects on PM2.5. Population urbanization (P-URB), social urbanization (S-URB), and ecological urbanization (e-URB) initially facilitated and later mitigated the presence of PM2.5, demonstrating inverted N-shaped patterns. The processes of economic urbanization (E-URB) and land urbanization (L-URB) resulted in a rise in PM2.5 levels, and the correlation between these factors has yet to achieve a point of improvement in air quality. Hence, the spatiotemporal variation in PM2.5 in China is attributable to the convergence and interplay of several elements, including socioeconomic factors and governmental policies. The findings of this study provide a valuable reference point for the implementation of strategies aimed at mitigating PM2.5 pollution and enhancing air quality.