The Nexus Between Urbanization and Precipitation Chemistry in the Pearl River Delta, China: Decoupling Analysis

Rapid urbanization exacerbates atmospheric pollutant emissions, markedly altering precipitation chemistry. However, a comprehensive understanding of the equilibrium between multicomponent precipitation chemistry and urbanization remains unclear. This study characterizes the spatiotemporal urbanization dynamics of the Pearl River Delta (PRD) urban agglomeration from 2000 to 2020 by integrating multi-source datasets, including nighttime light (NTL) observations, Landsat-derived land cover, and gridded population products. The interplay between precipitation chemistry and urbanization was evaluated through a multidimensional equilibrium assessment framework integrating spatial heterogeneity, trade-off and synergy analysis, and decoupling diagnosis. The results indicated that interannual variation in precipitation ion concentrations initially increased and then decreased from 2000 to 2020. Spatially, major ion concentrations (SO42−, NO3−, Ca2+, and NH4+) generally declined across the region after 2009, with the most pronounced declines in peripheral PRD areas. Conversely, urbanization intensified, spreading from the central core to the periphery. NTL intensity exhibited the strongest explanatory power for major ion concentrations, suggesting that economic urbanization is more closely associated with spatial variability in precipitation chemistry. Over time, the relationship between precipitation ion concentrations and urbanization shifted from synergy to trade-off. After 2005, the dominant decoupling status of major ion concentrations relative to urbanization shifted from expansive negative decoupling (up to 62.93%) to strong decoupling (up to 75.86%). This shift indicates that the antagonism between urbanization and precipitation pollution has gradually transitioned toward equilibrium. However, this equilibrium exhibits spatiotemporal non-stationarity. These findings highlight the complex nonlinear interactions between urbanization and precipitation chemistry. Identifying spatial heterogeneity in decoupling between urbanization and environmental pollution facilitates the formulation of differentiated zoning management strategies.