Spatiotemporal Dynamics and Multiscale Driving Mechanisms of COD and Ammonia Nitrogen Emissions in China

This study provides a systematic examination of the spatiotemporal evolution of chemical oxygen demand (COD) and ammonia nitrogen (NH₃-N) emissions in China from 2004 to 2022, together with their major socioeconomic and industrial driving factors. Using the Spatial Durbin Model (SDM), the study evaluates both the direct impacts and spillover effects of economic variables, including GDP per capita, urbanization rate, and secondary industry output, on pollutant emissions. The results reveal a “rise-then-fall” trajectory of COD emissions, which reflects the combined influence of national environmental policies, such as the 13^th Five-Year Plan for Ecological Environmental Protection and the Urban Black and Odorous Water Remediation Program. Although these interventions and technological upgrades effectively reduced emissions, early-stage economic expansion initially intensified pollution. After 2019, however, COD emissions began to increase again, indicating that once GDP per capita reaches a certain level, the relationship between economic growth and pollution becomes positively correlated. This reversal is likely associated with accelerated industrialization and urbanization, which may have outpaced the capacity of existing pollution control systems. Spatially, COD and NH₃-N emissions exhibit strong clustering patterns, with the eastern coastal provinces forming pronounced high agglomeration zones. Meanwhile, the central and western regions have experienced growing emission pressure due to ongoing industrialization. Among the key drivers, secondary industry output plays a dominant role in shaping emission levels, while urbanization significantly exacerbates pollutant discharge. Overall, the findings provide empirical support for the formulation of region-specific and adaptive pollution control policies. This research contributes to a more nuanced understanding of the multiscale mechanisms driving water pollution in China and offers scientific guidance for harmonizing economic development and environmental sustainability, particularly in regions experiencing rapid industrialization.