Formation and Dissipation Mechanisms of a Heavy Pollution Event in east-central China under the Combined Influence of Topography and Cold Fronts

Topography and cold frontal passages are key factors influencing the formation and dissipation of air pollution in east-central China. Elucidating their synergistic mechanisms is of great significance for regional air pollution prevention and control. Using multi-source data, this study analyzed the evolutionary characteristics of a regional PM₂.₅ heavy pollution event during December 16–21, 2025, in east-central China, and revealed the physical mechanisms by which the coupling of post-cold-frontal subsidence and topographic forcing drove the formation and intensification of this heavy pollution event. The results indicate that this event was characterized by a wide effect range and long duration, with heavy pollution occurring in all cities of Anhui Province and its surrounding provinces, displaying a spatial distribution of heavier pollution in the west and lighter pollution in the east. The cumulative heavy pollution duration in central and western Anhui cities generally exceeded 30 hours, with a maximum of 44 hours. Most cities experienced two explosive increases in PM₂.₅ concentrations. The formation and dissipation mechanisms can be attributed to three factors: (1) Regional transport provided the background for pollution accumulation; (2) Large-scale subsidence following two cold frontal passages directly drove the two explosive increases in surface PM₂.₅ concentrations; (3) Topographic effects enhanced the convergence and accumulation of particles on the windward side of mountains, and by altering the spatial distribution of vertical motions, aggravated pollution on the northern and western sides of the Dabie Mountains, the mountainous areas of southern Anhui, and other hilly terrains.

The findings of this study contribute to improving the forecasting and early warning capabilities for heavy pollution events in east-central China. Operationally, forecasters should pay close attention to the variations in upstream PM₂.₅ concentrations and the evolution of the continental high-pressure system, which governs shifts in wind direction and pollutant transport pathways. Notably, heavy pollution episodes tend to occur in the post-cold-frontal phase, under the control of the continental high pressure.