Effects of Plateau–Mountain and basin topography on winter meteorology and PM2.5 pollution in the Sichuan basin

The Sichuan Basin (SCB) frequently suffers from large-scale and persistent PM 2.5 pollution during autumn and winter due to its unique topography. However, the combined effects of the large-scale plateau topography and the major mountains surrounding the basin (PM) and the basin’s enclosed terrain on regional air quality, particularly their underlying mechanisms, have yet to be fully elucidated. Building upon previous work that addressed the Tibetan Plateau (TP) and basin topography separately, this study conducted topographic sensitivity simulations using an atmospheric chemistry–meteorology coupled modeling system to isolate and quantify the effects of these two topographic features on meteorological conditions and PM 2.5 concentrations. The results show that both topographies worsen regional PM 2.5 pollution by modulating meteorological conditions, with the enclosed basin topography exerting an effect 1.4 times as strong as that of the PM terrain. Specifically, the PM terrain weakens winds at 1–2.5 km, making them lower than those below 1 km and thereby cutting off the vertical mixing pathway; it also reduces near-surface wind speed by 0.3 m s −1 and lowers the boundary layer height by 103 m, suppressing atmospheric diffusion conditions and causing PM 2.5 increases of approximately 40–50 μg m −3 in the western, southern, and eastern parts of the SCB. The enclosed basin topography reduces wind speed by 0.7 m s −1 and boundary layer height by 183 m, and increases specific humidity by 1.1 g kg −1 , leading to PM 2.5 increments of 50–70 μg m −3 across most regions from the central to southern SCB. Furthermore, the meteorological signature of the PM terrain peaked in the afternoon, while that of the basin terrain remained relatively uniform; however, the two topographies exhibited consistent regulation of PM 2.5 . These findings provide a quantitative mechanistic understanding of topography-driven pollution in the SCB and offer a theoretical basis for developing targeted regional air quality management strategies.