Impact of deep basin terrain on PM2.5 distribution and its seasonality over the Sichuan Basin, Southwest China

The terrain effect on atmospheric environment is poorly understood in particular for the polluted region with underlying complex topography. Therefore, this study targeted the Sichuan Basin (SCB), a deep basin with severe PM₂.₅ pollution enclosed by the eastern Tibetan Plateau (TP), Yunnan-Guizhou Plateaus (YGP) and mountains over Southwest China, and we investigated the terrain effect on seasonal PM₂.₅ distribution and the meteorological mechanism based on the WRF-Chem simulation with stuffing the basin topography. It is characterized that the three-dimensional distribution of topography-induced PM₂.₅ concentrations over the SCB with the seasonal shift of regional PM₂.₅ averages from approximately 30 μg m⁻³ in summer to 90 μg m⁻³ in winter at surface layer and from summertime 10 μg m⁻³ to wintertime 30 μg m⁻³ in the lower free troposphere. Such basin-forced PM₂.₅ changes presented the vertically monotonical declines concentrated within the lower troposphere below 3.6 km in spring, 2.3 km in summer, 2.6 km in autumn and 4.8 km in winter. Impacts of deep basin aggravated PM₂.₅ accumulation within the SCB and transport toward the surrounding plateaus contributing approximately 50–90% to PM₂.₅ levels over the regions of eastern TP and northern YGP. In the SCB, atmospheric thermal structure in the lower troposphere could build a vertical convergence layer between the boundary layer and free troposphere, acting as a lid inhibiting air diffusion, which was regulated by the terrain effects on interactions of westerlies and Asian monsoons, especially the wintertime strong warm lid deteriorating air pollution in the SCB. Furthermore, warm and humid air conditions within the basin prompted sulfur oxidation ratio by +0.02 and nitrogen oxidation ratio by +0.22 effectively producing the secondary PM₂.₅ in atmospheric environment.