Exploring the ozone pollution over the western Sichuan Basin, Southwest China: The impact of diurnal change in mountain-plains solenoid.

The Sichuan Basin (SCB), to the east of the Tibetan Plateau (TP), experiences severe ozone (O) pollution. Unfavorable atmospheric diffusion conditions are considered the main causes of heavy air pollution over the basin. However, the meteorological impact of thermally driven mountain-plains solenoid (MPS) between the TP and SCB on O pollution has not been reported. Here we show the MPS driving the diurnal O changes in the atmospheric boundary layer over the SCB based on surface and high-resolution vertical observations, ERA5 reanalysis data, and the WRF-Chem model. The MPS shifts between upslope and easterly flows along the eastern slope of the TP and SCB during the day and downslope westerly flows to the western SCB at night. The daytime MPS flows drive the westward transport of O-rich air mass in the atmospheric boundary layer from the polluted SCB and accumulate high O levels from the western edge of the SCB to the eastern slope of TP, subsequently aggravating O pollution in this region. After sunset, the MPS drainage flows carry air containing elevated O eastward downslope along the eastern slope of the TP into the nocturnal residual layer, enhancing the O concentrations aloft over the western SCB. The high-level O in the residual layer is transported downstream by nocturnal prevailing winds and contributes significantly to the next-day surface O buildup in the downwind region through daytime vertical mixing (~30 μg m h). The present study reveals a transport mechanism driven by the MPS with coupling diurnal changes in the atmospheric boundary layer, which redistributes O over the basin and exacerbates O pollution along the western edge of the basin. This study has important implications for understanding meteorological drivers on atmospheric environment underlying the complex terrain.