Simulation of ozone pollution events in Sichuan Basin under the alternating effects of continental high pressure and subtropical high pressure circulation

The Sichuan Basin (SCB), located in Southwest China, often has serious ozone (O₃) pollution due to its special topography, complex meteorological conditions and emissions. The frequent occurrence of extreme high temperature has recently aggravated the biogenic O₃ precursors emission and the O₃ pollution in the SCB. To quantify the O₃ pollution caused by high temperature, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used to study the formation mechanism and transport process of the O₃ pollution event in the SCB in July 2022, after updating emission information with TROPOMI satellite data and MEGAN model initial data. The results show that during the pollution period, the SCB was affected by the alternating effects of the circulation changes caused by the eastward movement of the northern continental high pressure and the westward movement of the western Pacific subtropical high pressure. The continuous control of the high pressure formed extremely high temperature, which led to the accelerated generation of O₃ in the SCB. The southeastern continental high pressure and the northwestern subtropical high pressure converged in the SCB, forming a “zigzag”-shaped circulation, which prevented O₃ from being transported out of the SCB. The results of integrated process rate analysis (IPR) show that the negative contribution of horizontal advection accelerates the O₃ output from the cities in the middle of the SCB to the surrounding cities, resulting in the rapid spread of pollution over the SCB, the O₃ accumulation in the high altitude of the basin, and the positive contribution of chemical generation in the south of the SCB.