Elevated 3D structures of PM 2.5 and impact of complex terrain-forcing circulations on heavy haze pollution over Sichuan Basin, China

Shu, Zhuozhi; Liu, Yubao; Zhao, Tianliang; Xia, Junrong; Wang, Chenggang; Cao, Le; Wang, Haoliang; Zhang, Lei; Zheng, Yu; Shen, Lijuan; Luo, Lei; Li, Yueqing

Deep basins create uniquely favorable conditions for causing air pollution, and the Sichuan Basin (SCB) in Southwest China is such a basin featuring frequent heavy pollution. A wintertime heavy haze pollution event in the SCB was studied with conventional and intensive observation data and the WRF-Chem model to explore the 3D distribution of PMinline-formula2.5 to understand the impact of regional pollutant emissions, basin circulations associated with plateaus, and downwind transport to the adjacent areas. It was found that the vertical structure of PMinline-formula2.5 over the SCB was characterized by a remarkable hollow sandwiched by high PMinline-formula2.5 layers at heights of 1.5–3 km and a highly polluted near-surface layer. The southwesterlies over the Tibetan Plateau (TP) and Yunnan-Guizhou Plateau (YGP) resulted in a lee vortex over the SCB, which helped form and maintain heavy PMinline-formula2.5 pollution. The basin PMinline-formula2.5 was lifted into the free troposphere and transported outside of the SCB. At the bottom of the SCB, high PMinline-formula2.5 concentrations were mostly located in the northwestern and southern regions. Due to the blocking effect of the plateau terrain on the northeasterly winds, PMinline-formula2.5 gradually increased from northeast to southwest in the basin. In the lower free troposphere, the high PMinline-formula2.5 centers were distributed over the northwestern and southwestern SCB areas, as well as the central SCB region. For this event, the regional emissions from the SCB contributed 75.4 %–94.6 % to the surface PMinline-formula2.5 concentrations in the SCB. The SCB emissions were the major source of PMinline-formula2.5 over the eastern regions of the TP and the northern regions of the YGP, with contribution rates of 72.7 % and 70.5 %, respectively, during the dissipation stage of heavy air pollution over the SCB, which was regarded as the major pollutant source affecting atmospheric environment changes in Southwest China.

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Shu, Zhuozhi / Liu, Yubao / Zhao, Tianliang / et al: Elevated 3D structures of PM2.5 and impact of complex terrain-forcing circulations on heavy haze pollution over Sichuan Basin, China. 2021. Copernicus Publications.

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