Abstract
Abstract. 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 PM2.5 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 PM2.5 over the SCB was characterized by a remarkable hollow sandwiched by high PM2.5 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 PM2.5 pollution. The basin PM2.5 was lifted into the free troposphere and transported outside of the SCB. At the bottom of the SCB, high PM2.5 concentrations were mostly located in the northwestern and southern regions. Due to the blocking effect of the plateau terrain on the northeasterly winds, PM2.5 gradually increased from northeast to southwest in the basin. In the lower free troposphere, the high PM2.5 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 PM2.5 concentrations in the SCB. The SCB emissions were the major source of PM2.5 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.
Highlights
Haze pollution has caused serious environmental problems, especially in the densely populated and economically developed regions in China, which have high levels of fine particulate matter (PM2.5) (Guo et al, 2014; P. Li et al, 2015; Gu and Yim, 2016; Lin et al, 2018)
The statistical metrics of comparisons between simulated and observed meteorological variables are given in Table 3, including the mean bias (MB), mean error (ME), and root mean squared error (RMSE)
The statistical verification of the simulated surface PM2.5 concentrations are shown in Table 4 with the normalized mean bias (NMB), normalized mean error (NME), mean fractional bias (MFB), and mean fractional error (MFE) in two levels of light PM2.5 pollution (75–150 μg m−3) and heavy PM2.5 pollution (>150 μg m−3)
Summary
Haze pollution has caused serious environmental problems, especially in the densely populated and economically developed regions in China, which have high levels of fine particulate matter (PM2.5) (particulate matter with an aerodynamic diameter equal to or less than 2.5 μm) (Guo et al, 2014; P. Li et al, 2015; Gu and Yim, 2016; Lin et al, 2018). With strong anthropogenic emissions and favorable meteorological conditions, four main regions with frequent heavy haze pollution have been identified, centered over the North China Plain (NCP) (Tao et al, 2012; Ye et al, 2016; Zhang et al, 2016; Huang et al, 2017), the Yangtze River Delta (YRD) in East China The unique terrain effect generates the asymmetries of meteorological and air pollutant distribution (Zhang et al, 2019), with a remarkable difference in PM2.5 concentrations between the eastern and western regions over the SCB (Chen and Xie, 2012; Ning et al, 2018b).
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