A modelling study of the terrain effects on haze pollution in the Sichuan Basin

The Sichuan Basin (SB), covering 260,000 km(2) immediately to the east of the Tibetan Plateau (TP) with a large drop exceeding 3000 m in elevation over a short horizontal distance, is a region of high haze pollution in China. The terrain effects of this unique deep basin on haze pollution have not been well investigated. During 14-19 January 2014, heavy haze pollution engulfed the SB with high PM2.5 concentrations. By using the WRF-Chem model, the topographic effects on haze pollution in the SB and the underlying mechanisms were investigated by two sets of simulation with and without the basin topography. The simulation results show that the SB topography lead to an increase of 48 mu g m(-3) in the basin-averaged surface PM2.5 contributing about 44% to the PM2.5 concentrations during heavy haze pollution, indicating an important role of the basin topography in worsening air pollution. The surface PM2.5 level enhancements varied spatially from 0 to 30 mu g m(-3) in the eastern basin to 60-120 mu g m(-3) in the western basin, respectively corresponding to 0-20% and 50-70% relative to the concentrations simulated without the SB topographic condition, reflecting the stronger topographic effect of the TP in the western SB. The topographic effects intensify the haze pollution via reducing wind speed, raising air temperature and humidity in the lower troposphere, as well as dropping boundary layer height in the basin. Resulted from the impact of TP on mid-latitude westerly winds, a typical leeside vortex of immediately upstream TP over the basin was developed with accentuating PM2.5 accumulations in the basin, implying a strong influence of TP on air quality in the SB.