Observed And Simulated Research On Climate Characteristic Of Air Quality And The Topographic Induced Effects In Sichuan Basin

The spatial and temporal variations of atmospheric visibility and hazy episodes and its driving factors in the Sichuan Basin over recent 5 decades were analyzed by using the observed meteorological data in the Sichuan basin as well as the ambient areas. Utilizing the FNL (Final Operational Global analysis) data between 1999 and 2013, the variations in atmospheric structure and boundary layer over the Sichuan Basin were studied. Finally, based on the WRF-Chem Model, the severe haze episodes happened during 12-20 January,2014 were simulated in two simulation experiments with and without the Sichuan basin, to investigate the changes of air pollutants driven by the topographic forcing effects, and quantitative evaluate the contribution of topography in haze days and near surface layers. The results are summarized as follows:(1) Over recent 50 years, visibility in the Sichuan Basin, has exhibited significantly reduction with the trend rate of about 0.91km.(10a)-1. Dry atmospheric extinction coefficient tended to increase. Increasing of population density and core-fired leaded to more emissions of fine particulate. Thus haze days in the basin increased seriously, with the trend rate at about 9.6 d.(10a)-1. Haze pollution is most serious in January, and relative clean in July compared with other months. High humidity and weak wind speed are the key meteorological factors causing the haze pollution.(2) Terrain effect on the atmospheric environment in the basin mainly exhibits in:1) An obvious seasonal change of atmospheric vertical structure, with sinking airflow over the basin and the strong updrafts near ground; 2) Due to the effect of plateau leeward slope, weak wind speed with zonal deviation of about 1m.s-1; 3) Wind vertical shear and weak vertical mixing from 900hPa to 850hPa; 4) The inversion layers near basin surface and the inversion layer off the ground induced by subsidence warming, making the stable boundary layer. Those effects are all not conducive to transport and diffuse of atmospheric aerosol in the basin, causing the accumulation of pollutants for haze pollutions.(3) The modeling of heavy haze pollution in the Sichuan Basin during January 12～20,2014 and its sensitivity test showed that the terrain effect could reduce near surface wind speed by about 1.5 m.s-1, increase the 2m temperature by 10℃ and lower the boundary layer height by about 100-150 m. The effect of Sichuan basin could increase 150ug.m-3 in near surface PM2.5 concentrations with the contribution rate of about 70%. The terrain effect on the air pollutant accumulation is more apparent in haze episodes. Topographic effects are more significant in the layers of 2500 m～3000 m with the concentration ratio reaching a maximum of about 85.3% at 2700 m. A temperature inversion layer off the ground is built by air subsidence warming induced by subsidence of westerlies in the leeward slope of Tibetan Plateau, and the surface ground inversion structure is reinforced by air sinks near the ground, strengthening the stable atmosphere, which is the key factor for this heavy air pollution episode in Sichuan basin.