Abstract
In addition to weather conditions and pollutant emissions, the degree to which topography influences the occurrence and development of haze pollution in downtown Beijing and the mechanisms that may be involved remain open questions. A series of atmospheric chemistry simulations are executed by using the online-coupled Weather Research and Forecasting with Chemistry (WRF-Chem) model for November-December 2015 with different hypothetical topographic height scenarios. The simulation results show that topography exerts an important influence on haze pollution in downtown Beijing, particularly the typical development of haze pollution. A possible mechanism that underlies the response of haze pollution to topography is that the mountains that surround Beijing tend to produce anomalous southerly winds, high relative humidity, low boundary layer heights, and sinking motion over most of Beijing. These conditions favor the formation and development of haze pollution in downtown Beijing. Furthermore, the reduction percentage in PM2.5 concentrations due to reduced terrain height in the southerly wind (S) mode is almost three times larger than that in the northerly wind (N) mode. In the context of the regional topography, the simple S and N modes represent useful indicators for haze prediction in Beijing to some extent, especially over medium to long time scales.
Highlights
Beijing is located in the northern tip of the North China Plain and surrounded by mountains along the west, north and northeast
A previous study has suggested that the “harbor” effect on the westerlies in the eastern lee of the Tibetan Plateau’s large topography may be an important factor that influences the regional distribution of haze frequency in eastern China[46]
The results show that increases in terrain height may lead to a sinking vertical vortex in the lower troposphere on the eastern lee of the regional topography, which promotes the development of sinking motion over downtown Beijing (Fig. 7)
Summary
Beijing is located in the northern tip of the North China Plain and surrounded by mountains along the west, north and northeast. Beijing and eastern China have suffered from severe haze or smog days frequently in recent years. These events are characterized by high particle mass concentrations and low visibility. Severe haze pollution, exemplified by the persistent haze days in January 2013, represent a substantial threat to human health and traffic safety These phenomena have stimulated great interest in studying the haze pollution in Beijing or eastern China as a whole. A previous study has suggested that the “harbor” effect on the westerlies in the eastern lee of the Tibetan Plateau’s large topography may be an important factor that influences the regional distribution of haze frequency in eastern China. This study aims to investigate the effects of topography on the occurrence and development of haze pollution using numerical sensitivity experiments
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