Abstract:  A heavy haze event occurred in the plain area of Beijing from January 10 to 14, 2013. During this period, the visibility was always less than 2 km; air pollution reached and exceeded severe level; the primary pollutant is PM_2.5. The process was synthetically analyzed based on the synoptic situation, the conventional and intensive observational data from weather stations and the observational PM_2.5 concentration from the urban and rural station. The results show that there exists relatively straight zonal circulation with west-northwest air current control and without obviously southward cold air in upper level over Beijing during this haze process. It is weak pressure system on the ground which is not favorable to diffusion and transportation of pollutants. The atmospheric stratification is stable; wind speed is low (the daily mean wind speed is less than 2 m·s^-1); relative humidity (RH) is high (the mean daily RH is more than 70%); the frequency of temperature inversion is high and its intensity is strong. Thus, the ability of vertical and horizontal diffusion of air pollutants within the boundary layer is very weak. Because of the heavy emission intensity, the high humidity and stable weather background in Beijing-Tianjin-Hebei region, the concentration of pollutants are influenced by the complex terrain and local circulation. Combined actions of local pollutants emission and regional transportation as well as the physical or chemical reactions of fine particle PM _2.5 make the pollutants concentration increasing sharply and maintaining with a high concentration in urban and plain areas of Beijing. The high concentration of PM _2.5 has a significant impact on light scattering extinction, which results in low visibility and severe haze weather.

Key words: Haze, Serious pollution, Meteorological condition, Pollutant transportation