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A severe haze episode occurred in winter in the North China Plain (NCP),and the phenomenon of an explosive growth and sharp decline in PM2.5 (particulate matter with an aerodynamic diameter equal to or less than 2.5 μm) concentration was observed.To study the systematic causes for this phenomenon,comprehensive observations were conducted in Beijing from November 26 to December 2,2015;during this period,meteorological parameters,LIDAR data,and the chemical compositions of aerosols were determined.The haze episode was characterized by rapidly varying PM2.5 concentration,and the highest PM2.5 concentration reached 667 μg/m3.During the haze episode,the NCP was dominated by a weak high-pressure system and continuously low PBL (planetary boundary layer)heights,which are unfavorable conditions for the diffusion of pollutants.The large increases in the concentrations of SNA (SO42-,NO3-and NH4+) during the haze implied that the formation of SNA was the largest contribution.Water vapor also played a vital role in the formation of haze by promoting the chemical transformation of secondary pollutants,which led to higher PM2.5 concentrations.The spatial distributions of PM2.5 in Beijing at different times and the backward trajectories of the air masses also indicated that pollutants from surrounding provinces in particular,contributed to the higher PM2.5 concentration.