Abstract:A serious haze process from 10th to 16th in January 2013 of Beijing was analyzed. During the period of haze, aerosol vertical distribution was retrieved from the MPL-4B-IDS series lidar system with Fernald algorithm. In order to illustrate the formation mechanism of this pollution episode, the surface aerosol measurement at the same site was compared with the lidar result. It was found that during the observation period, haze occurred in 89.4% of the time, among which 39.8% were recognized as heavy haze episodes. The extinction coefficients strongly correlated with surface ρ (PM2.5 ) (r=0.95). During the study, the average PBL height was only 293m, when 91% of the time it was less than 500m and restrained the effective diffusion of pollutants significantly. Extinction coefficient of each vertical height reached more than 1.5km-1above the ground. By comparing the analysis of the aerosol depolarization ratio, it was found that the atmospheric extinction above the city was caused by the combined effect of particulate aerosols and atmospheric moisture. The aerosol optical depth (AOD) at 532nm in this severe haze event was high and affected by the relative humidity greatly. In lower RH, the value of AOD was contributed by particulate aerosols, and atmospheric moisture made a significant contribution due to the hygroscopicity of fine particle when the RH was high. Additionally, stability of the PBL situation and pollutants emitted from regional area were the main reasons of this severe haze event, and high humidity made the haze more worse.