Abstract: Based on the hourly PM_2.5 concentration observed at eighty environmental monitoring stations and conventional meteorological data,the spatial-temporal characteristics of PM_2.5 concentration during three typical severe pollution events over the Beijing-Tianjin-Hebei region in January of 2013 were analyzed.The effect of external pollution sources on the pollutants transport in the Beijing area was investigated by analyzing the characteristics of wind field during the period of rapidly increasing PM_2.5 concentration.The results indicate that there are three high-value centers of PM_2.5 concentration over the Beijing-Tianjin-Hebei region in January of 2013,and they are located at the Shijiazhuang-Baoding,Langfang,and Tangshan areas in Hebei province,respectively.The external pollution sources for the Beijing area mainly come from the middle and southern regions of Hebei province such as the Shijiazhuang-Baoding and Langfang areas.Pollutants can be transported from long distance by the southerly winds in the boundary layer,and the convergence line and inversion structure in the boundary layer strengthen the accumulation of pollutants in the Beijing area.Under the persistent stable weather condition,PM_2.5 pollutants are transported and accumulated in front of the Yanshan and Taihang mountains.This leads to PM_2.5 concentration in the Beijing area larger than that in the middle and southern regions of Hebei province,and that in the suburban areas higher than that in the urban areas of Beijing.