Abstract: In this paper,a coupled MM5-CAMx modeling system and meteorological observation data were used to analyze a PM₁₀ air pollution event in Beijing.The PSAT technique was applied to source apportion Beijing PM₁₀ contributions to source emission from different regions in North China.The result of modeling system indicated that PM₁₀ trans-boundary transportation especially the emission from Hebei and Tianjin was the trigger of this PM₁₀ pollution event in Beijing.Meteorological observation diagnose confirmed that this PM₁₀ pollution event in Beijing was influenced by regional meteorological factors,and concluded that the high PM₁₀ concentration process in this event could be divided into two phases.In phase 1,trans-boundary transportation by southwest wind which increased the PM₁₀ concentration in Beijing was the cause of this PM₁₀ pollution event.In phase 2,even pressure and weak low pressure field which caused the PM₁₀ convergence from Beijing local emission maintained this process of high PM₁₀ concentration.In addition,PM₁₀ trans-boundary transportation from Tianjin by southeast wind also promoted the high PM₁₀ concentration in phase 2.