In this paper,the WRF-Chem model was used to analyze the temporal and spatial variations of the near-surface concentrations of ozone(O₃),nitrogen dioxide(NO₂) and fine particulate matter(PM_2.5) over the Beijing-Tianjin-Hebei region in August 2007.The simulation results were compared with surface observations.It was indicated that the model could reproduce the temporal and spatial variations of O₃ and PM_2.5 reasonably well and successfully reflect several pollution events in August 2007.However,the model's performance was relatively poor for NO₂.The correlation coefficients between the model simulations and the observations were 0.69—0.86 for O₃,0.44—0.49 for PM_2.5 and 0.27—0.43 for NO₂.In August,the monthly mean concentration of O₃ in Beijing and Tianjin was relatively low at about 30×10^-9 whereas relatively high O₃ concentration (about 60×10^-9) was predicted in the west of the Beijing-Tianjin-Hebei region and over the Bohai Bay.The PM_2.5 showed higher concentration levels in the south than in the north within the Beijing-Tianjin-Hebei region with the monthly mean concentrations ranging from 120 to 240 μg/m³.The monthly mean O₃ concentration at 14:00 BST in August was about 60×10^-9 in Beijing and Tianjin,which was lower than those in the surrounding areas.The mean PM_2.5 concentrations at 14:00 BST showed higher levels(100—120 μg/m³) in the southern parts of the Hebei province and the Bohai Bay areas.An O₃ pollution episode occurred on August 17 when the air temperature in Beijing reached 33 ℃ at 14:00 BST,resulting in high concentration of O₃(80×10^-9—110×10^-9).Meanwhile,due to the combined effects of local emission,chemical reaction and trans-boundary transport,the PM_2.5 concentration in the west and the north of the Bohai bay was as high as over 120 μg/m³,in which the secondary aerosols formed through chemical reactions,the primary anthropogenic aerosols released directly and sea salt aerosols accounted for about 50—100 μg/m³,10—20 μg/m³ and 1—7 μg/m³,respectively.Thus,the secondary aerosols were the main contributor to PM_2.5 in these areas during this episode.