Characteristics of two serious air pollution episodes (9-15 January,as the winter case;and 30 June to 1 July,as the summer case),which occurred in Beijing in 2013 were investigated and compared using multi-method observations and numerical simulations.During these two air pollution episodes,PM2.5 concentrations varied significantly within Beijing,with PM2.5 concentrations in south parts of Beijing being significantly higher than in north areas.Typically,heavy air pollution episodes begin in the south parts and disperse towards the north parts of Beijing.Clearly,synoptic patts and the stability of atmospheric circulation patts were the main factors controlling air pollution in Beijing.During the winter case,a warm center above 900 hPa occurred over Beijing.Meanwhile,in the summer case,although there was only a weak inversion,the convective inhibition energy was strong (over 200 J/kG).This clearly influenced the duration of the air pollution event.Except for the local accumulation and secondary atmospheric reactions in both cases,regional straw bings contributed a lot to the PM2.5 concentrations in summer case.Using the CAM× model,we established that regional transport contributed almost 59％ to the PM2.s averaged concentration in Beijing in the winter case,but only 31％ in the summer case.Thus,the winter case was a typical regional air pollution episode,while the summer case resulted from local accumulation straw bumings transportation and strong secondary atmospheric reactions.Given that air pollution is a regional problem in China,consistent and simultaneous implementation of regional prevention and control strategies is necessary to improve regional air quality.