Year-round measurements of the mass concentration and optical properties of fine aerosols (PM2.5) from June 2009 to May 2010 at an urban site in Beijing were analyzed. The annual mean values of the PM2.5 mass concentration, absorption coefficient (Ab), scattering coefficient (Sc) and single scattering albedo (SSA) at 525 nm were 67 ± 66 μg/m3, 64 ± 62 Mm−1, 360 ± 405 Mm−1 and 0.82 ± 0.09, respectively. The bulk mass absorption efficiency and scattering efficiency of the PM2.5 at 525 nm were 0.78 m2/g and 5.55 m2/g, respectively. The Ab and Sc showed a similar diurnal variation with a maximum at night and a minimum in the afternoon, whereas SSA displayed an opposite diurnal pattern. Significant increases in the Ab and Sc were observed in pollution episodes caused by the accumulation of pollutants from both local and regional sources under unfavorable weather conditions. Aerosol loadings in dust events increased by several times in the spring, which had limited effects on the Ab and Sc due to the low absorption and scattering efficiency of dust particles. The frequency of haze days was the highest in autumn because of the high aerosol absorption and scattering under unfavorable weather conditions. The daily PM2.5 concentration should be controlled to a level lower than 64 μg/m3 to prevent the occurrence of haze days according to its exponentially decreased relationship with visibility.