The optical properties of aerosols and their chemical composition, including water-soluble ions, organic carbon (OC), and elemental carbon (EC) in PM 2.5 and PM 10 , were measured from 26 May to 30 June of 2012 at an urban site in Beijing. The daily average concentrations of PM 2.5 and PM 10 were 103.2 and 159.6μg/m 3 , respectively. On average, the OC and EC contributed 20.1% and 4.3%, respectively, to PM 2.5 and 16.3% and 3.9%, respectively, to PM 10 . Secondary ions (SO42−, NO3−, and NH4+) dominated the water-soluble ions and accounted for 57.9% and 62.6% of PM 2.5 and PM 10 , respectively. The wind dependence of PM 2.5 , OC, SO42−, and NO3− implied that the pollution sources mainly came from south and southeast of Beijing during the summer. The monthly mean values of the scattering coefficient (σ sc ) and absorption coefficient (σ ab ) at 525nm were 312.9 and 28.7Mm −1 , respectively, and the mean single-scattering albedo (ω) was 0.85. The wind dependence of σ sc revealed that this value was mainly influenced by regional transport during the summer, and the relationship between σ ab and wind indicated that a high σ ab resulted from the joint effects of local emissions and regional transport. The reconstructed σ sc that was derived from the revised IMPROVE equation agreed well with the observations. The contribution of different chemical species to σ sc was investigated under different pollution levels, and it was found that secondary inorganic aerosols accounted for a large part of σ sc during pollution episodes (35.7%), while organic matter was the main contributor to σ sc under clean conditions (33.6%).