Ground-based measurements of aerosol optical properties were conducted during Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006) (11 August to 9 September 2006) at a suburban site ~30 km south of Beijing. Averaged over the measurement campaign (arithmetic mean +/- standard deviation), the total scattering coefficients (sigma s ) were 469 +/- 374 Mm-1 (450 nm), 361 +/- 295 Mm-1 (550 nm), and 249 +/- 206 Mm-1 (700 nm) and the absorption coefficient (sigma a ) was 51.8 +/- 36.5 Mm-1 (532 nm). The average Ångström exponent was 1.42 +/- 0.19 (450 nm/700 nm) and the average single scattering albedo (omega 532) was 0.86 +/- 0.07 (532 nm) with minimum values as low as 0.5. Pronounced diurnal cycles were observed in sigma s , sigma a , and omega 532 and can be explained by boundary layer mixing effects. Additionally, an enhancement of absorbing particles in the early morning (0500-0800 local time) was observed; this may be attributed to soot emissions from traffic activity. When the measured air masses originated in the north and passed over Beijing, the single scattering albedo was generally low (omega 532 < 0.8), which indicates that the local emissions of particulate matter in Beijing were dominated by primary particles from combustion sources (soot). The southerly inflow to Beijing had typically very high sigma s and higher than average omega 532 values, suggesting a large amount of secondary aerosol (e.g., sulfate and oxidized organics). Overall, the results suggest that a majority of the particle pollution in Beijing is transported into the city from the south.