Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydro-logical cycle and climate. CCN properties were measured and characterized during the CAREBeijing-2006 campaign at a regional site south of the megacity of Beijing, China. Size-resolved CCN efficiency spectra recorded for a super-saturation range of S = 0.07 % to 0.86 % yielded average ac-tivation diameters in the range of 190 nm to 45 nm. The cor-responding effective hygroscopicity parameters (κ) exhibited a strong size dependence ranging from ∼0.25 in the Aitken size range to ∼0.45 in the accumulation size range. The campaign average value (κ = 0.3 ± 0.1) was similar to the values observed and modeled for other populated continental regions. The hygroscopicity parameters derived from the CCN measurements were consistent with chemical composition data recorded by an aerosol mass spectrometer (AMS) and thermo-optical measurements of apparent elemental and or-ganic carbon (EC and OC). The CCN hygroscopicity and its size dependence could be parameterized as a function of only AMS based organic and inorganic mass fractions (f org , f inorg) using the simple mixing rule κ p ≈ 0.1 · f org + 0.7 · f inorg . When the measured air masses originated from the north and passed rapidly over the center of Beijing (fresh city pol-lution), the average particle hygroscopicity was reduced (κ = 0.2 ± 0.1), which is consistent with enhanced mass fractions of organic compounds (∼50 %) and EC (∼30 %) in the fine particulate matter (PM 1). Moreover, substantial fractions of externally mixed weakly CCN-active particles were observed at low supersaturation (S = 0.07 %), which can be explained by the presence of freshly emitted soot particles with very low hygroscopicity (κ < 0.1). Particles in stagnant air from the industrialized region south of Beijing (aged regional pol-lution) were on average larger and more hygroscopic, which is consistent with enhanced mass fractions (∼60 %) of solu-ble inorganic ions (mostly sulfate, ammonium, and nitrate). Accordingly, the number concentration of CCN in aged air from the megacity region was higher than in fresh city out-flow ((2.5–9.9) × 10 3 cm −3 vs. (0.4–8.3) × 10 3 cm −3 for S = 0.07–0.86 %) although the total aerosol particle number con-centration was lower (1.2 × 10 4 cm −3 vs. 2.3 × 10 4 cm −3). A comparison with related studies suggests that the fresh outflow from Chinese urban centers generally may contain more, but smaller and less hygroscopic aerosol particles and thus fewer CCN than the aged outflow from megacity re-gions.