Abstract. The cloud condensation nuclei (CCN) properties of atmospheric aerosols were measured on 1–30 May 2011 at the HKUST (Hong Kong University of Science and Technology) Supersite, a coastal site in Hong Kong. Size-resolved CCN activation curves, the ratio of number concentration of CCN (N_CCN) to aerosol concentration (N_CN) as a function of particle size, were obtained at supersaturation (SS) = 0.15, 0.35, 0.50, and 0.70% using a DMT (Droplet Measurement Technologies) CCN counter (CCNc) and a TSI scanning mobility particle sizer (SMPS). The mean bulk size-integrated N_CCN ranged from ~500 cm⁻³ at SS = 0.15% to ~2100 cm⁻³ at SS = 0.70%, and the mean bulk N_CCN / N_CN ratio ranged from 0.16 at SS = 0.15% to 0.65 at SS = 0.70%. The average critical mobility diameters (D₅₀) at SS = 0.15, 0.35, 0.50, and 0.70% were 116, 67, 56, and 46 nm, respectively. The corresponding average hygroscopic parameters (κ_CCN) were 0.39, 0.36, 0.31, and 0.28. The decrease in κ_CCN can be attributed to the increase in organic to inorganic volume ratio as particle size decreases, as measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The κ_CCN correlates reasonably well with κ_{AMS_SR} based on size-resolved AMS measurements: κ_{AMS_SR} = κ_org × f_org + κ_inorg × f_inorg, where f_org and f_inorg are the organic and inorganic volume fractions, respectively, κ_org = 0.1 and κ_inorg = 0.6, with a R² of 0.51.

In closure analysis, NCCN was estimated by integrating the measured size-resolved NCN for particles larger than D₅₀ derived from κ assuming internal mixing state. Estimates using κ_{AMS_SR} show that the measured and predicted N_CCN were generally within 10% of each other at all four SS. The deviation increased to 26% when κ_AMS was calculated from bulk PM₁ AMS measurements of particles because PM₁ was dominated by particles of 200 to 500 nm in diameter, which had a larger inorganic fraction than those of D₅₀ (particle diameter < 200 nm). A constant κ = 0.33 (the average value of κ_{AMS_SR} over the course of campaign) was found to give an N_CCN prediction within 12% of the actual measured values. We also compared N_CCN estimates based on the measured average D₅₀ and the average size-resolved CCN activation ratio to examine the relative importance of hygroscopicity and mixing state. N_CCN appears to be relatively more sensitive to the mixing state and hygroscopicity at a high SS = 0.70% and a low SS = 0.15%, respectively.