Size-resolved cloud condensation nuclei (CCN) activity and closure analysis at the HKUST Supersite in Hong Kong

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.