Cloud condensation nuclei (CCN) and HR-ToF-AMS measurements at a coastal site in Hong Kong: size-resolved CCN activity and closure analysis

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 similar to 500 cm(-3) at SS=0.15% to similar 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 (D-50) at SS=0.15, 0.35, 0.50, and 0.70% were 116, 67, 56, and 46 nm, respectively. The corresponding average hygroscopic parameters (kappa(CCN)) were 0.39, 0.36, 0.31, and 0.28. The decrease in kappa(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 kappa(CCN) correlates reasonably well with kappa(AMS_SR) based on size-resolved AMS measurements: kappa(AMS_SR) = kappa(org) x f(org) + kappa(inorg) x f(inorg), where forg and finorg are the organic and inorganic volume fractions, respectively, kappa(org) = 0.1 and kappa(inorg) = 0.6, with a R-2 of 0.51. In closure analysis, N-CCN was estimated by integrating the measured size-resolved N-CN for particles larger than D-50 derived from kappa assuming internal mixing state. Estimates using kappa(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 kappa(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 D-50 (particle diameter <200 nm). A constant kappa = 0.33 (the average value of kappa(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 D-50 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.