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

• Main Authors: J. W. Meng, M. C. Yeung, Y. J. Li, B. Y. L. Lee, C. K. Chan
• Format: Article
• Language: English
• Published: Copernicus Publications 2014-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/14/10267/2014/acp-14-10267-2014.pdf

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 (<i>N</i>_CCN) to aerosol concentration (<i>N</i>_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 <i>N</i>_CCN ranged from ~500 cm⁻³ at SS = 0.15% to ~2100 cm⁻³ at SS = 0.70%, and the mean bulk <i>N</i>_CCN / <i>N</i>_CN ratio ranged from 0.16 at SS = 0.15% to 0.65 at SS = 0.70%. The average critical mobility diameters (<i>D</i>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 × <i>f</i>_org + &kappa;_inorg × <i>f</i>_inorg, where <i>f</i>_org and <i>f</i>_inorg are the organic and inorganic volume fractions, respectively, &kappa;_org = 0.1 and &kappa;_inorg = 0.6, with a <i>R</i>² of 0.51. <br><br> In closure analysis, <i>N</i></sub>CCN</sub> was estimated by integrating the measured size-resolved <i>N</i></sub>CN</sub> for particles larger than <i>D</i>₅₀ derived from κ assuming internal mixing state. Estimates using &kappa;_{AMS_SR} show that the measured and predicted <i>N</i>_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 <i>D</i>₅₀ (particle diameter < 200 nm). A constant κ = 0.33 (the average value of &kappa;_{AMS_SR} over the course of campaign) was found to give an <i>N</i>_CCN prediction within 12% of the actual measured values. We also compared <i>N</i>_CCN estimates based on the measured average <i>D</i>₅₀ and the average size-resolved CCN activation ratio to examine the relative importance of hygroscopicity and mixing state. <i>N</i>_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.