Abstract
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 (NCCN) to aerosol concentration (NCN) 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 NCCN ranged from ~500 cm−3 at SS = 0.15% to ~2100 cm−3 at SS = 0.70%, and the mean bulk NCCN / NCN 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 (κCCN) were 0.39, 0.36, 0.31, and 0.28. The decrease in κCCN 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 κCCN correlates reasonably well with κAMS_SR based on size-resolved AMS measurements: κAMS_SR = κorg × forg + κinorg × finorg, where forg and finorg are the organic and inorganic volume fractions, respectively, κorg = 0.1 and κinorg = 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 κAMS_SR show that the measured and predicted NCCN were generally within 10% of each other at all four SS. The deviation increased to 26% when κAMS 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 κAMS_SR over the course of campaign) was found to give an NCCN prediction within 12% of the actual measured values. We also compared NCCN 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. NCCN 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.
Highlights
Atmospheric aerosols can act as cloud condensation nuclei (CCN) and affect cloud formation by influencing the CCN number concentration (NCCN) and the size of cloud droplets
For most of the time, the total NCCN at SS of 0.15, 0.35, 0.50 and 0.70 % were below 800, 3000, 5000 and 5600 cm−3 respectively, and number concentration (NCN) was below 10 000 cm−3
Both NCCN and NCN in this study are lower than those observed in July 2006 in Guangzhou, a www.atmos-chem-phys.net/14/10267/2014/
Summary
Atmospheric aerosols can act as cloud condensation nuclei (CCN) and affect cloud formation by influencing the CCN number concentration (NCCN) and the size of cloud droplets. Whether aerosol particles will eventually form cloud droplets under a set atmospheric condition mainly depends on their size, chemical composition, and mixing states. Predicting NCCN usually involves measuring the aerosol size distribution and making assumptions about the chemical composition associated to mixing state. Bulk chemical compositions and an assumption of internal mixing state (i.e., particles are identical mixtures of all participating species) are often used in predicting NCCN (Moore et al, 2012a; Wang et al, 2010). Meng et al.: CCN activity and closure analysis at the HKUST Supersite in Hong Kong
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