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Size-resolved and bulk activation properties of aerosols in the North China Plain

by Z. Z. Deng, C. S. Zhao, N. Ma, P. F. Liu, L. Ran, W. Y. Xu, J. Chen, Z. Liang, S. Liang, M. Y. Huang, X. C. Ma, Q. Zhang, J. N. Quan, P. Yan, S. Henning, K. Mildenberger, E. Sommerhage, M. Sch??fer, F. Stratmann, A. Wiedensohler show all authors
Atmospheric Chemistry and Physics ()

Abstract

Size-resolved and bulk activation properties of aerosols were measured at a regional/suburban site in the North China Plain (NCP), which is occasionally heavily polluted by anthropogenic aerosol particles and gases. A Cloud Condensation Nuclei (CCN) closure study is con- ducted with bulk CCN number concentration (NCCN) and calculated CCN number concentration based on the aerosol number size distribution and size-resolved activation proper- ties. The observed CCN number concentration (NCCN-obs) are higher than those observed in other locations than China, with average NCCN-obs of roughly 2000, 3000, 6000, 10 000 and 13 000cm−3 at supersaturations of 0.056, 0.083, 0.17, 0.35 and 0.70%, respectively. An inferred critical dry diam- eter (Dm) is calculated based on the NCCN-obs and aerosol number size distribution assuming homogeneous chemical composition. The inferred cut-off diameters are in the ranges of 190–280, 160–260, 95–180, 65–120 and 50–100nm at supersaturations of 0.056, 0.083, 0.17, 0.35 and 0.7%, with their mean values 230.1, 198.4, 128.4, 86.4 and 69.2nm, re- spectively. Size-resolved activation measurements show that most of the 300nm particles are activated at the investigated super- saturations, while almost no particles of 30nm are activated even at the highest supersaturation of 0.72%. The activation ratio increases with increasing supersaturation and particle size. The slopes of the activation curves for ambient aerosols are not as steep as those observed in calibrations with am- monium sulfate suggesting that the observed aerosols is an external mixture of more hygroscopic and hydrophobic par- ticles. The calculated CCN number concentrations (NCCN-calc) based on the size-resolved activation ratio and aerosol num- ber size distribution correlate well with the NCCN-obs, and show an average overestimation of 19%. Sensitivity studies of the CCN closure show that the NCCN at each supersatu- ration is well predicted with the campaign average of size- resolved activation curves. These results indicate that the aerosol number size distribution is critical in the prediction of possible CCN. The CCN number concentration can be reli- ably estimated using time-averaged, size-resolved activation efficiencies without accounting for the temporal variations.

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