Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-253
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
27 Mar 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Dependence of the hygroscopicity parameter κ on particle size, humidity and solute concentration: implications for laboratory experiments, field measurements and model studies
Zhibin Wang1, Yafang Cheng1,2, Nan Ma1,3, Eugene Mikhailov4, Ulrich Pöschl1, and Hang Su2,1 1Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz 55020, Germany
2Institute for Environmental and Climate Research (ECI), Jinan University, Guangzhou 511443, China
3Leibniz-Institute for Tropospheric Research, Leipzig 04318, Germany
4St. Petersburg State University, 7/9 Universitetskaya nab, St. Petersburg 199034, Russia
Abstract. The hygroscopicity parameter κ has been intensively used in the investigation of the water uptake, cloud condensation nuclei (CCN) activity and chemical composition of atmospheric aerosol particles. A representative value of κ is often assigned to individual species or sources. Such treatment may lead to confusion in closure studies of κ derived from hygroscopic growth factor measurements (κgf) and CCN activity measurements (κCCN), and in studies of aerosols at the sub-10 nm size range. Here we show that for particles of the same dry composition, κ may differ as a function of water content, solute concentration and particle size. The concentration- and size-dependence of κ are demonstrated for representative inorganic and organic compounds, i.e., ammonium sulfate (AS), sodium chloride (NaCl) and sucrose. Our results illustrate that an absolute closure between κgf and κCCN should not be expected, and how the deviations observed in field and laboratory experiments can be quantitatively explained and reconciled. The difference between κgf and κCCN increases as particle size decreases reaching up to 40 % and 30 % for 10 nm AS and NaCl particles, respectively. Moreover, we show that the deviations of κCCN vary from ~ 10 % for 30 nm and ~ 40 % for 200 nm, indicating a strong dependence on the Köhler models and thermodynamic parameterizations used for instrument calibration (e.g., effective water vapor supersaturation in CCN counter). By taking these factors into account, we can largely explain apparent discrepancies between κgf and κCCN values reported in the scientific literature. Our results help to understand and interpret κ values determined at different water vapor ratios and at different size ranges (especially sub-10 nm). We highlight the importance of self-consistent thermodynamic parameterizations when using AS for calibration aerosol and taking it as a reference substance representing inorganics in closure study between chemical composition and hygroscopicity of aerosol particles.

Citation: Wang, Z., Cheng, Y., Ma, N., Mikhailov, E., Pöschl, U., and Su, H.: Dependence of the hygroscopicity parameter κ on particle size, humidity and solute concentration: implications for laboratory experiments, field measurements and model studies, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-253, in review, 2017.
Zhibin Wang et al.
Zhibin Wang et al.
Zhibin Wang et al.

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