Dependence of the hygroscopicity parameter κ on particle size, humidity and solute concentration: implications for laboratory experiments, field measurements and model studies

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 (κ_gf) and CCN activity measurements (κ_CCN), 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 κ_gf and κ_CCN should not be expected, and how the deviations observed in field and laboratory experiments can be quantitatively explained and reconciled. The difference between κ_gf and κ_CCN 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 κ_CCN 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 κ_gf and κ_CCN 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.