The mass accommodation coefficient αmass of water vapor in NaCl solutions has been studied for realistic lower atmospheric conditions. To determine αmass, a combination of experimental data and computational fluid dynamics (CFD) modeling has been applied. Experiments were performed at the Leipzig Aerosol Cloud Interaction Simulator (LACIS), a laminar flow diffusion tube for measurements of both hygroscopic growth and cloud condensation nuclei (CCN) activation. Quasi-monodisperse sodium chloride particles with 54 nm and 108 nm in diameter have been used as condensation nuclei. To characterize particle growth, mean droplet diameters have been measured at the outlet of LACIS with a white-light optical particle spectrometer. Measurements were performed for different saturation ratios in the range between 1.0 and 1.02. Experiments have been modeled using the Computational Fluid Dynamics Code (CFD-Code) FLUENT6 combined with the Fine Particle Model (FPM). For determination of αmass, measured droplet diameters have been compared with calculated ones. The accommodation coefficient in the theoretical calculations was varied to achieve a quantitative comparison with the measurements. Experimental data shown in this study are consistent with αmass > 0.30. Therefore our results support previous studies applying different experimental techniques.