The hygroscopic properties of aerosol particles, composed of two different commercially available humic acids, have been investigated in the temperature range 293-295 K as a function of relative humidity using an electrodynamic balance. The measured mass growth factors of Sigma-Aldrich humic acid sodium salt (NaHA) and leonardite humic acid (LHA) at 90% relative humidity are 1.87 +/- 0.02 and 1.20 +/- 0.05, respectively. There is a general agreement between the results described in this study with previous TDMA results, which are undertaken with smaller particle ensembles and shorter aerosol-water equilibration times. The measured mass growth factors of Sigma-Aldrich humic acid sodium salt (NaHA) and leonardite humic acid (LHA) at 90% relative humidity are 1.87 +/- 0.05 and 1.20 +/- 0.05, respectively. The UNIFAC model was used to calculate the relationship between water activity and the concentrations of a modelled LHA molecule, and thus the hygroscopicity. The LHA molecule was modelled by both the Steelink monomer and the Northeastern-Temple-Birmingham humic acid monomer structural models. (C) 2010 Elsevier Ltd. All rights reserved.