Thermodynamic Properties of Aqueous Aerosols to High Supersaturation: I--Measurements of Water Activity of the System Na+-Cl--NO-3-SO24-H2O at 298.15 K

Water activity/concentration relationships of aqueous NaCl, NaN03 , Na2S04 and their mixtures from dilute concentration to high supersaturation have been determined at room temperature in an electrodynamic balance. Using a dynamic measurement technique based on the evaporation of the droplets, a set of water activity measurements of a solution droplet can be obtained in less than an hour. The water activities of the mixtures are compared with theoretical predictions of the Pitzer-Simonson-Clegg (PSC) model, the Zdanovskii-Stokes-Robinson (ZSR) equation, and the Kusik and Meissner (KM) model. Comparisons of the data with model predictions indicate that the PSC, ZSR, and KM approaches agree well among themselves and with the experimental data. All three models give similar maximum (± 0.02) and standard (0.01) deviations in mass fraction of the solute (mfs). These deviations are close to the experimental error of ± (0.014).02). The agreement of the models and the need for single-component water activities at differenttemperatures, as input for all models, suggest that a high priority should be given to such experiments in the future.