Study of water activities of aerosols of mixtures of sodium and magnesium salts

Sodium and magnesium are major components of sea salt aerosols. Their salts of chlorides, sulfates and nitrates are hygroscopic. In this research, water activities of aqueous solutions of NaCl–MgCl2 (molar ratio 1 : 1), NaNO3–Mg(NO3)2 (1 : 1), Na2SO4–MgSO4 (1 : 1), NaCl–Mg(NO3)2 (1 : 1 and 3 : 1), NaCl–MgSO4 (1 : 1), and NaNO3–MgSO4 (1 : 1) were measured from dilute concentrations to high supersaturations using an electrodynamic balance. We employed a dynamic method, which is based on continuous measurements of the balancing voltage of the particle as a result of a step decrease of the relative humidity (RH) of the feed to the electrodynamic balance. Good agreements are found between the predictions of the ZSR (Zdanovskii–Stokes–Robinson) and the KM (Kusik and Meissner) models and the experimental data, with the exception of Na2SO4–MgSO4 solutions. Analysis of the time-series data reveals that Na2SO4–MgSO4 solution has a lower evaporation rate than other solutions. We postulate that this is due to internal mass transfer limitation when the particle forms a gel at high concentration. Partial crystallization of NaCl–MgCl2, NaCl–Mg(NO3)2 (3 : 1), and NaCl–MgSO4 mixtures occurs in the measurements taken at low RHs. The particles continue to lose water as RH decreases further. Other systems, including NaCl–Mg(NO3)2 (1 : 1), do not crystallize even at RH=30%. Crystallization of NaCl is suppressed when sufficient amount of nitrate is present.