Water uptake by aerosol: Water activity in supersaturated potassium solutions and deliquescence as a function of temperature

Atmospheric aerosol models often calculate water uptake by particles based on information on salt deliquescence and electrolyte mixing rules. Therefore improvements in deliquescence predictions and in the water-activity relations used in mixing rules can improve estimates of size distributions. The primary goals of this work are to derive equations for the variation in the mutual deliquescence relative humidity (MDRH) with temperature (T) for a particle containing an arbitrary number of hydrated and anhydrous salts and to extend the range of validity of water–activity relations reported for binary solutions of KNO3, KCl, KBr, and K2SO4 to greater supersaturations. Equations for MDRH(T) are derived based on the equilibrium conditions that define deliquescence and the Gibbs-Duhem relation. MDRH is calculated from solubility data and the ZSR mixing rule to generate reference values for the equations and to evaluate their reliability. The ZSR mixing rule is also used to predict concentrations of KNO3, KCl, KBr, and K2SO4 in highly supersaturated binary solutions. Polynomial fits are presented that relate water activity to concentration for these solutions. The deliquescence equations derived here predict MDRH(T) for hydrate systems with sufficient accuracy for temperature excursions of about 25 K. MDRH(T) usually follows the trend of DRH(T) for the most soluble salt in the system, while the temperature dependence of saturation concentration can be neglected for some salt systems. A linear temperature dependence for saturation concentration appears necessary to capture MDRH(T) for systems containing Ca(NO3)2·4H2O and CaCl2·6H2O. Consideration of deliquescence for a representative sea-salt system indicates that minor hydrated components have an important influence on the uptake of water by sea-salt particles and should be considered in equilibrium aerosol models. Results of this study improve estimates of water uptake by atmospheric particles containing hydrates and potassium salts.