Thermodynamic Model of the System H⁺−NH₄⁺−SO₄^2-−NO₃^-−H₂O at Tropospheric Temperatures

Abstract

A multicomponent mole-fraction-based thermodynamic model is used to represent aqueous phase activities, equilibrium partial pressures (of H2O, HNO3, and NH3), and saturation with respect to solid phases (H2SO4 and HNO3 hydrates, (NH4)2SO4(cr), (NH4)3H(SO4)2(cr), NH4HSO4(cr), (NH4)2SO4·2NH4NO3(cr), (NH4)2SO4·3NH4NO3(cr), and NH4HSO4·NH4NO3(cr)) in the system H+−NH4+−SO42-−NO3-−H2O. The model is valid from 328 to <200 K, dependent upon liquid-phase composition. Parameters for H2SO4−H2O, HNO3−H2O, and (NH4)2SO4−H2O interactions were adopted from previous studies, and values for NH4NO3−H2O obtained from vapor pressures (including data for supersaturated solutions), enthalpies, and heat capacities. Parameters for ternary interactions were determined from extensive literature data for salt solubilities, electromotive forces (emfs), and vapor pressures with an emphasis upon measurements of supersaturated H2SO4−(NH4)2SO4−H2O solutions. Comparisons suggest that the model satisfactorily represents partial pressures of both NH3 and H2SO4 above acidic sulfate mixtures in addition to that of HNO3, and salt solubilities and water activities.