Thermodynamic Model of the System H+−NH4+−SO42-−NO3-−H2O at Tropospheric Temperatures

Abstract

A multicomponent mole-fraction-based thermodynamic model is used to represent aqueous phase activities, equilibrium partial pressures (of H 2 O, HNO 3, and NH 3 ), and saturation with respect to solid phases (H 2 SO 4 and HNO 3 hydrates, (NH 4 ) 2 SO 4(cr), (NH 4 ) 3 H(SO 4 ) 2(cr), NH 4 HSO 4(cr), (NH 4 ) 2 SO 4 ·2NH 4 NO 3(cr), (NH 4 ) 2 SO 4 ·3NH 4 NO 3(cr), and NH 4 HSO 4 ·NH 4 NO 3 (cr) ) in the system H + −NH 4 + −SO 4 2- −NO 3 - −H 2 O. The model is valid from 328 to <200 K, dependent upon liquid-phase composition. Parameters for H 2 SO 4 −H 2 O, HNO 3 −H 2 O, and (NH 4 ) 2 SO 4 −H 2 O interactions were adopted from previous studies, and values for NH 4 NO 3 −H 2 O 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 H 2 SO 4 −(NH 4 ) 2 SO 4 −H 2 O solutions. Comparisons suggest that the model satisfactorily represents partial pressures of both NH 3 and H 2 SO 4 above acidic sulfate mixtures in addition to that of HNO 3, and salt solubilities and water activities.