Model II: Summary and Abstract


The inorganic element of Model II is an equilibrium thermodynamic model of the system H+ - NH4+ - SO42− - NO3 - H2O. It is valid from 328 K to about 200 K, and includes the following species:

See the model description page for an explanation of how organic compounds, and an additional liquid phase, are included in the chemical system.


The development of the model is described in the following paper:

S. L. Clegg, P. Brimblecombe, and A. S. Wexler (1998) A thermodynamic model of the system H+ - NH4+ - SO42− - NO3 - H2O at tropospheric temperatures. J. Phys. Chem. A, 102, 2137-2154.

Abstract: A multicomponent mole-fraction-based thermodynamic model is used to represent aqueous phase activities, equilibrium partial pressures (of H2O, HNO3, NH3 and H2SO4) and saturation with respect to fifteen solid phases. 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 vapour 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 vapour 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 sulphate mixtures in addition to that of HNO3, and salt solubilities and water activities.


Since the paper was published the thermodynamic model has been extended as follows:

The above extensions are also present in Model I.

We have added aqueous NH3 to the model for all calculations except the "Simple" type. This allows systems that are alkaline to be treated – those in which the total ammonia present (NH4+ + NH3) is only partially neutralised by H+. However, the model is not intended to be applied to systems containing high concentrations of aqueous NH3 relative to other dissolved solutes.

The Gibbs energies of formation of the NH3(g) and NH3(aq) species are based on the Henry's law constant and acid dissociation constants used by Clegg and Brimblecombe (1989) (J. Phys. Chem. 93, 7237-7248). The heat capacity changes for the reactions (ΔCp°) are not known for low temperatures. Consequently E-AIM results will be most accurate for temperatures above about 273 K. The dissociation of water in the aqueous phase is also modelled for some compositions. The activity coefficient of the OH produced by the dissociation is at present estimated using only the Debye-Huckel limiting law expression in E-AIM. Gas liquid partitioning is only affected by this reaction at pH very close to neutral. The Gibbs energy of formation of the OH ion was calculated using standard values of the equilibrium constant, and ΔH° and ΔCp° for the reaction.

In E-AIM the molality-based activity coefficient of dissolved NH3 is assumed to be unity at all concentrations and temperatures. The known "salting-out" influence of ions such as SO42− (sulphate) and Na+ (sodium) on aqueous NH3 has not yet been included in the model. This means that, for non-acidic systems in which the concentration of NH3 is significant relative to that of NH4+, the equilibrium vapour pressures of ammonia above concentrated solutions will be too low – by as much as 60% for an ammonium sulphate solution at 80% relative humidity and room temperature, for example. However, the effect was only seen at H+ concentrations below about 10-5 mol kg-1. For more acid systems, in which the dissociation of NH4+ is negligible, the lack of salting out parameters does not affect the accuracy of equilibrium pNH3 predictions or calculated ammonia partitioning.

See Clegg and Brimblecombe (1989) for a discussion of the available data, calculations of ammonia partial pressures and the acid dissociation of the NH4+ ion, and an activity coefficient treatment using the Pitzer molality-based model.