The data are entered in a series of fields, and options are set using radio buttons or check boxes.
The presence of aqueous phase ammonia (NH3) as a species in the model allows systems that are alkaline to be treated - those in which the total ammonia present (NH4+ + NH3) is only partially neutralised by H+ thus leaving an excess of NH3. However, the model is not intended to be applied to systems containing high concentrations of aqueous NH3 relative to other dissolved solutes (these are unlikely to occur in the atmosphere), and the input data are tested for this.
See the Model II description for brief details of how NH3(aq) has been included in the model, and limitations of the approach.
Pressing the "Show Options" button displays additional controls that affect the calculation, but which may be of interest to only small numbers of users. Option (1) allows NH4+ dissociation (NH4+ = NH3 + H+) and water dissociation (H2O = H+ + OH−) to be switched off. The reactions only affect speciation and phase partitioning over a limited range of pH (i.e., for non-acidic systems), and may not be significant in the calculation being carried out. Switching off these reactions can also be useful in sensitivity studies.
For example, consider a system containing 1.0E-6 moles of H2SO4, and the same amount of HNO3, in 1 m3 at 298.15 K and a fixed RH of 50%. If no HNO3 is allowed to partition into the vapour phase, then at equilibrium the amount of HNO3 in the liquid phase remains at 1.0E-6 mol, and the equilibrium pHNO3 is 0.1125E-3 atm. If partitioning is enabled, then 0.99988E-6 mol of HNO3 resides in the gas phase, at a partial pressure of 0.2408E-7 atm. The default is that partitioning is enabled. Check the boxes to prevent partitioning for each trace gas, as required.
For neutral and alkaline systems containing dissolved sulphate, the equilibrium partial pressures of H2SO4 are often so low (less than 10-30 atm) that it is not possible for the model to partition the acid between the condensed phase(s) and the gas phase. In these cases, only the equilibrium partial pressure of the acid (and the equivalent number of moles) is reported, even if the H2SO4 box on the input page is not checked.