Model II: Inputs for Aqueous Solution Calculations

The data are entered in a series of fields, and options are set using radio buttons or check boxes.

• Temperature: enter an absolute temperature (Kelvin) between the specified limits. Users should be aware that thermodynamic data for electrolyte solutions and their mixtures are sparse at low temperatures - below, say, 253.15 K to 273.15 K. The model predictions for some systems may therefore be extrapolations well beyond the available data. The papers describing the model should be consulted for details.
• Composition of the Condensed Phase: enter the molalities of each ion, NH3, and/or organic compound, ensuring that charge balance is correct to at least one part in 10⁴. Note that 'molality' is defined as moles per kg of pure solvent (water), and not moles per kg of solution.

  Inorganic Composition: enter the numbers of moles of each ion, and any ammonia, per m³ of atmosphere. Ensure that the charge balance is correct to at least one part in 10⁴.

  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.

  If the system contains hydrophobic organic compounds (which are either constrained to the hydrophobic phase, or are likely to partition into it) the amounts are still entered as the number of moles of each compound per kg of total liquid water.

• Solid Phases: at low relative humidities aerosols may exist in a metastable liquid state (i.e., solids have not precipitated even though the liquid phase droplets are saturated or supersaturated with respect to them). The properties of such solutions can be investigated with the model by checking the boxes to prevent the formation of the indicated inorganic and/or organic solid phases.