### Model IV: Inputs for Comprehensive 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, according to the composition of the solution. These limits are:
• 180 K to 330 K for systems containing only acids or their mixtures.
• 180 K to 330 K for systems containing two or more of the ions H+, NH4+, SO42−, or NO3.
• 263.15 K to 330 K for all other systems.
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.
• Water Content of the System. This can be specified in three different ways:
• fixed relative humidity: enter the value (as a fraction, not a percentage) between the limits specified on the input page. This quantity is equivalent to the ambient partial pressure of water divided by that over pure water at the same temperature. Above 273.15 K the latter quantity is, of course, well established. At lower temperatures the model uses vapour pressures of pure supercooled water given by Murphy and Koop (Q. J. R. Meteorol. Soc. 131, 1539-1565, 2005).
• constant total water: the total amount of water in the system, i.e. the vapour plus condensed phases, is maintained at a constant value. This is entered as the total number of moles of water associated with 1 m3 of dry air at the current temperature. This approximates closely to "moles per m3" at all but the highest temperatures and relative humidities, for which the amount of water vapour causes the system volume to exceed 1 m3 by a few percent.
• vapour pressure over ice: the water vapour pressure in the system is fixed to that above ice at the current temperature, calculated using the thermodynamic properties listed in the paper of Friese and Ebel (J. Phys. Chem. A 114, 11595-11631, 2005). This option is only valid below 273.15 K, and is set with a checkbox.
• 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. Note that model results are not affected by scaling the numbers of moles only where the both the relative humidity is fixed and the trace gases are not being partitioned into the vapour phase.

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.

• Organic Compounds: if organic compounds have been added to the system (by pressing the Manage Compounds button and then selecting or creating the compounds) enter the numbers of moles of species per m3 of atmosphere in the box provided. There may also be options associated with the compounds, such as the ability to switch dissociation on or off, or to restrict the compounds to one of the two possible liquid phases (aqueous and hydrophobic).
• Trace Gases: the model can report the equilibrium partial pressures of the indicated trace gases (HNO3, HCl, NH3, H2SO4, and any volatile organic compounds present in the system) that would exist above the condensed phase. It can also calculate the actual partitioning of these species between the aerosol and vapour phases.

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 0.13E-3 atm. If partitioning is enabled, then 0.9999E-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.

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

This option is limited to systems that contain only acids or their mixtures, or the following ions: H+, NH4+, SO42−, and/or NO3. It is not possible to calculate the properties of supersaturated mixtures containing either Na+, or both NH4+ and Cl. Solids containing these ions are therefore omitted from the list on the problem input page.