### Model IV: Inputs for Simple 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.
• Relative Humidity: enter a relative humidity (as a fraction, not percentage) between the specified limits. 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 water given by Murphy and Koop (Q. J. R. Meteorol. Soc. 131, 1539-1565, 2005).
• Ionic Composition: enter the numbers of moles of each ion per m3 of atmosphere, ensuring that charge balance is correct to at least one part in 104.

The present "simple" type of problem is one where an assemblage of ions is equilibrated to a fixed relative humidity, and the partial pressures of trace gases that would exist above the condensed phase are reported. In this case the scaling of the numbers of moles does not affect the result. Thus, for a fixed relative humidity of 0.515 at 298.15 K, equilibrating 2 moles of H+ and 1 of SO42−, or 2x10-6 moles H+ and 1x10-6 moles SO42−, gives the same answer: an aqueous solution of about 7.5 molal H2SO4.

• 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).
• 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.