### Types of Calculation

The on-line models carry out a variety of computations ranging from water, ion, and organic solute activities in aqueous solutions and liquid mixtures, to aerosol/vapour partitioning calculations and the formation of solids. These calculations can be done for one or more individual cases, or for a range of values of a selected variable such as temperature or relative humidity. The results of such multiple calculations can be output in column form or graphed.

The following types of calculation are available:

1. Simple: a single solute (i.e., an organic compound or electrolyte), or a mixture of solutes, is equilibrated to a specified temperature and relative humidity. Chemical composition is entered as the number of moles of each species. The user can choose to disable the formation of individual solids. The model output consists of the solution composition at equilibrium, including the amount of liquid water present, and the amounts of any solids (salts, organic compounds, and their hydrates) that have formed. Depending on the properties of the organic components of the system (if any) a hydrophobic phase may also be present. Equilibrium partial pressures of volatile components (for example NH3 and acids such as HNO3 and HCl, depending on the model) are also calculated. A single problem is entered at a time.
2. Comprehensive: this is similar to the "simple" case above, but with more facilities and options available to the user. Here the electrolyte/organic/water system is considered to exist in 1 m3 of atmosphere. Again, it can be equilibrated to a fixed relative humidity, or to a specified total amount of water. In the latter case the equilibrium distribution of water between the vapour and condensed phases will be determined - thus the relative humidity is a quantity calculated by the model. Partitioning of volatile components into the vapour phase is also calculated, at the option of the user. The formation of solids can be switched off individually, in the same way as for the "simple" calculation, in order to investigate the properties of supersaturated solutions.
3. Aqueous Solution and Liquid Mixture: here the model determines the equilibrium thermodynamic properties of a system whose composition is specified in moles of each ion and/or organic compound per kg of water. Solute and water activities are calculated, together with the presence of a hydrophobic liquid phase (which may occur if any hydrophobic organic compounds are present) and any solids. The formation of individual solids can be switched off, if required. Equilibrium partial pressures of volatile inorganic and organic components (for example NH3 and various acid gases) are also determined.
4. Batch: this is the most flexible mode of use of the model, in which one or more problems can entered (one per line) in an html text box. All of the above types of problem - "simple", "comprehensive" and "aqueous solution" - can be entered. The options controlling each calculation are given as numerical codes. Results can be output in normal or column form, or as graphs.
5. Köhler curves (Models II to IV only): here the model is used to calculte the equilibrium saturation (or supersaturation) of water over liquid droplets of specified size. The value of the saturation S can rise above 100% for very small droplets, i.e., the water vapour pressure becomes greater than the equilibrium value for a pure water plane surface, due to the Kelvin effect. Köhler curves are often plotted as % saturation of water against the logarithm of particle radius.

Particle composition can be entered in a number of ways: moles of ions, moles or masses of chemical components, and dry particle radius together with the volume fractions of the salts present. It is also possible to include a single organic compound in these calculations, which can be either non-dissociating or a mono- or di-carboxylic acid. The properties of this organic compound are entered on the Köhler calculation page. There is one further way of entering chemical composition: particle radius, volume fraction of solid(s), and kappa (κ) value(s). In this case the calculations are carried out using the κ–Köhler theory of Petters and Kreidenweis.

6. Parametric: this type of calculation allows the effect of the variation of a single quantity, such as temperature or some element of the ionic composition for example, to be investigated. The flexibility of the input is similar to the "comprehensive" type. The variation of the chosen model quantity is specified by the user as the maximum and minimum values of the range, and the number of points to be calculated.

For example, if relative humidity were being varied with a range of 0.8 to 0.9, with 3 points to be calculated, then the model would determine the equilibrium properties of the system at relative humidities of 0.8, 0.85 and 0.9. The output of model results can be in normal or column form, or as graphs, at the option of the user. Note that the "lapse rate" parametric option allows the effect of the adiabatic movement of an air parcel, with its consequent changes in temperature and relative humidity, to be examined.