The model inputs are essentially the same as those for the comprehensive type calculation, but with the ionic composition of the system varied over a user-specified range. First, the initial system composition (i.e., amounts of each ion and ammonia present) is entered by the user, followed by a pair of ions that will be substituted one for the other, and then the range of amounts. In order to maintain overall charge neutrality, only a cation can be substituted for a cation, and an anion for an anion.
The organic composition of the system (if any compounds have been included) can not be varied
Take the following example: starting with a system containing 2 moles of H+, and 1 mole of SO42−, we substitute NO3− for the SO42− from 50 mole% of the original 1 mole of SO42− present up to 100 mole%. In carrying out the substitution we have to take account of the differing charge magnitudes of the two anions. The initial substitution in this example is for 50 mole%, or 0.5 moles. To maintain charge balance, this is replaced by 1.0 moles of NO3−. The final substitution, of 100 mole% of the SO42−, yields a system containing 2 moles of H+ and 2 moles of NO3−.
In the same way as for the other parametric calculations, the output is flexible with a choice between the normal mode used for the single-problem calculations, column mode (useful for pasting of results into other programs), and graphs. The input fields and output quantities are described below.
normal: the results for each problem will be output sequentially in verbose form (the same way as for single calculations) to a single html page.
column: the results for the complete set of calculations will be output as columns, one per variable. Because of the large number of variables, the columns are separated into several groups and are output to a single html page one after the other. The variable names (i.e., the column headers) are the same as those used in batch type calculations.
graph: all the results are made available for graph plotting. The user selects the x and y variables from an html page that appears when the calculations are complete, and the graph will be displayed in the browser window.
Temperature. This is fixed at 298.15 K for Model III.
fixed relative humidity: as temperature is being varied in these calculations, a fixed RH corresponds to a varying partial pressure of water (higher as temperature increases). Enter RH as a fraction (not a percentage) between the limits specified on the input page. Relative humidity 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. For lower temperatures the model uses vapour pressures of pure supercooled water as given by Carslaw et al. (J. Phys. Chem. 99, 11557-11574, 1995).
total water: the total amount of water in the system, i.e. the vapour plus condensed phases, is maintained at a constant value. Enter the total water content as the total number of moles of water associated with 1 m3 of dry air at the initial 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.
liquid water: the total amount of water in the condensed phase (generally the aqueous phase and/or any solids containing waters of hydration) is maintained at a constant value. Enter the water content as the total number of moles of liquid water associated with 1 m3 of dry air at the initial temperature. Volatile species such as NH3 and the acid gases will be allowed to partition into the vapour phase, if desired, but not water.
First enter the initial composition of the system, as the numbers of moles of each ion, and any ammonia, per m3 of atmosphere. Ensure that charge balance is correct to at least one part in 104. Note that scaling the numbers of moles, i.e. multiplying all the values by some fixed factor, does not affect the results where 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 III 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 + NH3) 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.
Next, choose the "initial" ion - the cation or anion that is going to be substituted by another. Remember that the ion must be present in a non-zero amount in the composition entered above.
Now select the substituting ion, which must be of the same charge type (cation or anion) as the initial ion.
Other Chemical Components: 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 each compound, such as the ability to switch dissociation on or off, or to restrict the compound to one of the two possible liquid phases (aqueous and hydrophobic).
Trace Gases: the model can either report the equilibrium partial pressures of the inorganic trace gases (NH3, HNO3, HCl, and H2SO4), and any organic trace gases, that would exist above the condensed phase, or calculate the actual partitioning of the 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 is 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.
Solid Phases: at low relative humidities aerosols may exist in a metastable liquid state (i.e., solids have not precipitated even though the 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 individual solid phases.
Normal: this is the form of verbose output used for single problems. A description of the calculated quantities is given here.
Column: the output is the same as that for the "column" option in batch calculations. Details of the quantities output, and definitions of the column headers, are given here.
Graph: the user will be presented with an html page from which to select the x and y variables to be plotted. Only non-zero quantities will be included in the lists of variables. Graphical output is also available for "batch" calculations. Further assistance is given on the graph selection pages, to which users are automatically directed at completion of calculations for which graphical output has been selected.