The model inputs are similar to those for a comprehensive type calculation. Here the system being modelled consists of a 1 m3 air parcel whose temperature and relative humidity are varied from their starting values by adiabatic movement between two atmospheric pressures (corresponding to a change in altitude). The output of results is flexible, with a choice between the normal mode used for the single-problem calculations, column mode (useful for pasting the results into other programs), and graphical mode. 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.
initial pressure: a pressure (in mbar) between the specified limits. This is the first value in the range for which model calculations will be carried out.
final pressure: a pressure (in mbar) between the specified limits. This is the last value in the range for which model calculations will be carried out.
number of points: the number of different pressures for which calculations will be carried out. The minimum number is 2, i.e. the initial and final pressures only, and the maximum is 100.
Pressures will be equispaced across the range. So, for example, if pressure is varied from 500 to 600 mbar, with 3 points calculated, then the model determines the equilibrium properties of the system at 500, 550 and 600 mbar pressure. At the initial pressure the temperature and relative humidity are assigned the values entered on this page (see below). For each further calculation the temperature is adjusted based upon pressure change and the wet adiabatic lapse rate.
The current relative humidity is determined from the ambient water partial pressure, which is always a fixed proportion of the total pressure, and the vapour pressure of pure water at the current temperature. The exchange of water vapour with the condensed phase (aerosol) is assumed to be negligible. Because of this, if a relative humidity of 100% is encountered at any point, the calculation will be terminated.
Temperature. Enter the temperature corresponding to the initial pressure above.
Note that the temperature limits for this model differ according to system composition, and are:Relative Humidity. Enter the relative humidity, as a fraction, corresponding to the initial pressure above.
Inorganic Composition: enter 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 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 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.
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, HCl, HNO3, 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 0.13E-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 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.
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.
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.