The model inputs are essentially the same as those for the comprehensive type calculation, but with temperature varied over a range specified by the user. There is flexibility in the output of results, with a choice between the normal mode used for the single-problem calculations, column mode (useful for pasting 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. Three values are required:
initial temperature: an absolute temperature (Kelvin) between the specified limits. This will be the first value in the range for which model calculations will be carried out.
final temperature: an absolute temperature between the specified limits. This will be the last value in the range for which model calculations will be carried out.
number of points: the number of different temperatures for which results will be calculated. The minimum number is 2, i.e. the initial and final temperatures only, and the maximum is 100.
Temperatures will be equispaced across the range. So, for example, if temperature is varied from 280 K to 310 K, with 4 points calculated, then the model determines the equilibrium properties of the system at T = 280, 290, 300 and 310 K. 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.
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 ice plus 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.
vapour pressure over ice: the water vapour pressure in the system can be fixed to that above ice at the current temperature (only valid below 273.15 K). See Clegg and Brimblecombe (J. Chem. Eng. Data 40, 43-64, 1995) for details. This option is set using a check box.
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, 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 0.1125E-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.
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.
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.