The data are entered in a series of fields, and options are set using
radio buttons or check boxes.
- Temperature: the model is, at present, only valid at 298.15 K.
All calculations are carried out at this temperature.
- Water Content of the System. This can be specified
in one of two ways:
- fixed relative humidity: enter the value as a
fraction (not a percentage) between the limits specified on the input page.
This quantity is equivalent to the ambient partial pressure of water
divided by that over pure water at the same temperature.
- constant total water: the total amount
of water in the system, i.e. the vapour plus condensed phases,
is maintained at a constant value. This is entered as the total
number of moles of water associated with 1 m3 of
dry air at the current temperature. This approximates closely to "moles per m3"
at all but the highest relative humidities, for which the
amount of water vapour causes the system volume to exceed 1
m3 by a very small amount.
- 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 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 + H+) 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.
- 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).
- Trace Gases: the model can either report the equilibrium partial
pressures of the indicated trace gases (HNO3, HCl, NH3,
and H2SO4 plus any volatile organic compounds
present) that would exist
above the condensed phase. It can also calculate the actual partitioning of these
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 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 in the
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 the indicated
solid phases.
Users should note that Model III is, for many compositions, only valid up
to saturation with respect to the relevant inorganic solids. This applies particularly
to solutions containing Na+ and Cl-, and Na+ and SO42-. However,
for (NH4)2SO4 -
H2SO4 - H2O mixtures the range of
validity is much the same as for Model II. The model parameterisation for
NH4NO3 - H2O and NaNO3
-H2O also included data for supersaturated solutions, and it
is therefore valid to high molalities for these salts.