The data are entered in a series of fields, and options are set using
radio buttons or check boxes.
- Temperature: enter an absolute temperature (Kelvin) between the specified
limits, according to the composition of the solution. These limits are:
- 180 K to 330 K for systems containing only acids or their mixtures.
- 180 K to 330 K for systems containing two or more of the ions
H+, NH4+,
SO42−,
or NO3−.
- 263.15 K to 330 K for all other systems.
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.
- Water Content of the System. This can be specified
in three different 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. Above 273.15 K the latter quantity is, of course, well
established. At lower temperatures the model uses vapour
pressures of pure supercooled water given by Murphy and Koop (Q. J. R. Meteorol. Soc.
131, 1539-1565, 2005).
- 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 temperatures and relative humidities, for which the
amount of water vapour causes the system volume to exceed 1
m3 by a few percent.
- vapour pressure over ice: the water vapour
pressure in the system is fixed to that above ice at
the current temperature, calculated using the thermodynamic properties listed in
the paper of Friese and Ebel (J. Phys. Chem. A
114, 11595-11631, 2005). This option is only valid below 273.15 K, and
is set with a checkbox.
-
Inorganic Composition: enter the numbers of moles of each ion, and any ammonia, per
m3 of atmosphere. Ensure that the
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 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 report the equilibrium partial
pressures of the indicated trace gases (HNO3, HCl, NH3,
H2SO4, and any volatile organic compounds
present in the system) 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 0.13E-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 liquid 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
solids.
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.