### General Help

They calculate the equilibrium properties of chemical systems consisting of water, organic compounds, and two or more of the ions H+, NH4+, Na+, SO42−, NO3, Cl, and Br. In the case of mixtures of water and inorganic electrolytes the models are based upon extensive thermodynamic data for the binary (single electrolyte plus water) and ternary (two electrolytes plus water) subsystems.

Although the models are intended for atmospheric aerosol applications no account is taken of aerosol size or number, thus only total amounts of material per m3 are required as input. Some of the problem input pages allow amounts of solutes to be specified as molalities, i.e. moles per kg of liquid water, for calculations involving bulk aqueous solutions. For further details see the links below, and the explanatory material associated with the data input pages of the models.

• Overview of the project, and a brief summary of the models and what they can do.
• Description of the chemical systems that can be modelled, and the methods used (with references).
• Abstracts for the inorganic models Model I, Model II, Model III, and Model IV from the original research papers. These abstracts summarise the thermodynamic properties included in the models, and their ranges of validity.
• Types of calculation that can be done using the on-line models. Descriptions of the input and output quantities are linked to the data input pages. To select a calculation, go first to the main page of the model of interest.

#### Examples

If you are uncertain about using the models then try these simple calculations:

First, a system containing a single electrolyte and water:

1. Go to the main page of Model I, from the link on the E-AIM home page.

2. Enter "0.5" in the relative humidity field, and "1" in both the hydrogen and nitrate fields.

3. Click on the "Run" button. The result of this equilibration of 1 mole of HNO3 to 50% relative humidity at 298.15 K is an aqueous solution of 12.94 mol kg-1 HNO3. The equilibrium pHNO3 above the solution is 0.01563 atm. There are no solids.

4. Changing the temperature to 240 K and re-running the model yields solid HNO3 · 3H2O. The equilibrium pHNO3 above the solid (for 50% relative humidity) is given as 0.2458E-6 atm. This is calculated from the equilibrium between the solid and vapour phase HNO3 and H2O. As a further example, see what happens if the model is run again with HNO3 · 3H2O formation switched off.

Second, a system containing both an electrolyte and a water-soluble organic compound:

1. Go to the main page for Model III, from the link on the E-AIM home page. Press the button "Manage Organic Compounds" which can be found just after the input fields for the inorganic ions. This will take you to the Available Compounds page, where you can select compounds to be included in calculations and/or define your own.

2. From the drop-down selection box choose "Glutaric acid (fitted equation)" and press the button "Add to List". An entry for glutaric acid should then appear on the page, which means that it will now be included in the Model III chemical system. The entry includes a link that allows you to view or edit the assigned thermodynamic properties of glutaric acid, and one to remove it so that it is no longer part of the chemical system (though it still exists in the library, and can be selected again if necesssary).

The 6 character short names will be used to identify the acid and its anions (produced by dissociation) in the E-AIM model output.

3. Return to the Model III input page by pressing the button 'Return to Calculation'. You should find and an input box for the amount of glutaric acid after those for the inorganic ions.

4. Now define a similar problem to the one for the inorganic system, but this time at 298.15 K: enter "0.8" in the relative humidity field, "1" in both the hydrogen and nitrate fields, and "1" in the glutaric acid amount field. This defines a system containing 1 mole of HNO3 and 1 mole of glutaric acid at a relative humidity of 0.8 (ie., 80%).

5. Click the "Run" button. The result is an aqueous solution containing 4.05 mol kg-1 of HNO3 glutaric acid, and very low concentrations of the glutarate and hydrogen glutarate ions. This is because the dissociation of the organic acid is suppressed by the H+ from the nitric acid.

There are a few important things to note. First, there is no equilibrium gas phase partial pressure of glutaric acid listed. This is not because the acid is non-volatile, but because no Henry's law constant or liquid vapour pressure is present in the database for the acid. A value can easily be entered by returning to the Available Compounds page , clicking the link "View/Edit" and then entering the appropriate value in the volatility section of the page.

Second, for the amount of liquid water present an alternative estimate of the water activity (or relative humidity) of 0.787 has been obtained using the Zdanovskii-Stokes-Robinson method. To do this, the uncharged glutaric acid is treated as one ZSR solute, and the HNO3 and other ions as the second solute. The total of 13.696 moles of water is then distributed between the two solutes (treating them as two separate solutions) to obtain the same water activity in each one. This water activity is 0.787.

#### Incorrect Input Data

The model checks user input for errors, such as invalid or out-of-range numbers and the charge balance of the ions. A short explanatory message will be written to the screen if one is detected. When this occurs the data will have to be re-entered correctly. Model results are also checked for thermodynamic self consistency and, if a problem is found, a warning message will be output. The tests include ensuring that the equilibria in the system between gas, liquid and solid phases, and dissociation equilibria in the aqueous phase, are obeyed.

Actvities of organic compounds in the liquid phase(s) will also be checked to ensure that they are <1.0, relative to an activity coefficient reference state of the pure liquid compound. A value of exactly 1.0 can only occur for the pure liquid compound, and a value of >1.0 indicates that a phase separation should occur.

More information is given on the pages detailing the output of each type of calculation.

#### Further Assistance

If, after reading the explanatory and help information provided, the use of the site or some aspect of the model results remain unclear then please contact one of the authors.

#### Using the Models on Your Own Computer

Executable code for running the models in batch mode from data files can be made available to users. The code can be provided for PCs running Windows, and Intel or AMD based servers running 64 bit linux.