Content

Entering the Data

1st Calculation

Select "Graph" as the form of output, and then enter the values and options under the following headings: Ambient Conditions Select a fixed relative humidity (the default), and enter 0.70 (i.e., 70%). Ionic Composition in Moles For the initial composition of the system enter Hydrogen = 1E-7, Ammonium = 2E-7, Nitrate = 3E-7. Then select NH4+ for the "Initial ion" and Na+ as the "Substituting ion" from the drop down lists. Enter a start value of 0.0, an end value of 100, and 50 for the number of points. Trace Gases There are no entries under this heading. Solid Phases There are no entries under this heading. Click on the "Run" button at the end of the page to do the calculation.

Note:  the above should be entered on the "variable chemical composition" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/mod3ion.php).

Viewing and Interpreting the Results

Consider what this set of inputs means: we are fixing the RH at 70% and varying the composition from nitric acid plus ammonium nitrate to nitric acid plus sodium nitrate.

First, we examine the amount of liquid water in the aerosol phase as a function of the total amount of NH4⁺ in the system:

1st Graph:  select the variables and enter the options as given below.
X Variable: "total moles of NH4" Range: leave blank Scale: linear (the default)	Y Variable: "moles of H2O(aq)" Range: leave blank Scale: linear (the default)
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame.

There is no liquid water on both the left portion of the graph (where the system is mostly HNO3 and NaNO3) and at the extreme right (where the system contains only HNO3 and NH4NO3). The amount of water peaks near 1.0E-7 total moles of NH4⁺, which corresponds to a 1:1 ratio of NH4⁺ to Na⁺. What is the explanation for this behaviour? To answer this question, look at the amount of solid NaNO3(s) in the system:

2nd Graph:  select the variables and enter the options as given below.
X Variable: "total moles of NH4" Range: leave blank Scale: linear (the default)	Y Variable: "moles of NaNO3" Range: leave blank Scale: linear (the default)
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame.

The extreme left of the graph corresponds to a system composition of 1.0E-7 moles HNO3 with 2.0E-7 moles of NaNO3. At this point the particle is solid sodium nitrate and all the nitric acid is in the gas phase. As ammonium is substituted for sodium, the amount of solid sodium nitrate decreases but – as we know from the previous graph – the particle contains no liquid water until about 0.7E-7 moles of total NH4⁺. At this point the amount of NaNO3(s) starts to decrease more rapidly as it dissolves into the liquid water in the particle. At about 1.0E-7 moles of total NH4⁺, the solid NaNO3(s) disappears and the particle is completely aqueous.

We can verify this by looking at the amount of dissolved Na⁺(aq) ion:

3rd Graph:  select the variables and enter the options as given below.
X Variable: "total moles of NH4" Range: leave blank Scale: linear (the default)	Y Variable: "moles of Na+(aq)" Range: leave blank Scale: linear (the default)
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame.

On the left, there is no Na⁺(aq) because the particle is solid. When the aqueous phase forms, the moles of Na⁺(aq) increase as more of the NaNO3(s) dissolves. Finally, after all the solid NaNO3 has dissolved in the aqueous phase the moles of Na⁺(aq) decreases as NH4⁺ continues to be substituted for Na⁺.

Last, we look at the ammonia present in the gas phase:

4th Graph:  select the variables and enter the options as given below.
X Variable: "total moles of NH4" Range: leave blank Scale: linear (the default)	Y Variable: "moles of NH3(g)" Range: leave blank Scale: linear (the default)
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame.

Starting on the left, the amount of NH3(g) increases as the NH4⁺ that is substituted for Na⁺ all goes into the gas phase. Between 0.7E-7 and 1.0E-7 moles of total NH4⁺, the aqueous phase forms and some of the NH4⁺ dissolves in the aqueous phase but the amount of NH3(g) (and consequently its partial pressure) remains constant. Recall the gas/liquid equilibrium expression for NH3(g) from Lesson 6b:

pNH3 = (Ka(NH4⁺) / KH'(NH3)) × aNH4⁺ / aH⁺

The constant partial pressure of NH3(g) implies that the aqueous phase activities of NH4⁺ and H⁺, or their ratio, are also constant. What is happening between 0.7E-7 and 1.0E-7 moles of total NH4⁺ is that as more NH4⁺ is added more NaNO3(s) dissolves and the solute concentrations in the aqueous phase remain the same, even though the total amount of liquid increases.

Above 1.0E-7 moles of total NH4⁺, the solid NaNO3(s) is no longer present, and further substitutions of NH4⁺ for Na⁺ again increase the amount of NH3(g). Finally, at the far right of the graph, there is no more Na⁺ in the system. All the components are volatile, the particle phase disappears and the compounds are present entirely in the gas phase as HNO3(g) and NH3(g).

You should now review the conclusions on the main page of this lesson.