Lesson 3b: reducing the relative humidity

### Content

The gas/particle partitioning of HNO3 and NH3 at a low relative humidity yields a system in which no particle phase exists.

### Part 1

Return to the data input page in the right hand browser window, and fill in the form as follows:

 1st Calculation Enter the values and select the options under the following headings: Ambient Conditions Relative humidity = 0.70 (i.e., 70%). Ionic Composition in Moles Hydrogen = 1E-7, Ammonium = 1E-7, Nitrate = 2E-7. 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 "comprehensive" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3b.php).

#### Interpreting the Results

You can see that at this relative humidity there is no aqueous phase, and no solids. All of the hydrogen ion, ammonium and nitrate that was input now exists in the gas phase as HNO3(g) and NH3(g). A check of the mole balances in the system shows that there is 2E-7 moles of nitrate (as HNO3(g)), which is equal to the total amount of nitrate input. There are also 1E-7 moles of NH3(g) which is equal to the amount of ammonium input. The total hydrogen ion input, 1E-7 moles, is equal to the 2E-7 moles present as HNO3(g), minus the 1E-7 moles produced by the dissociation of the ammonium to produce NH3(g). (Here you may wish to return to the results section of Lesson 3a to review the equilibria involved.)

We saw in Lesson 1c that a system containing NH4+ and NO3, but with no partitioning into the vapour phase, remains as an aqueous solution down to a relative humidity of at least 65%. Thus, we would not expect there to be a solid in the present result. Why is there no aqueous phase either? In Part 2, below, we will run a further calculation to explain this. Note that the partial pressures of HNO3(g) and NH3(g) for the present gas-only result are 4.786E-9 and 2.393E-9 atm, respectively.

### Part 2

Next, re-run the calculation from Part 1, but preventing partitioning of HNO3 and NH3 into the gas phase:

 2nd Calculation Enter the values and select the options under the following headings: Ambient Conditions Relative humidity = 0.70 (i.e., 70%). Ionic Composition in Moles Hydrogen = 1E-7, Ammonium = 1E-7, Nitrate = 2E-7. Trace Gases Check the boxes for HNO3 and NH3. 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 "comprehensive" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3b.php).

#### Interpreting the Results

We now have a aqueous phase as expected, containing 5.11 mol kg-1 of H+(aq) and NH4+(aq), and 10.22 mol kg-1 of NO3(aq). In the "Gases" section we can see that the partial pressures of HNO3 and NH3 that would be in equilibrium with this aqueous phase are 1.910E-5 and 8.127E-13 atm, respectively. These values are the key to explaining why there was no aqueous phase in the previous calculation: there the equilibrium HNO3 partial pressure - with all the material present in the gas phases - was much lower (4.786E-9 atm). This means that, if the present aqueous phase were allowed to evaporate to come to equilibrium with 1 m3 of gas phase the amounts of ions in the system would not be enough to produce the equilibrium HNO3 partial pressure for that solution composition. Because of this, any such aqueous phase will evaporate entirely as we found in Part 1.

You can confirm the above result by looking at the gas phase amounts (moles) given on the output page: the equilibrium HNO3 partial pressure of 1.910E-5 atm is equivalent to 7.809E-4 moles which is greater than the total amount of nitrate in the system by a large amount. Overall, the results of the calculations show that the existence of an aerosol phase will depend on both the total amount of material present, and on the relative humidity.

Proceed to Lesson 3c, or return to the main page for this lesson.