Lesson 3d: evaporation of a solid aerosol


The partitioning of HNO3 and NH3 between the gas phase and a solid aerosol at low relative humidity is investigated, and the transition relative humidity above which no aerosol exists is determined.

Entering the Data

We now look at the same HNO3/NH4NO3 system as in the previous two parts of this lesson, but at a lower relative humidity. Select this link to open the data input page for "comprehensive" calculations using Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3b.php). Fill in the form as follows:

1st Calculation
  1. Enter the values and select the options under the following headings:

    Ambient Conditions
    Relative humidity = 0.50 (i.e., 50%).

    Ionic Composition in Moles
    Hydrogen = 1E-6, Ammonium = 1E-6, Nitrate = 2E-6.

    Trace Gases
    There are no entries under this heading.
  2. Solid Phases
    There are no entries under this heading.

  3. 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

Lesson 2 showed how a solid NH4NO3(s) particle takes up water (deliquesces) at about 61.2% relative humidity at 298.15 K, and exists as a liquid particle above that. At the 50% relative humidity in the present calculation we are well below the deliquescence point so it is not surprising that the aerosol phase is all solid even though we have HNO3 present in addition to the salt.

Solid ammonium nitrate has an equilibrium vapor pressure product of ammonia and nitric acid of 4.356E-17 atm2 at this temperature. This value is the equilibrium constant for the reaction NH4NO3(s) = HNO3(g) + NH3(g), which we discussed in Lesson 2. The partial pressures listed in the "Gases" section of the results output are calculated directly from the amounts of HNO3(g) and NH3(g) present, and we can check that 2.5773E-8 × 1.6901E-9 does indeed equal 4.356E-17 atm2. This agreement confirms that the system is at equilibrium.

At these low relative humidities, where we do not expect an aqueous phase to form, the criterion for the existence of a solid aerosol is a simple one: it will form if the partial pressure product pHNO3 × pNH3 of the gases in the atmosphere exceeds the equilibrium value for the solid. If the actual partial pressure product is lower then this, then at high relative humidities an aqueous phase may form though this still depends on the amounts of material present. As the relative humidity increases, the ammonium nitrate is more dilute in solution and the resulting equilibrium vapor pressure product is lower. You can always find a humidity sufficiently close to 100% where a aqueous phase will form - after all, consider cloud and fog droplets in which solute concentrations are very low indeed.

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