Lesson 6b: a mixed aqueous solution at 70% relative humidity


Here we study the same system as in Lesson 6a, but increase the relative humidity so that it exceeds the deliquescence relative humidity of sodium nitrate.

Entering the Data

We again equilibrate a system containing 0.1 micromoles each of nitric acid, ammonium nitrate, and sodium nitrate. Return to the data input page in the right hand browser window, and fill in the form as follows:

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

    Ambient Conditions
    Relative humidity = 0.70 (60%).

    Ionic Composition in Moles
    Hydrogen = 1E-7, Ammonium = 1E-7, Sodium = 1E-7, Nitrate = 3E-7.

    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

Now there is an aqueous phase but no solids. The Na+(aq) ion resides in the aqueous phase with an equivalent amount of NO3(aq) ion because it is non-volatile, and the other constituents of the mixture are partitioned between the gas and aqueous phases, rather than existing entirely in the gas phase as NH3 and HNO3. In this example the ambient relative humidity is greater than the deliquescence relative humidities of both sodium and ammonium nitrate salts, causing an aqueous solution to be formed. The gases NH3 and HNO3 partition into this solution according to the equations:

pHNO3 = aH+ × aNO3 / KH(HNO3)


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

where the equilibrium constants are defined in Lesson 1a and Lesson 3a. Thus here, and for any case in which a liquid aerosol exists, the ammonia and HNO3 will exist in both the gas phase and the aqueous phase (as the ions H+(aq), NH4+(aq), and NO3(aq)).

Proceed to Lesson 6c to learn how partitioning in the system changes as a function of ion composition, or return to the main page for this lesson.