|Lesson 3d: evaporation of a solid aerosol|
|Note: the above should be entered on the
"comprehensive" calculations page of Model III
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.