Lesson 1c: reducing the relative humidity 
1^{st} Calculation 

Note: the above should be entered on the "simple" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3a.php). 
The change in concentrations of the ions also, of course, entails a change in the activities. The mole fraction activities of NH4^{+}(aq) and NO3^{−}(aq) are both equal to 0.01171 × 0.5667 = 0.006636 (compared to 0.005694 × 0.6402 = 0.0036453 for the 99% relative humidity case). The activity coefficients have decreased slightly from 99% to 98% relative humidity.
In the "Gases" section the equilibrium partial pressures 1.6848E12 atm (HNO3), and 2.8718E07 atm (NH3), are both larger than at 99% relative humidity. Thus, as the solution becomes more concentrated the equilibrium partial pressures of these gases increase.
In the "Solids" section we now have an entry that was not present at 99% relative humidity: the saturation ratio of solid NH4NO3 is 0.01111. This quantity is related to the equilibrium constant for the dissolution of NH4NO3(s), Ks(NH4NO3), as follows:
NH4NO3(s) = NH4^{+}(aq) + NO3^{−}(aq)
Ks(NH4NO3) = aNH4^{+} × aNO3^{−} = (xNH4 fNH4) × ( xNO3 fNO3)
where Ks(NH4NO3) is equal to the activity product of the ions NH4^{+} and NO3^{−} in a solution saturated with respect to solid NH4NO3(s). Equilibrium constants Ks for all solids are functions of temperature, but not the composition of the system. The activity of the pure solid phase does not appear as a denominator in the equation above as it is unity by definition. The saturation ratio, given in the model output, is equal to the actual value of the activity product in a solution (0.006636^{2} in the present example) divided by Ks(NH4NO3). A saturation ratio of unity (1.0) would mean that the solution was completely saturated with respect to the salt, and that any further increase in concentration would result in the precipitation of the solid.
2^{nd} Calculation 

Note: the above should be entered on the "simple" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3a.php). 
In the "Gases" section the equilibrium partial pressure of NH3 is 4.6063E07 atm, and that of HNO3 is 8.2501E11 atm. This shows that the equilibrium partial pressures of these gases rise steeply as the solution becomes more concentrated.
In the "Solids" section we see that the saturation ratio of solid NH4NO3 is 0.8724 at this relative humidity. This is quite close to unity (complete saturation), and suggests that an aqueous solution of NH4NO3 will become saturated with respect to the solid at a relative humidity not too far below 65%. This behaviour will be examined further in a later lesson.
3^{rd} Calculation 

Note: the above should be entered on the "simple" calculations page of Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3a.php). 
In the "Gases" section the equilibrium partial pressure of HNO3 is given as 0.6509E4 atm, which is more than three orders of magnitude greater than that at 99% relative humidity (Lesson 1a). We would therefore expect that an aqueous HNO3 droplet would be unlikely to survive in an atmosphere at low relative humidity: its very high equilibrium HNO3 partial pressure would cause it to evaporate.
This result also illustrates the important general principle that gas/aerosol partitioning varies strongly with relative humidity, tending towards the aerosol phase at high relative humidities and the gas phase at low ones.
You should now proceed to Lesson 1d, or return to the main page for this lesson.