Lesson 3c: evaporation of a liquid aerosol |
1st Calculation |
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Note: the above should be entered on the
variable "relative humidity or total water" parametric calculations page of
Model III |
First, plot the number of moles of aqueous phase NO3−(aq) to determine how this varies with relative humidity, and at what point the aqueous phase ceases to exist:
1st Graph: select the variables and enter the options as given below. | |
X Variable: "relative humidity" Range: leave blank Scale: linear (the default) |
Y Variable: "moles of NO3-(aq)" Range: leave blank Scale: linear (the default) |
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame. |
As the relative humidity decreases from 95%, so also does the moles of aqueous phase NO3−(aq). This is because the lowering of the relative humidity requires the equilibrium aqueous phase to have a lower water activity, and therefore higher ion concentrations. These in turn result in higher equilibrium partial pressures of HNO3(g) and NH3(g) and consequently the transfer of material to the gas phase. You can verify this, if necessary, by plotting the molality of NO3−(aq) present in the aqueous phase ("mNO3-" as Y axis variable), or HNO3 in the gas phase ("moles of HNO3(g)" as Y axis variable).
At about 86% relative humidity the number of moles of aqueous NO3− reaches zero. This is the transition point between gas phase NH3 and HNO3 existing alone at lower relative humidities and the same compounds in equilibrium with an aqueous phase in more humid atmospheres.
2nd Calculation |
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Note: the above should be entered on the
variable "relative humidity or total water" parametric calculations page of
Model III |
2nd Graph: select the variables and enter the options as given below. | |
X Variable: "relative humidity" Range: leave blank Scale: linear (the default) |
Y Variable: "moles of NO3-(aq)" Range: leave blank Scale: linear (the default) |
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame. |
The graph shows that the moles of NO3−(aq) is not just larger than before, as you would expect, but now remains greater than zero at all relative humidities between 70% and 95%. This is because the additional amount of material in the system is enough for the aqueous phase to release the amounts of HNO3 and NH3 required to produce the equilibrium partial pressures at each relative humidity.
3rd Calculation |
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Note: the above should be entered on the
variable "relative humidity or total water" parametric calculations page of
Model III |
3rd Graph: select the variables and enter the options as given below. | |
X Variable: "relative humidity" Range: leave blank Scale: linear (the default) |
Y Variable: "mNO3-(aq)" Range: leave blank Scale: linear (the default) |
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame. |
The graph shows that the transition relative humidity below which no aqueous phase particle exists (and molality is therefore zero) has now increased to about 90%. This is what we would expect: the aqueous phase becomes more dilute with increasing relative humidity, and the partial pressures of trace gases at equilibrium with it become lower. Therefore, when the total amounts of ions in the system are small a aqueous phase will exist only at relative humidities for which the amounts of HNO3(g) and NH3(g) are also small. In contrast, as the total amount of material increases the relative humidity of transition falls as there are then enough ions to sustain the higher equilibrium partial pressures of HNO3(g) and NH3(g) over the more concentrated solutions.
In summary, there is a transition relative humidity above which a aqueous phase exists, and below which the ions are present only as the gases HNO3(g) and NH3(g). This transition relative humidity depends on the total amount of material in the system. In general, the transition will be dependent upon the system composition and also the temperature, but we have not investigated these effects here.
Proceed to Lesson 3d to examine equilibrium between trace gases and solid particles, or return to the main page for this lesson.