### E-AIM Tutorial

 Lesson 3: Gas/aerosol Partitioning in a Simple System

### Introduction

In earlier lessons we examined single electrolyte solutions and their water content and phase state (solid or liquid) as a function of relative humidity. We used the simple version of E-AIM Model III, which only considers species in the particle phase. In this lesson, we will examine partitioning between the condensed and vapour phases.

### Preparation

Before starting, ensure that this browser window occupies only the left half of your screen. You should leave enough space for another browser window where you can enter data into E-AIM and read the results. If your screen is too small for two windows, print out this tutorial and use this window to enter data and read E-AIM's results. In these lessons we assume that you will have two browser windows open.

Select this link to open a second browser window containing the data input page for "comprehensive" calculations using Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3b.php). Arrange the windows on your screen so that both are visible and the left window contains this text.

### The Lesson

This consists of the four sets of calculations described in the links below, which should be done in the order listed.

### Conclusion

You have completed Lesson 3, and learned that:
• In a system containing a volatile salt the existence of a condensed phase, either solid or liquid, depends on the vapour pressure product of the salt (here pHNO3 × pNH3), the quantity of the compound present, and the relative humidity.
• The solid salt, which is most likely to be present at low relative humidity, has an equilibrium vapour pressure product that varies with temperature. Only if the product of the actual partial pressures of the gases exceeds this value will the solid form.
• At relative humidities above its deliquescence point a solid aerosol will take up water and exist as an aqueous droplet. The vapour pressure product in equilibrium with the droplet is always lower than that over the solid, and decreases with increased relative humidity as the droplet becomes more dilute. As the total amount of ionic material in the system decreases the transition relative humidity at which the HNO3(g) and NH3(g) gases condense to form an aqueous droplet goes up.

Now proceed to Lesson 4, which examines the properties of aerosol solutions containing more than one electrolyte.