Introduction

Lesson 1 showed that atmospheric particles containing soluble salts also contain a substantial amount of water which varies with the relative humidity. In this lesson we explore the conditions under which these particles are completely solid, and the transition from solid to liquid state.

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 "simple" calculations using Model III (https://www.aim.env.uea.ac.uk/aim/model3/model3a.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 three sets of calculations described in the links below, which should be done in the order listed.

a.  A solid salt aerosol at low relative humidity

b.  Deliquescence of a dry salt aerosol

c.  Salt solubility and deliquescence

Conclusion

You have completed Lesson 2, and learned that:

• For a solid composed of volatile components, such as NH4NO3, there is an equilibrium between the solid and the partial pressure product of the components. This product is a function of temperature only.
• The phase state of a particle (i.e., solid or liquid) is a function of the relative humidity.
• There is a transition between solid and liquid state at a single relative humidity. This is known as the deliquescence point. For a single salt it varies only with temperature. At relative humidities below the deliquescence point, the particles are solid whereas above it they are fully liquid. We have only examined a single solid here (NH4NO3). However, the deliquescence point varies widely for different salts.
• The concentration of an aqueous solution of a salt at the deliquescence point is the same as the solubility of the salt.

Now proceed to Lesson 3 to explore how solid phases form in aqueous aerosols under different conditions.