Introduction

Atmospheric particles contain electrolytes, for example the salts NH4NO3 and NaCl and acids such as H2SO4 and HNO3, which are soluble in water. Their presence can lead to some or all of the particle existing as an aqueous solution, depending on the atmospheric conditions.

The amount of water in an aerosol particle depends on the nature of the electrolytes, the quantities present, and on the ambient relative humidity and temperature. The aerosol water determines the concentrations of the dissolved electrolytes and consequently the ion activities. These in turn control equilibria of volatile species such as NH3 and HNO3 with the surrounding atmosphere, and the formation of solid salts within the aerosol particle.

This lesson will help you to understand the relationship between the water content of a particle and relative humidity, and the influence on ion activities, for systems containing a single electrolyte at constant temperature. In part (a) of the lesson the output of the model, and the meaning of the various quantities presented, will be explained.

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 (http://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 four sets of calculations described in the links below, which should be done in the order listed.

a.  Aqueous solutions at a fixed relative humidity

b.  Increasing the amounts of electrolytes

c.  Reducing the relative humidity

d.  Properties over a range of relative humidities

Conclusion

You have completed Lesson 1, and learned:

• How to enter data into E-AIM and interpret the results for simple electrolyte/water systems.
• The uptake of water by a soluble electrolyte, and therefore its concentration in the aqueous phase, is controlled by the ambient relative humidity. The activity of any aqueous phase species is equal to its activity coefficient multiplied by its mole fraction, and in a system at equilibrium the water activity is equal to the relative humidity (expressed as a fraction).
• The concentration of an aqueous particle, at a fixed relative humidity, is independent of the amount of solute. However, this concentration (or uptake of water) can differ markedly between solutes (NH4NO3 and HNO3 in the examples here). These differences are greatest at low relative humidities.
• The equilibrium partial pressures of water and soluble gases such as NH3 and HNO3, and the saturation ratios of solids, can be calculated from the water and ion activities and the appropriate equilibrium constants.
• The activity coefficients of ions in solution tend to unity as the relative humidity tends to 100% and the solution becomes very dilute. In the very concentrated solutions that exist at moderate to low relative humidities activity coefficients of ions vary widely. For example, ion activity coefficients in NH4NO3 and HNO3 solutions here show opposite trends with increasing concentration.

Now proceed to Lesson 2, which examines how solid phases form in aqueous aerosols at different relative humidities.