Introduction

Atmospheric particles contain a wide range of compounds of which inorganic electrolytes, such as ammonium sulphate and sodium chloride, are the most soluble. In a humid atmosphere the particulate salts may exist either as solid crystals or as aqueous droplets. Volatile acids and alkalis such as nitric acid and ammonia are also present as gases. The partitioning of these compounds between the solid, aqueous, and vapour phases determines their environmental effects and lifetime in the atmosphere, and is a complex function of temperature, relative humidity, and concentration.

The Extended Aerosol Inorganics Model (E-AIM) programs were developed by Simon Clegg and Anthony Wexler to help atmospheric scientists predict the water content, phase state (solid or liquid), and partitioning of the inorganic components of aerosol systems. These programs can also assist students in understanding this partitioning behaviour and how it is related to the chemical thermodynamics of the compounds and their mixtures with water. These topics are covered in many air pollution and atmospheric science courses, and a basic understanding of thermodynamic equilibrium is assumed.

Each of the four E-AIM models on this site consists of a set of equilibrium constants for all possible reactions between the water, aqueous ions, solids, and gases. These equilibrium constants are combined with sets of equations to calculate the activity coefficients of the liquid phase species. The models determine the equilibrium composition of each system, which is specified as an initial ionic composition, a fixed relative humidity (RH) or total amount of water, and temperature.

Using the Tutorial

Resize your browser window so that it occupies the left hand half of your screen. Then click on one of the lessons below. We recommend following the lessons in the order given. (Note that Lesson 1 explains the meaning of the printed output of the model in detail.)