TABLE OF CONTENTS Aerosols are suspensions of small particles in gases. They are formed by the conversion of gases to particles or by the disintegration of liquids or solids. They may also result from the resuspension of powdered material or the breakup of agglomerates. Formation from the gas phase tends to produce much finer particles than do disintegration processes (except when condensation takes place directly on existing particles). Particles formed directly from the gas are usually smaller than 1 ?m in diameter (1 micron = 1 micrometer = 10 ?4 cm, designated by the symbol 1 ?m).
The many words employed to describe particulate systems attest to their ubiquity and to the impression they have made on humans from early times. Smoke, dust, haze, fume, mist, and soot are all terms in common use with somewhat different popular meanings. Thus dust usually refers to solid particles produced by disintegration processes, while smoke and fume particles are generally smaller and formed from the gas phase. Mists are composed of liquid droplets. Soot usually refers to small carbon particles generated in fuel combustion but is now frequently used to describe very fine solid particles of silica and other inorganic oxides generated intentionally in industrial processes. In this text, however, we will rarely employ these special terms because of the difficulty of exact definition and the complexity of many real systems composed of mixtures of particles. Instead, we employ the generic term aerosol to describe...
