Definitions
An aerosol is defined as a suspension of solid or liquid particles in a gas. This includes both the particles and the suspending gas, which is usually air. The name aerosol is thought to have been first used by F.G. Donnan during World War I to describe clouds of microscopic particles in air. This term was an analogy to a liquid colloid suspension called a hydrosol. A primary aerosol has particles that are introduced directly into the gas and secondary aerosols are formed when gas-to-particle conversion occurs.
There are several measures of aerosol concentration. The most important in the area of environmental science and health is the mass concentration (M), defined as the mass of particulate matter per unit volume with units such as μg/m3. Also commonly used is the number concentration (N), the number of particles per unit volume with units such as number/m3 or number/cm3.
The size of particles has a major influence on their properties and the aerosol particle radius or diameter (dp) is a key property used to characterise aerosols. If all the particles in an aerosol are the same size it is known as monodisperse and this type of aerosol can be produced in the laboratory. Most aerosols however are polydisperse, i.e. they have a range of particle sizes. While liquid droplets are nearly always spherical, solid particles have a variety of shapes and to understand their properties, a equivalent diameter is used. The equivalent diameter is the diameter of a regular particle which has the same value of some physical property as the irregular particle. The equivalent volume diameter (de) is defined as the diameter of a sphere having the same volume as that of the irregular particle. Also commonly used is the aerodynamic diameter.
Read more about this topic: Aerodynamic Diameter
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