A suspension of airborne particles, whether liquid droplets or solids, is generally referred to as an aerosol. As toxicants, airborne particles are more complex than pollutant gases and vapors because of the additional variables of particle size and mixed chemical composition. Size affects particle motion and the probabilities for physical phenomena such as coagulation, dispersion, sedimentation, and impaction onto surfaces. It is not possible to fully characterize a given particle by a single size parameter. For example, a particle's aerodynamic properties depend on density and shape as well as linear dimensions.
Typically, aerosols are composed of particles of many different sizes and are therefore called "heterodisperse" or "polydisperse." Different aerosols have different degrees of size dispersion. It is therefore necessary to specify at least two parameters in characterizing aerosol size: a measure of central tendency, such as a mean or median, and a measure of dispersion, such as a geometric standard deviation.
Many properties of particles, other than their linear size, can greatly influence their airborne behavior and their effects on the environment and health. For spherical particles, the surface increases with the square of the diameter. However, for an aerosol of given mass concentration, the total aerosol surface increases with decreasing particle size. Particle volume varies as the cube of diameter, therefore the few largest particles in an aerosol tend to dominate its volume concentration, and a particle's shape affects its motion and deposition probabilities. A particle's velocity in response to gravitational or inertial forces increases as the square root of its density. The diameter of a unit-density sphere having the same terminal settling velocity as the particle under consideration is equal to its aerodynamic diameter. Aerodynamic diameter is determined by the actual particle size, the particle density, and an aerodynamic shape factor.
Aerosols are generally classified in terms of their processes of formation. Dust is an aerosol formed by mechanical subdivision of bulk material into airborne fines having the same chemical composition. Dust particles are generally solid and irregular in shape and have diameters greater than 1 micrometer. A fume is an aerosol of solid particles formed by condensation of vapors formed at elevated temperatures. The primary particles are generally very small (less than 0.1 micrometer) and have spherical or characteristic crystalline shapes. Since they may be formed in high number concentrations, they often rapidly coagulate, forming aggregate clusters of low overall density.
Smoke is formed by condensation of combustion products, generally of organic materials. The particles are generally liquid droplets with diameters of less than 0.5 micrometer. Mist is droplet aerosol formed by mechanical shearing of a bulk liquid; for example, by atomization, nebulization, bubbling, or spraying. The droplet size can cover a very large range, usually from about 2 micrometers to greater than 50 micrometers.
Fog is an aqueous aerosol formed by condensation of water vapor on atmospheric nuclei at high relative humidities. The droplet sizes are generally greater than 1 micrometer. Smog is a popular term for a pollution aerosol derived from a combination of smoke and fog. The term is now commonly used for any atmospheric pollution mixture.
Aerosols have integral properties that depend upon the concentration and size distribution of the particles. Some integral properties such as light-scattering ability or electrical charge depend on other particle parameters as well. Some of the important integral properties are:
- Number concentration, which is the total number of airborne particles per unit volume of air, without distinction as to their sizes.
- Surface concentration, which is the total external surface area of all the particles in the aerosol, may be of interest when surface catalysis or gas adsorption processes are of concern. Aerosol surface is one factor affecting light-scatter and atmospheric-visibility reductions.
- Mass concentration, which is the total mass of all the particles in the aerosol, is frequently of interest. The mass of a particle is the product of its volume and density. If all of the particles have the same density, the total mass concentration is simply the volume concentration times the density. In some cases, such as "respirable," "thoracic," and "inhalable" dust sampling, the parameter of interest is the mass concentration over a restricted range of particle size.
(SEE ALSO: Ambient Air Quality [Air Pollution]; Hazardous Air Pollutants; Inhalable Particles [Sulfates]; National Ambient Air Quality Standards; Smog [Air Pollution]; Sulfur-Containing Air Pollutants [Particulates]; Total Suspended Particles [TSP])