The Impact of Aerosols
The impact of aerosols on our daily lives is large, as our activities are performed in an atmospheric sea containing gases and particles (Table 1.1). The particles, liquid and solid, organic and inorganic, viable and nonviable, influence the environment. Natural particle phenomena include cloud formation, the role of particles in the water cycle, the shaping of land by wind, pollination of plants, and the distribution of seeds and spores. Human uses of aerosols include the atomization of fuels prior to combustion, the application of paints, cosmetics, medicines, insecticides, and lubricants; and scientific uses.
Unfortunately, aerosols often cause problems which resist eradication. Among these are infectious diseases including the common cold, influenza, viral pneumonia, measles, mumps, and tuberculosis. Other diseases in which inhaled particles often play a central role are bronchitis, pulmonary emphysema, asthma, diffuse interstitial fibrosis, alveolitis, silicosis, anthracosilicosis, berylliosis, farmers lung, byssinossis, lung cancer, and nasal cancer.
Size Regimes
The great diversity in particle size, shape, and composition makes it impossible to describe aerosol behavior simply. As a starting point, one can divide aerosols into regimes (Table 1.2). These regimes, which encompass given size ranges, are each associated with sets of equations that describe the physical behavior of aerosols. An important dimensionless parameter, the Knudsen number, Kn, which relates the particle radius, rp, to the molecular mean free-path of the suspending gas, λg, is given by:
{\rm{Kn}} = {{{\lambda _{\rm{g}}}} \over {{{\rm{r}}_{\rm{p}}}}}<$$>\end{equation}?>![]()
Cloud—Any free (not spatially confined) aerosol system with a definite overall shape and size. Rain clouds and smoke rings are examples.
Colloid—A dispersion of liquid or solid particles in a gas, liquid, or solid medium that has all of the following properties: slow settling, large surface to volume ratio, invisibility to the unaided eye, and producing scattering of a light beam. Examples include smoke, milk, and gelatin.
Several reference books on aerosols have been published. The basic theoretical reference is a work by Nicholai A. Fuchs (1964) entitled The Mechanics of Aerosols, which was translated from Russian into English by R.E. Daisley and Marina Fuchs and edited by C.N. Davies. A variety of additional books, some general and some specialized, are presented in Table 1.3. Although not exhaustive, the listed references cover most problems that arise in studies with aerosols.