AEROSOL

A suspension of particles, liquid or solid, in a gas. The term implies a degree of permanence in the suspension, which puts a rough upper limit on particle size at a few tens of micrometers at most
 (1 micrometer = 0.00004 in). Thus in proper use the term connotes the ensemble of the particles
and the suspending gas.The atmospheric aerosol has two major components, generally referred to as coarse and fine particles, with different sources and different composition. Coarse particles result
from mechanical processes, such as grinding. The smaller particles are ground, the more surface they have per unit of mass. Creating new surface requires energy, so the smallest average size that can be created by such processes is limited by the available energy. It is rare for such mechanically generated particles to be less than 1 mm (0.00004 in.) in diameter. Fine particles, on the other hand, are formed by condensation from the vapor phase. For most substances, condensation is difficult from a uniform gaseous state; it requires the presence of pre-existing particles on which the vapors can deposit. Alternatively, very high concentrations of the vapor are required, compared with the concentration in equilibrium with the condensed material.Hence, fine particles form readily in combustion processes when substances are vaporized. The gas is then quickly cooled. These can then serve as nuclei for the formation of larger particles, still in the fine particle size range, in the
presence of condensable vapors. However, in the atmosphere such particles become rapidly more scarce with increasing size, and are relatively rare in sizes much larger than a few micrometers. At about 2 mm (0.00008 in.), coarse and fine particles are about equally abundant.Using the term strictly, one rarely samples the atmospheric aerosol, but rather the particles out of the aerosol.
The presence of aerosols is generally detected by their effect on light. Aerosols of a uniform particle size in the vicinity of the wavelengths of visible light can produce rather spectacular optical effects. In the laboratory, such aerosols can be produced by condensation of the heated vapors of certain oils
on nuclei made by evaporating salts from heated filaments. If the suspending gas is cooled quickly, particle size is governed by the supply of vapor compared with the supply of nuclei, and the time available for condensation to occur. Since these can all be made nearly constant throughout the gas, the resulting particles are quite uniform. It is also possible to
produce uniform particles by spraying a dilute solution of a soluble material, then evaporating the solvent. If the spray head is vibrated in an appropriate frequency range, the drops will be uniform in size, with the size controlled by the frequency of vibration and the rate of flow of the spray. Obviously, the final particle size is also a function of the concentration of the sprayed solution.