Page 51 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

274 Aerosols

workplace and air pollution. They are sometimes haz- ibility is a common sight. In most modern cities, smoke
ardous as explosive mixtures. plumes have largely been eliminated with pollution.
Both liquid and solid material can be suspended in a gas When smoke formation accompanies traces of noxious
by a variety of mechanisms. Aerosols produced under lab- vapors, it may be called a fume—for example, a metallic
oratory conditions or by specific generating devices may oxide developing with sulfur in a melting or smelting pro-
have very uniform properties that can be investigated rel- cess. The term fume is also used in a more general way to
atively easily by physical and chemical instrumentation. describe a particle cloud resulting from mixing and chem-
Natural aerosols found in the atmosphere are mixtures ical reactions of vapors diffusing from the surface of a
of materials from many sources that are highly heteroge- pool of liquid.
neous in composition and physical properties. Their char-
acterization has required the application of a variety of 3. Mists
measurement techniques and has been a major activity in
Suspensions of liquid droplets by atomization or vapor
modern aerosol science.
condensation are called mists. These aerial suspensions
often consist of particles larger than 1 µm in diameter, and
I. PHENOMENOLOGICAL ASPECTS relatively low concentrations are involved. With evapora-
tion of the droplets or particle formation by condensation
A. Classification of a vapor, higher concentrations of very small particles in
the submicrometer size range may be observed. In general,
1. Dusts
mists refer principally to large-particle suspensions such
Dusts are clouds of solid particles brought about by me- that historically particle size is the principal property dis-
chanical disintegration of material, which is suspended by tinguishing mists from smokes. If the mist has sufficiently
mixing in a gas. Examples include clouds of particles from high particle concentration to obscure visibility, it may be
the breakup of solids in crushing, grinding, or explosive called fog. Hazes in the atmosphere usually contain rel-
disintegration and the disaggregation of powders by air atively high concentrations of very small particles with
blasts. Dust clouds are often dramatic in form as storms absorbed liquid water. The name smog (smoke combined
rising from the earth’s surface and traveling hundreds of with fog) refers to a particulate cloud normally observed
miles. Generally, dusts are quite heterogeneous in com- over urban areas, where pollutants mix with haze and react
position and have poor colloidal stability with respect to chemically to contribute condensed material to the partic-
gravitational settling because they are generally made up ulate mixture.
of large particles. Yet, the lower range of their particle size
distribution may typically be submicroscopic. 4. Colloidal Stability
The term aerosol has been associated with F. G. Don-
2. Smokes
nan in connection with his work on smokes during World
In contrast to dusts, smokes cover a wide variety of aerial War I. An aerosol is regarded as an analogy to a liquid
dispersions dominated by residual material from burning, colloidal suspension, sometimes called a hydrosol. These
other chemical reactions, or condensation of supersatu- suspensions are relatively stable, with very low gravita-
rated vapors. Such clouds generally consist of smaller tional settling speeds and slow rates of coagulation. The
particles than dusts and are composed of material of low stability to gravitational settling is the principal criterion
volatility in relatively high concentrations. Because of the for defining an aerosol.
small size of the particles, smokes are more stable to grav- Low settling rate in itself is not adequate for defining
itational settling than dusts and may remain suspended for an aerosol. Additional criteria have emerged. For example,
an extended period of time. Examples include particulate the thermal agitation or Brownian motion of particles is
plumes from combustion processes, chemical reactions an important characteristic of aerosol particles. Brownian
between reaction gases such as ammonia and hydrogen motion becomes a factor for particle behavior of particles
chloride or ozone and hydrocarbon vapors, oxidation in less than 0.5 µm in diameter. Brownian motion essentially
a metallic arc, and the photochemical decomposition of provides the theoretical linkage between the idealized be-
materials such as iron carbonyl. An important measure havior of molecules and small particles. The mechanical
of smoke is particle size; the distribution in size is con- theory of large molecules and spheres in gases applies
strained to be smaller than 10 micrometers (µm) in diam- well to the behavior of very small particles in the submi-
eter to less than a tenth of a µm. Smokes normally have croscopic size range. This characterization is central to the
4
3
high concentrations, often exceeding 10 particles/cm . evolution of a large segment of particle science and tech-
In the atmosphere, smoke from chimneys obscuring vis- nology. Indeed, it forms the basis for explaining features of