Encyclopedia of Environmental Science and Engineering, Volume I and II

TABLE 1

Measures of particle size

Definition of characteristic diameters

Physical meaning and corresponding

measuring method

geometric size ()/,(bl


 233111222 blt)/,(),/(/ / /),,{(blt^13 / l

b t lb lb bt l

tt/)}6

Feret diam. unidirectional diameter: diameter of particles

at random along a given fixed line, no

meaning for a single particle.

Martin diam. unidirectional diameter: diameter of particles

as the length of a chord dividing the

particle into two equal areas.

equivalent diam. equivalent projection area diam.

(Heywood diam.)

diameter of the circle having the same area as

projection area of particle, corresponding

to diam. obtained by light extinction.

equivalent surface area diam.

(specific surface diam.) (s/p)1/2

diameter of the sphere having the same

surface as that of a particle, corresponding

to diam. obtained by absorption or

permeability method.

equivalent volume diam.

(6v/p)1/3

diameter of the sphere having the same

volume as that of a particle,

corresponding to diam. obtained by

Coulter Counter.

breadth: b

length: l

Stokes diam. diameter of the sphere having the same

gravitational setting velocity as that of a

particle,

Dst  [18 mvt/g(rp  rf)Cc]1/2, obtained by

sedimentation and impactor.

(continued)

b

l

t

AEROSOLS

An aerosol is a system of tiny particles suspended in a gas.

Aerosols or particulate matter refer to any substance, except

pure water, that exists as a liquid or solid in the atmosphere

under normal conditions and is of microscopic or submicrosco-

pic size but larger than molecular dimensions. There are two

fundamentally different mechanisms of aerosol formation:

• nucleation from vapor molecules (photochemis-
  try, combustion, etc.)


• comminution of solid or liquid matter (grinding,
  erosion, sea spray, etc.)

Formation by molecular nucleation produces particles of

diameter smaller than 0.1 m m. Particles formed by mechani-

cal means tend to be much larger, diameters exceeding 10 m m

or so, and tend to settle quickly out of the atmosphere. The

very small particles formed by nucleation, due to their large

number, tend to coagulate rapidly to form larger particles.

Surface tension practically limits the smallest size of particles

that can be formed by mechanical means to about 1 m m.

PARTICLE SIZE DISTRIBUTION

Size is the most important single characterization of an aero-

sol particle. For a spherical particle, diameter is the usual

reported dimension. When a particle is not spherical, the size

can be reported either in terms of a length scale characteristic

of its silhouette or of a hypothetical sphere with equivalent

dynamic properties, such as settling velocity in air.

Table 1 summarizes the physical interpretation for a

variety of characteristic diameters. The Feret and Martin

diameters are typical geometric diameters obtained from

particle silhouettes under a microscope.

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