Page 71 - 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

294 Aerosols

while the small particles bend with the high-speed flow as extinction energy, is equal to the scattered energy in the
it moves radially around the receiving tube. Fractions of spherical wave plus the energy of absorption.
different sizes are then deposited on separate filters. The light intensity is the conventional measure of the
Instead of using the virtual impactor approach, North energy of scattered light and, in cgs units, has units of
2
American air monitoring programs in the 1980s and later erg/cm sec. The intensity of scattered light is proportional
have adopted “simpler” reference methods that use the to the intensity of the incident light beam I 0 and the radial
weighing of filters in the laboratory. The filters are ob- distance expressed as:
tained from samplers equipped with an inlet device that
2
I = I 0 F(θ, φ, λ) x
provides for a sharp cut-point in particle entry for sam-
ples of particles <10 µm diameter or <2.5 µm diameter, where θ, φ are angular coordinates, λ the wavelength of
which are operated over a fixed time period of 24 hours. light, and x = 2πr/λ. In general, F(θ, φ, λ) depends on
The inlet fractionation is facilitated either by a carefully the wavelength of the incident beam and on the size, shape,
designed cyclone or by an impactor. The combination of and optical properties of the particle but not onr, the radial
the two samplers can give estimates of mass concentration distance from the particle. For spherical particles, there is
for fine-particle and coarse-particle concentrations. no dependence.
Recent advances in continuous mass monitors may re- The scattering function can be determined from theory
place the labor-intensive filter methods for air monitor- for certain important special cases. The performance of
ing. One of the promising devices for continuous moni- optical single-particle counters depends on the variation
toringisthetaperedelementoscillatingfiltermeasurement of the scattering function with angular position.
(TEOM). This method measures the change in natural os- Rayleigh scattering for x 1 and the large-particle ex-
cillation frequency of a suspended filter, which ideally is tinctionlawfor x
1provideusefullimitingrelationships
proportional to the mass of particles collected. TEOM in- for the efficiency factor. Aerosol light scattering, however,
struments have been deployed in the 1990s at selected sites is often limited by particles whose size is of the same or-
in North America and are undergoing extensive intercom- der as the wavelength of light in the optical range from
parison with the gravimetric filter method. 0.1–1 µm in diameter. In this range, Rayleigh’s theory is
not applicable since different parts of the particle inter-
act with different portions of an incident wave. Yet, such
5. Light Extinction and Optical Devices
particles are still too small for the large-particle scattering
Small particles scatter and absorb light. This phenomenon theory to be applicable. In such a situation the theory of
has been used to investigate aerosol behavior extensively Mie is applied. Expressions for the scattering and extinc-
since Tyndall’s work in the nineteenth century. In more tion are obtained by solving Maxwell’s electromagnetic
recent years, instruments have been built to take advantage theory for the regions inside and outside a homogeneous
of light interactions to deduce particle size distributions. sphere with suitable boundary conditions. Mie found that
Toappreciatehowsuchdeviceswork,weintroducecertain the efficiency factors are functions of x and the index
basic principles of light interaction with airborne material. of refraction alone. The calculations must be carried out
Basically, the scattering and absorption of light by in- numerically, and the results have been tabulated for spe-
dividual particles depend on their size and shape, their cific values of the refractive index.
index of refraction, as well as the wavelength of incident The intensity function in itself is not sufficient to char-
light. The total scattering and absorption from a beam of acterize the scattered light. Also needed are the polariza-
light by an aerosol cloud corresponds to the summation tion and phase of the scattered light. For measurement ap-
of the scattering from all particles of different size and plications including instrument design, the parameters of
refractive index. If the particle cloud is dilute enough, the most interest are the intensity function and the scattering
effects of multiple light scattering can be disregarded, and efficiency.
the summation of single particles suffices to describe the
interaction. 6. Single-Particle Optical Analyzers
The light attenuation process can be analyzed by con-
sidering a single particle of arbitrary size and shape ir- Particle sizing by means of light scattering on single parti-
radiated by a planar electromagnetic wave. The effect of cles was understood by 1900. In the meantime, the subject
the presence of the particle is to diminish the amplitude has been steadily developed. Since about 1960, optical
of the plane energy wave. At a distance large compared particle counters using white light illumination have been
with the particle diameter and the wavelength, the scat- commercially available. After the invention of the laser
tered energy appears as a spherical wave centered on the principle, several attempts were made to replace the white
particle and possessing a phase different from the inci- light illumination of scattering devices by coherent and
dent beam. The total energy lost by the plane wave, the monochromatic laser light illumination.