Page 72 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: FWD Revised Pages
Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

Aerosols 295

To examine the influence of the optical properties and The attenuation of a light beam is given in terms of
the shape of a particle on the response of a light-scattering the extinction coefficient, sometimes called the attenu-
device, it is helpful to introduce some definitions of equiv- ation coefficient or turbidity, and it is a key measure
alency in particle size. Particle-scattering cross section is of the optical behavior of particulate systems. In terms
related to the geometric radius or diameter by some mea- of the separate contributions for particle scattering and
sured relation, which depends on a calibration refraction absorption,
index and spherical shape of the particles. For a nonspher-
b ext = b sp + b ap
ical particle, a light-scattering diameter is defined that is
related to equivalent spheres, but the particle shape is de- if the contribution of gas absorption and scattering is dis-
scribed by an optical shape factor. Thus, an optical counter regarded. Here, the particle scattering coefficient b sp and
measures a light-extinction cross section and not particle the absorption coefficient b ap are functions of wavelength
size directly. Unless the counter is calibrated for a given of incident radiation. The particle light-scattering coeffi-
aerosol particle material, its optical size generally will not cient has been measured by a variety of instruments. One
be the same as the geometric size. simple device that was invented many years ago is the
The size resolution ideally depends on the monotonic intergrating nephelometer. The absorption coefficient is
nature of the response curve. Since the counter response difficult to determine but has been obtained through anal-
is strongly dependent on the index of refraction of parti- ysis of transmissometer data or inferred from absorptivity
cles of a given size, the size discrimination will be quite measured from filter-collected material.
variable for an aerosol of heterogeneous composition.
Single-particle optical analyzers are especially useful 8. Remote Sensing Techniques
for continuous measurement of particles of uniform phys-
ical properties. However, as discussed earlier, uncertain- The development of the relationships between scattered
ties develop in the measurement of particle clouds that light and aerosols has stimulated the use of radiation trans-
are heterogeneous in composition because the refractive fer theory for remote sensing of particles in planetary at-
index may vary from particle to particle. Thus, in mak- mospheres. Highly sophisticated experimental and theo-
ing atmospheric aerosol measurements, workers have as- retical techniques have emerged for the interpretation of
sumed an “average” refractive index characteristic of the observations of sunlight and artificial light sources in the
mixture to estimate a calibration curve or have reported earth’s atmosphere. A description of their application de-
data in terms of the equivalent particle diameter for a pends on further development of the concepts of radiant
standard aerosol, such as suspended polystyrene latex energy transfer.
spheres. According to one simple concept, remote sensing de-
pendsonlightattenuationasafunctionoflightpathlength.
The reduction in the intensity of the light beam passing
7. Light Transmission and Nephelometry
through an aerosol is obtained by integrating between any
The extinction of a light beam or incident radiation asso- two points along the beam, L 1 and L 2 ,
ciated with a cloud of particles basically involves a mea- −τ
I 2 = I 1 e
sure of a moment of the particle number–size distribution

roughly proportional to the surface concentration. The ex- where the optical thickness τ = b ext dz is a dimension-
tinction of a light beam is well-developed basis for semi- less quantity (z is the path length of light); b ext has been
quantitative measurement of particles suspended in gases. kept inside the integral sign to show that it can vary in
Devices for these purposes, including smoke photometers, space with the aerosol particle concentration.
have been available commercially for many years. They For aerosols in the atmosphere, the light-scattering co-
may take many forms, one of which is the transmissome- efficient empirically is found to be proportional to particle
ter, or opacity instrument. mass concentration in the range of diameter below 10 µm.
Transmissometers in simplest form consist of a light This relationship is useful for coarse estimates of airborne
source and a detector located some distance along the axis particle concentrations.
of the light beam. Among their applications, these devices Remote sensing is achieved by the three groups of tech-
are commonly used at airports to provide data in visual niques listed in Table III: (1) ground-based passive optical
ranging conditions and are used to measure particle load- sensors, (2) airborne passive sensors (aircraft, balloon, or
ings in smokestacks. When calibrated for the type of par- artificial satellite), and (3) active sensor systems, employ-
ticles present in this aerosol, a semiquantitative measure ing a controlled or manmade light source. The instrumen-
of the particulate emissions can be made, with knowledge tation designed and built for such purposes is diverse and
of the volumetric gas flow. ingenious. It has occupied the thoughts of astronomers,