Particulate Emission Control - Mathematics and Statistics for the Environment - page 491

17.2 PARTICULATE SIZE CHARACTERISTICS AND GENERAL CHARACTERISTICS
As we have said, particulate air pollution consists of solid and/or liquid matter in air or gas. Airborne
particles come in a range of sizes. From near molecular size, the size of particulate matter ranges
upward and is expressed in micrometers, µm—one millionth of a meter. For control purposes,
the lower practical limit is about 0.01 µm. Because of the increased difficulty in controlling their
emission, particles 3 µm or smaller are defined as ine particles. Unless otherwise specified, con-
centrations of particulate matter are by mass. Liquid particulate matter and particulates formed
from liquids (very small particles) are likely to be spherical in shape. In order to express the size
of nonspherical (irregular) particles as a diameter, several important relationships are discussed.
These include

• Aerodynamic diameter
• Equivalent diameter
• Sedimentation diameter
• Cut diameter
• Dynamic shape factor

17.2.1 a erodynamiC d iameter
Aerodynamic diameter ( d a ) is the diameter of a unit density sphere (i.e., density = 1.00 g/cm 3 ) that
would have the same settling velocity as the particle or aerosol in question. Note that because the
USEPA is interested in how deeply a particle penetrates into the lung, it is more interested in nomi-
nal aerodynamic diameter than in the other methods of assessing size of nonspherical particles.
Nevertheless, a particle's nominal aerodynamic diameter is generally similar to its conventional,
nominal physical diameter.

17.2.2 e quivalent d iameter
Equivalent diameter ( d e ) is the diameter of a sphere that has the same value of a physical property
as that of the nonspherical particle and is given by

1/3

6
π

V

e =

d (17.2)

where V is volume of the particle.

17.2.3 s edimentation d iameter
Sedimentation diameter or Stokes' diameter ( d s ) is the diameter of a sphere having the same ter-
minal settling velocity and density as the particle. In regards to density particles, it is called the
reduced sedimentation diameter, making it the same as aerodynamic diameter. The dynamic shape
factor accounts for a nonspherical particle settling more slowly than a sphere of the same volume.

17.2.4 C ut d iameter
Cut diameter ( d c ) is the diameter of particles collected with 50% efficiency (i.e., individual effi-
ciency ε i = 0.5) and that half penetrate through the collector (i.e., penetration P t = 0.5):

Pt i = 1 - ε i (17.3)