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Module 3: Characteristics of Particles - Aerodynamic Diameter
Lesson Material
Practice
Problems
Objective
- Calculate the aerodynamic diameter of particles.
Particles emitted from air pollution sources and formed by natural
processes have a number of different shapes and densities as indicated
in Figure 1.
The photomicrograph below shows a variety of spherical particles and irregularly shaped particles collected on a polycarbonate filter.
Defining particle size for spherical particles is easy; it is simply the diameter of the particle. For non-spherical particles, the term "diameter" does not appear to be strictly applicable. For example, what is the diameter of a flake of material or a fiber? Also, particles of identical shape can be composed of quite different chemical compounds and, therefore, have different densities. The differences in shape and density could introduce considerable confusion in defining particle size.
In air pollution control, it is necessary to use a particle size definition that directly relates to how the particle behaves in a fluid such as air. The term "aerodynamic diameter" has been developed by aerosol physicists in order to provide a simple means of categorizing the sizes of particles having different shapes and densities with a single dimension. The aerodynamic diameter is the diameter of a spherical particle having a density of 1 gm/cm3 that has the same inertial properties [i.e. terminal settling velocity (discussed later)] in the gas as the particle of interest.
The aerodynamic diameter for all particles greater than 0.5 micrometer can be approximated using the following equation. Refer to aerosol textbooks to determine the aerodynamic diameter of particles less than 0.5 micrometer.
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Where:
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Where:
Particle density affects the motion of a particle through a fluid and is taken into account in Equation 1. The Stokes diameter for a particle is the diameter of the sphere that has the same density and settling velocity as the particle. It is based on the aerodynamic drag force caused by the difference in velocity of the particle and the surrounding fluid. For smooth, spherical particles, the Stokes diameter is identical to the physical or actual diameter.
Inertial sampling devices such as cascade impactors are used for particle sizing. These sampling devices determine the aerodynamic diameter. The term "aerodynamic diameter" is useful for all particles including fibers and particle clusters. It is not a true size because "non-spherical" particles require more than one dimension to characterize their size.
Note: For the remainder of these modules, the terms particle diameter and particle size refer to the aerodynamic diameter unless otherwise stated. The terms PM10 and PM2.5 also use the aerodynamic diameter formats. (PM10 and PM2.5 are discussed later in this Module.)
Particles that appear to have different physical sizes and shapes can have the same aerodynamic diameter as illustrated below.
Conversely some particles that appear to be visually similar can have somewhat different aerodynamic diameters as illustrated below.
Practice Problems
Aerodynamic Diameter
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.