Classical Aerodynamic Diameter

The diameter of a unit density sphere, p° = 1 g cm-3, having the same terminal velocity as the particle is defined as its classical aerodynamic diameter, Dca The classical aerodynamic diameter is then given by 

Dividing (9.108) by (9.106), one can then find the relationship between the classical aerodynamic and the Stokes diameter as 

For spherical particles we replace Ds, with Dp in this equation to find that 

For a spherical particle of nonunit density the classical aerodynamic diameter is different from its physical diameter and it depends on its density. Aerosol instruments like the cascade impactor and aerodynamic particle sizer measure the classical aerodynamic diameter of atmospheric particles, which is in general different from the physical diameter of the particles even if they are spherical. 

Aerodynamic Diameter Calculate the aerodynamic diameter of spherical particles of diameters equal to 0.01, 0.1, and 1 pm. Assume that their density is 1.5 gem-3, which is a typical average density for multicomponent atmospheric particles. 

---