The aerodynamic diameter of an irregular particle is defined as the diameter of the spherical particle with a density of 1000 kg/m and the same settling velocity as the irregular particle.
Neglecting the slip correction, the particle settles at the terminal velocity proportional to the square of the aerodynamic diameter, d:
where
: = standard particle density (1000 kg/m).
This equation gives the aerodynamic diameter:
One can apply the aerodynamic diameter to particulate pollutants or to inhaled drugs to predict where in the respiratory tract such particles deposit. Pharmaceutical companies typically use aerodynamic diameter, not geometric diameter, to characterize particles in inhalable drugs.