Aerosol#Aerodynamic diameter

The aerodynamic diameter is a measure to describe the behavior of a particle or body, which is denser than the surrounding gas. Examples: the sinking of a dust particle or a cobweb thread in stagnant air, the falling of a raindrop, spring ball or dandelion seed, dust separation by centrifugal force in a cyclone vacuum cleaner. When a particle accelerates vertically or centrifugally by gravity or centrifugal force of a rotating flow, it eventually reaches a relative velocity with respect to the surrounding gas, in which reverse the accelerating force and the flow resistance, ie The particles will not be further accelerated, with respect to the gas terminal velocity setting another depending on the shape, size and density difference of the particle.

The aerodynamic diameter of a particle is now defined as the geometric diameter of a spherical, rigid comparison particle of density 1 g / cm ³, which has the same settling velocity in air at normal pressure and temperature would have as the observed particles ( see also: Stokes equation).

The aerodynamic diameter is about his connection with Sink - speed and respirability among others important for the categorization of fine dust. The weighting function of fine grain size classes for PM10 or PM2.5 is defined by the aerodynamic diameter, the proportion of included in the evaluation of particle mass is at 10 mm or 2.5 mm in about 50%.

• Aerodynamics