Comparing Laser Diffraction and Aerodynamic Sizing Techniques

Traditional aerosol measurement techniques such as cascade impaction measure the size of particles based on their behaviour in an air-stream. Particles are therefore classified aerodynamically and, in the case of cascade impaction, the mass of particles with a given aerodynamic diameter is reported. The Mass Median Aerodynamic Diameter (MMAD) is then reported as the average particle size.

In assessing how laser diffraction and aerodynamic techniques compare, it is important to understand how the aerodynamic diameter is derived. This is defined as “the diameter of the equivalent sphere of unit density which falls though air at the same terminal velocity as the particle under study”. Laser diffraction will therefore agree with aerodynamic techniques when measuring spherical particles of unit density, for example water droplets. If the particle density is lower than unity (e.g. if the particles are porous) then the aerodynamic size will generally be smaller than the volume equivalent size reported by laser diffraction. The same is true if non-spherical particles are measured. Here, the particles tend to orientate themselves such that the smallest cross-sectional area faces the direction of the airflow. As such, the aerodynamic diameter reported for high-aspect ratio particles will always be lower than the volume-based diameter.

It is possible to correlate the median particle size reported by laser diffraction with that reported by aerodynamic techniques if the true particle density is know

where ρ is the particle density and K is a constant which can be considered to be a shape factor. For liquid water droplets ( ρ /K) will be unity and the two techniques can be directly compared. For dry powder particles ( ρ /K) can be significantly less than unity.