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Aerodynamic Diameter

What is particle size?

It’s conceptually easy to assign a size to a spherical particle:

Measuring particle size with a ruler

It is however difficult to do this with an aggregate:

Aggregate Size

Equivalent diameters are used for particle metrology, derived from other measurands:

  • Electrical mobility diameter (dₘ), based on the deflection of a charged particle under an electric field in a sheath flow; small particles are deflected more (classifiable by the Differential Mobility Analyzer, DMA)

  • Aerodynamic diameter (dₐ ), based on the particles’ ability to change direction (i.e. their inertia) in a flow of gas; small particles are easier to deflect (classifiable by the Aerodynamic Aerosol Classifier, AAC)

The choice of metric is often arbitrary, governed by convention and the available measurement technologies.

Complex particles can even be classified by both aerodynamic diameter and mobiliy diameter by putting classifiers in series (known as a tandem classifier experiment). Below are shown some soot particles which have been first classified by an AAC, then by an DMA to further narrow down their structure into long chain-like aggregates. This kind of insight is invaluable for studying engineered nanoparticles like carbon nanotubes, or hazardous fibres such as asbestos.

Classifying by AAC and DMA in tandem

(data from, and courtesy of, Johnson, Zhang et al 2019)

Aerodynamic Diameter

The concept of aerodynamic size is perhaps best explained by the use of an impactor. Particles accelerate through a nozzle region, and the largest particles, unable to follow the gas streamlines due to their inertia, are impacted on a plate, whilst the particles with smallest aerodynamic diameter do follow the streamlines, and emerge.


Impactors are often used to collect samples of aerosol, sometimes with multiple impactors with varying size cuts arranged in the form of a cascade impactor (such as an Anderson Cascade Impactor)

The aerodynamic diameter of an irregular particle is defined as the diameter of a spherical particle with a density of 1000 kg/m³ and the same settling velocity as the irregular particle.

Aerodynamic diameter is the size metric of choice for many aerosol scientists, especially those working in inhalation, health effects & filtration. This is because aerodynamic diameter directly affects how in a filter, or where in a lung, particles are deposited.

Aerodynamic size increases with increasing particle density. Dense spheres have a larger aerodynamic diameter:

Different aerodynamic diameters with different densities

Classification by Aerodynamic Diameter

There are several instruments which can tell you what the aerodynamic particle size distribution (APSD) of an aerosol is, such as the Aerodynamic Particle Sizer® (APS™) and the Electrical Low Pressure Impactor (ELPI®). Indeed, the AAC can do this as well at unprecedented resolution and accuracy, when configured alongside a condensation particle counter (CPC) as a Scanning Aerodynamic Sizer Spectrometer (SASS).

However rather than just determining the sizes of particles in an aerosol stream, it is often desirable to be able to classify them for further online analysis.

For many years, the state of the art only allowed for “high pass” or “low pass” classification of aerosol particles by aerodynamic diameter, by the use of impactors (or cyclones), or “virtual impactors” – until the Cambustion AAC arrived nothing allowed a narrow range of sizes to be so classified:

Impactor, Virtual Impactor and AAC

The unique ability of the AAC to select particles by aerodynamic diameter over a narrow range whilst they still remain suspended as an aerosol allows size-selected lung exposure studies (for example, to inhlaer delivered drugs), or size selected in-vitro lung cell exposure to harmful aerosols:

AAC used in inhalation

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