For a perfect sphere, size can be unambiguously described by its geometric diameter. However, most aerosol particles are non-spherical, rendering a single geometric value insufficient. To address this, aerosol science employs the concept of characteristic diameters, or equivalent diameters. This framework defines a particle’s size based on a hypothetical sphere that exhibits an equivalent physical property, such as settling velocity, surface area, or light-scattering intensity. This approach is fundamental to interpreting data, as the choice of measurement instrument predetermines which equivalent diameter is being measured. The most common equivalent diameters are defined below.

• Geometric Diameters These diameters are derived directly from a two-dimensional projection or silhouette of a particle, typically observed under a microscope. The Feret diameter is a unidirectional measure determined by the distance between two parallel tangents on opposite sides of the particle silhouette along a fixed line. The Martin diameter is the length of a chord that divides the projected area of the particle into two equal halves.
• Equivalent Area Diameters These diameters relate the particle’s size to a sphere with an equivalent area. The equivalent projection area diameter, also known as the Heywood diameter, is the diameter of a circle having the same area as the particle’s projected silhouette. The equivalent surface area diameter is the diameter of a sphere that has the same total surface area as the particle.
• Equivalent Volume Diameter This is the diameter of a sphere having the same volume as the particle. This diameter corresponds to the size measured by instruments like the Coulter Counter, which measures volume by detecting changes in electrical resistance as a particle displaces a conductive liquid.
• Stokes Diameter This is the diameter of a sphere that has the same gravitational settling velocity as the particle in question. It is an important dynamic diameter measured by methods based on sedimentation and inertial impaction.
• Aerodynamic Diameter Closely related to the Stokes diameter, the aerodynamic diameter is defined as the diameter of a unit-density sphere (1 g/cm³) that has the same gravitational settling velocity as the particle. This is the standard diameter measured by impactors and other inertial separation devices.
• Electrical Mobility Equivalent Diameter This is the diameter of a sphere that possesses the same electrical mobility (velocity per unit electric field) as the particle. It is the characteristic size obtained by an electrical mobility analyzer.
• Equivalent Diffusion Diameter This diameter is defined for a sphere that exhibits the same diffusional loss, or penetration, as the particle when passing through a device like a diffusion battery. It is most relevant for very small particles where Brownian motion is significant.
• Equivalent Light Scattering Diameter This is the diameter of a standard calibration sphere, such as one made of Polystyrene Latex (PSL), that produces the same intensity of scattered light as the particle when illuminated by a light beam. This is the parameter measured by optical particle counters.

Each of the measurement techniques discussed in the following section is designed to measure one or more of the specific equivalent diameters defined here.