2.0 Characterization of Particle Size
Particle size is the most critical single parameter for characterizing an aerosol, as it dictates everything from a particle’s transport and deposition to its optical properties and health effects. For a perfect sphere, size is unambiguously defined by its diameter. However, most real-world aerosol particles are non-spherical, which presents a significant challenge for characterization. To address this, aerosol science employs a range of characteristic and equivalent diameters. These definitions describe a particle either by the geometric dimensions of its silhouette or by the diameter of a hypothetical sphere that would exhibit the same dynamic or physical properties.
The following table summarizes the key definitions of particle size used in aerosol science, their physical meaning, and the measurement methods that correspond to them.
| Diameter Definition | Physical Meaning | Corresponding Measuring Method |
| Feret diameter | An unidirectional diameter measured along a fixed line for particles at random orientation; has no meaning for a single particle. | Microscopy |
| Martin diameter | An unidirectional diameter defined as the length of a chord that divides the particle’s projected area into two equal halves. | Microscopy |
| Equivalent projection area dia. (Heywood diam.) | The diameter of a circle that has the same area as the particle’s projected silhouette. | Light extinction methods |
| Equivalent surface area dia. (specific surface diam.) | The diameter of a sphere possessing the same total surface area as the particle. | Gas adsorption or permeability methods |
| Equivalent volume dia. | The diameter of a sphere having the same volume as the particle. | Coulter Counter (electric resistance method) |
| Stokes diameter | The diameter of a sphere having the same density and gravitational settling velocity as the particle. | Sedimentation methods, Impactors |
| Aerodynamic diameter | The diameter of a sphere with unit density (1 g/cm³) that has the same gravitational settling velocity as the particle. | Sedimentation methods, Impactors |
| Electrical mobility equivalent dia. | The diameter of a sphere that exhibits the same electrical mobility as the particle. Defined by De = npeCc/3pmBe. | Electrical mobility analyzer |
| Equivalent diffusion dia. | The diameter of a sphere that has the same penetration (or loss rate due to diffusion) as the particle. | Diffusion battery |
| Equivalent light scattering dia. | The diameter of a standard sphere (e.g., polystyrene latex) that produces the same intensity of scattered light as the particle. | Light scattering instruments |
In practice, a crucial distinction exists between diameters derived from geometric analysis and those based on dynamic properties. Geometric diameters like the Feret and Martin diameters are typically obtained by analyzing microscopic images of collected particles. In contrast, dynamic diameters such as the Stokes and aerodynamic diameters are inferred from the particle’s behavior as it moves through a fluid. These “equivalent” diameters are often more relevant for predicting how a particle will be transported in the atmosphere or collected in sampling devices.
Defining the size of a single irregular particle is a foundational step, but understanding an entire aerosol population requires the use of statistical methods to describe the distribution of these sizes.