An aerosol is a system of solid or liquid particles suspended in a gas. These systems are formed through two primary mechanisms: the nucleation of new particles from vapor molecules and the comminution, or mechanical breakdown, of larger matter. Formation by molecular nucleation, often through combustion or photochemical reactions, typically produces particles smaller than 0.1 µm, while mechanical comminution processes, such as in grinding or sea spray, generate much larger particles, often exceeding 10 µm in diameter. Of all the properties that define an aerosol, particle size is the most critical characteristic, dictating its transport, lifetime, and interaction with light and biological systems. However, the definition and measurement of “size” are far from simple, particularly for the irregularly shaped particles common in the environment. This complexity has given rise to a diverse array of measurement methodologies, each founded on different physical principles.

The choice of a measurement technique is not a trivial one. Different instruments exploit different physical or dynamic properties of a particle, and as a result, they measure different “equivalent” diameters. For a non-spherical particle, the size reported by an optical instrument will often differ from that reported by an inertial or electrical one. Understanding these differences is fundamental to the accurate interpretation of aerosol data. This review will first establish a clear framework for the various definitions of particle size before critically evaluating the primary instruments designed to measure them.