Particle Size

A wide variety of methods have been used to measure the particle size of aerosols, and most of these have found an application in the specialized field of pharmaceutical product characterization. In general, these approaches result in the measurement of a particular diameter that may be defined according to the principle underlying the measurement [3].

Definitions of particle diameters derived by different methods have been described in detail [4]. The aerodynamic diameter is defined as the diameter of a unit-density sphere having the same settling velocity, generally in air, as the particle. This encompasses particle shape, density, and physical size, all of which influence the aerodynamic behavior of the particle. As a dynamic parameter, it can generally be linked with aerosol deposition and specifically with that in the lung [5].

An aerosol rarely consists of particles that are the same size, and usually a distribution of sizes around a mean is observed. The observed data may be fitted by statistical approximation to a distribution. The number of particles in a size range when plotted against the logarithm of the particle diameters frequently exhibit a normal (Gaussian) distribution. This is known as a log-normal distribution and is described by a parameter known as the geometric standard deviation. Theoretically, a monodisperse aerosol will exhibit a geometric standard deviation of 1; in practice, however, an accepted limit is 1.2 [6].

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