2. Particle Size Characterization and Distribution
The most important single characterization of an aerosol particle is its size. While a simple diameter suffices for a spherical particle, non-spherical particles require more complex definitions, often based on geometric properties or an equivalent spherical particle with identical dynamic or physical properties.
Defining Particle Size: Characteristic Diameters
A variety of standardized diameters are used to characterize aerosol particles, each with a specific physical meaning and corresponding measurement method.
| Diameter Type | Definition | Measurement Method |
| Stokes Diameter | Diameter of a sphere with the same gravitational settling velocity as the particle. | Sedimentation, Impactor |
| Aerodynamic Diameter | Diameter of a sphere with unit specific gravity (1 g/cm³) and the same settling velocity as the particle. | Sedimentation, Impactor |
| Equivalent Volume Diameter | Diameter of a sphere with the same volume as the particle. | Coulter Counter |
| Equivalent Surface Area Diameter | Diameter of a sphere with the same surface area as the particle. | Absorption, Permeability Method |
| Equivalent Projection Area Diameter | Diameter of a circle with the same projected area as the particle. | Light Extinction |
| Electrical Mobility Equivalent Diameter | Diameter of a sphere with the same electrical mobility as the particle. | Electrical Mobility Analyzer |
| Equivalent Diffusion Diameter | Diameter of a sphere with the same penetration as the particle. | Diffusion Battery |
Size Distributions
Particle size distributions describe the relative abundance of particles across a range of sizes. These can be defined on a number basis (using total particle number, N) or a mass basis (using total mass, M).
Many aerosol distributions are well-represented by the log-normal distribution, which is particularly useful as it precludes negative particle sizes. When plotted on log-probability paper, a log-normal distribution appears as a straight line. Key parameters of this distribution are:
- Geometric Mean Diameter (Dg) or Number Median Diameter (NMD): The particle size at the 50th percentile of the cumulative distribution.
- Geometric Standard Deviation (sg): A measure of the spread or width of the distribution. It is calculated from the slope of the line on a log-probability plot, often using the formula: sg = Dp(at 84.13%) / Dp(at 50%).
The distribution on a number basis can differ significantly from the distribution on a mass basis. The Mass Median Diameter (MMD) can be calculated from the NMD and sg using the following conversion formula:
ln(MMD) = ln(NMD) + 3(ln sg)²
Based on a given size distribution, various average diameters can be calculated to represent the central tendency of the particle population.
| Average Diameter Name | Common Use / Description |
| Number Mean Diameter (D1) | Simple arithmetic mean of diameters. |
| Surface Mean Diameter (D3) | Represents the mean volume-to-surface ratio. Also known as the Sauter mean diameter. |
| Mass Mean Diameter (D4) | Mean diameter weighted by particle mass (or volume). |
| Number Median Diameter (NMD) | Geometric mean; the 50th percentile by number. |
| Mass Median Diameter (MMD) | The 50th percentile by mass. |