1. Why Particle Size is the Master Variable
In aerosol science, size is the most important single characterization of an aerosol particle. However, there’s a challenge: most particles are not perfect spheres. Describing an irregularly shaped speck of dust with a single “diameter” isn’t straightforward.
To solve this, scientists use the concept of equivalent diameters. This clever approach describes an irregular particle by the diameter of a hypothetical sphere that behaves in the same way under specific physical conditions. It allows for consistent measurement and comparison.
Here are three common types of equivalent diameters:
| Diameter Type | What It Measures | Simple Analogy |
| Equivalent Volume Diameter | The diameter of a sphere with the same total volume as the particle. | “How much space does the particle actually take up?” |
| Stokes Diameter | The diameter of a sphere that falls through the air at the same speed as the particle. | “How does the particle behave when settling under gravity?” |
| Aerodynamic Diameter | The diameter of a sphere with a standard density (1 g/cm³) that settles at the same speed as the particle. | “A standardized ‘behavioral’ size that’s useful for comparing different types of particles.” |
The power of these equivalent diameters is that they allow scientists to compare apples and oranges. A fluffy, low-density soot particle and a small, dense metallic particle might have vastly different shapes and compositions, but they can have the same aerodynamic diameter if they settle in the air at the same speed. This allows for a standardized way to predict and measure particle behavior regardless of their origin.
This concept of an ‘equivalent’ behavioral size is critical, because as we’ll see next, a particle’s diameter is the primary factor determining which physical forces will dictate its movement through the air.