1. The Challenge of Defining a Particle’s “Size”
For a perfect sphere, its size is easily described by its diameter. However, most aerosol particles are irregular and not spherical. To solve this, scientists use the concept of equivalent diameters. This method describes an irregular particle’s size by comparing one of its measurable properties to that of a hypothetical sphere that shares the same property.
Here are three common types of equivalent diameters:
| Equivalent Diameter Type | What It Really Means (Physical Meaning) |
| Equivalent volume diameter | The diameter of a sphere that has the same volume as the irregular particle (measured by a Coulter Counter). |
| Stokes diameter | The diameter of a sphere that falls through the air at the same speed due to gravity as the particle (measured by sedimentation and impactor). |
| Aerodynamic diameter | The diameter of a sphere with a standard density (unit specific gravity) that falls at the same speed as the particle. This is a crucial concept for understanding air quality and particle transport (measured by methods like sedimentation and impactors). |
Defining the size of a single particle is the first step, but to understand an aerosol’s impact, we must understand the entire mix of sizes present in the air.