A particle’s size is its most important characteristic because it dictates how it behaves in the atmosphere—how it moves, how it responds to forces, and how it interacts with its environment.

• How It Moves Tiny particles (less than about 0.5 µm) are so small that their movement is dominated by collisions with air molecules, causing them to follow a random, zigzagging path known as Brownian motion (diffusion). In contrast, larger particles are dominated by gravity and have a distinct settling velocity, meaning they tend to fall and settle out of the air. This is why the extremely small particles formed by nucleation are governed by diffusion, while the much larger particles created by comminution are dominated by settling.
• How It Responds to Forces A particle’s inertia is described by its relaxation time—the time it takes to respond to a change in external forces, like a shift in airflow. A more intuitive way to think about this is a particle’s stop-distance: the distance it would travel if projected into still air before stopping. Larger particles have more inertia and a longer stop-distance, so they resist changes in air direction. This is why a large dust particle might fail to follow the air around a sharp corner and instead hit the surface. Smaller particles have very little inertia and follow air currents almost perfectly.
• How It Interacts with Light Particle size is critical for optical phenomena like smog and visibility. Particles with a diameter around 0.5 µm are the most effective at scattering visible light. Because of this, aerosols with a high concentration of particles in this size range have the greatest impact on reducing visibility.

To fully describe an aerosol’s behavior, scientists need a way to represent the full range of particle sizes found in a sample.