An aerosol is formally defined as a system of tiny particles suspended in a gas. You can think of it as the fine mist from a spray bottle or the dust motes you see dancing in a sunbeam. These suspended particles are everywhere, influencing everything from the air we breathe to the colors of a sunset. They form in two primary ways:

• Nucleation: This is a “bottom-up” process where particles form from the condensation of vapor molecules. It often occurs during combustion or photochemical reactions in the atmosphere. This method typically creates very small particles, with diameters smaller than 0.1 micrometers (μm). Because they are so numerous, these tiny particles tend to collide and stick together (a process called coagulation) very quickly.
• Comminution: This is a “top-down” process involving the mechanical breakdown of larger solid or liquid matter. Think of the grinding of rock into dust, sea spray being kicked up by the wind, or soil erosion. This method produces much larger particles, typically exceeding 10 μm.

The way a particle is formed gives a strong clue to its initial size, and it is this single characteristic—size—that governs almost every aspect of its behavior in the air.