Definition and Formation

An aerosol is a suspension of tiny solid or liquid particles in a gas. These particles, also referred to as particulate matter, are larger than molecular dimensions but are typically microscopic or submicroscopic. Pure water is excluded from this definition.

There are two distinct mechanisms of aerosol formation, which result in particles of characteristically different sizes:

• Nucleation from Vapor: This process, driven by photochemistry or combustion, involves the condensation of molecules from a gaseous state to form new particles. These particles are typically very small, with diameters of less than 0.1 µm. Due to their high number concentration, they tend to coagulate rapidly into larger particles.
• Comminution of Matter: This mechanical process involves the breakdown of larger solid or liquid matter through grinding, erosion, or spraying (e.g., sea spray). Particles formed this way are much larger, with diameters often exceeding 10 µm, and they tend to settle quickly from the atmosphere. Surface tension generally limits the smallest particle size achievable through mechanical means to approximately 1 µm.

Atmospheric Sources and Distribution

Atmospheric aerosols originate from a wide range of natural and anthropogenic sources. The distribution of these particles typically shows a minimum around a diameter of 1 µm, which separates the fine particles generated by molecular processes from the coarse particles generated by mechanical processes.

• Fine Particles (< 1 µm): Major sources include forest fires, industrial emissions, heavy automotive traffic, and atmospheric chemical reactions that lead to intense smog.
• Coarse Particles (> 1 µm): These are primarily generated by natural phenomena such as dust storms, sand storms, volcanic plumes, and sea salt from oceans.

The surface area distribution of aerosols from these sources varies significantly, as illustrated by data comparing continental backgrounds to intensely polluted urban environments.