The study of aerosols—systems of fine particles suspended in a gas—is of critical importance for professionals across environmental science, atmospheric chemistry, and industrial engineering. These ubiquitous particles play a central role in a vast range of phenomena, from influencing Earth’s climate and reducing atmospheric visibility to determining the efficacy of industrial processes and impacting human health. This monograph provides a comprehensive overview of the fundamental principles of aerosol science, covering particle characterization, dynamics, and the foundational measurement techniques used in the field.

An aerosol is formally defined as a system of tiny particles suspended in a gas. The term particulate matter refers to any substance, with the exception of pure water, that exists as a liquid or solid in the atmosphere under normal conditions. These particles are microscopic or submicroscopic in size, though they are larger than individual molecules. Aerosols are formed through two fundamentally different mechanisms:

1. Nucleation from Vapor: This bottom-up process involves the condensation of molecules from a vapor phase, often driven by photochemical reactions or combustion. This mechanism typically produces very fine particles with diameters smaller than 0.1 μm.
2. Comminution of Matter: This top-down process involves the mechanical breakdown of larger solid or liquid matter. Examples include grinding, wind-driven erosion of soil, and the formation of sea spray. Particles formed by these mechanical means tend to be much larger, with diameters often exceeding 10 μm. Critically, surface tension practically limits the smallest size of particles that can be formed by mechanical means to approximately 1 μm.

The initial formation mechanism is only the first step in an aerosol’s life cycle. Newly formed particles are subject to dynamic processes that rapidly alter their characteristics. The very small particles generated by nucleation, due to their high number concentration, tend to coagulate rapidly to form larger composite particles. Conversely, the large particles produced by comminution are more susceptible to gravity and tend to settle quickly out of the atmosphere. To understand and predict these distinct behaviors, the single most important characteristic of an aerosol particle that must be defined is its size.