aerosol

An aerosol is a suspension of fine solid particles or liquid droplets in a gas. Particle sizes span roughly from 1 nanometer to 100 micrometers in diameter, with practical divisions such as ultrafine (<0.1 μm), fine (0.1–2.5 μm), and coarse (2.5–10 μm). Aerosols include mineral dust, sea salt, smoke, soot, water and organic droplets, and sometimes biological materials like pollen or bacteria. Their optical and physical properties depend on composition, size distribution, shape, and density, which influence how they behave in air.

Aerosols originate from natural sources—volcanic eruptions, desert dust, sea spray, pollen, spores, and wildfires—and from human activities, including combustion, industrial emissions, vehicle exhaust, and consumer sprays. They form through condensation of vapors, fragmentation, and coagulation, creating complex mixtures that evolve with humidity, temperature, and air chemistry.

In the atmosphere, aerosols scatter and absorb sunlight, affecting visibility and the radiation balance of the Earth. They can act as cloud condensation nuclei or ice nuclei, influencing cloud formation and precipitation. Hygroscopic growth changes their size and optical properties with humidity. Particles settle by gravity, coagulate, or evaporate, producing changes in concentration and size distribution over time.

Health and environmental effects are notable, especially for fine and ultrafine aerosols. Inhalation can irritate the respiratory system and contribute to cardiovascular issues; PM2.5 is a common health metric. Measurements use optical counters and gravimetric methods, reporting number concentration, mass concentration, or size distribution. Regulation targets aerosol emissions to protect air quality and climate.

Applications include medicine, where aerosols deliver drugs by inhalation, and various industrial and consumer uses such as sprays and coatings.