What Are Aerosols?

Aerosols are suspensions of fine solid particles or liquid droplets dispersed in a gas, most commonly air. The defining characteristic is the two-phase nature of the system: a continuous gaseous phase carrying a dispersed condensed phase whose particles are small enough to remain airborne for extended periods. Particle diameters range from a few nanometers to roughly 100 micrometers, spanning nucleation clusters, combustion soot, sea spray, mineral dust, and industrial sprays. The field of aerosol science draws on fluid mechanics, thermodynamics, physical chemistry, optics, and electrical engineering, and it intersects with atmospheric physics, industrial process engineering, and biomedical research.

The behavior of an aerosol particle is governed by the interplay of gravitational settling, Brownian diffusion, inertial impaction, and electrostatic forces. For particles below about 0.1 micrometers in diameter, diffusion dominates transport; for particles above roughly 1 micrometer, gravitational settling and inertia become significant. This size-dependent transition defines the measurement and control strategies used across applications, from cleanroom filtration to respiratory drug delivery.

Particle Formation and Nucleation

Aerosol particles are generated either by mechanical dispersion of bulk matter, such as grinding, spraying, or wind erosion, or by gas-to-particle conversion through nucleation and condensation. Nucleation occurs when a supersaturated vapor spontaneously forms molecular clusters that grow into stable particles; heterogeneous nucleation on pre-existing surfaces or ions lowers the energy barrier and predominates in most atmospheric and industrial settings. Combustion aerosols form through high-temperature oxidation of fuel, producing carbonaceous soot aggregates and secondary organic aerosol (SOA) from photochemical reactions. The Journal of Aerosol Science, the primary publication venue for the discipline, covers nucleation kinetics, growth mechanisms, and the optical and electrical properties of freshly formed and aged particles.

Spraying and Atomization

Spraying converts a bulk liquid into a population of droplets by applying energy through pressure, ultrasound, electrostatics, or rotary motion. Pressure nozzles use the kinetic energy of the liquid jet to promote ligament breakup; twin-fluid atomizers introduce a second gas stream to enhance disintegration at lower liquid pressures. The resulting droplet size distribution depends on nozzle geometry, liquid viscosity and surface tension, and gas velocity. Electrospray ionization, which uses a strong electric field to disperse a liquid at a needle tip, produces monodisperse nanometer-scale droplets and is the basis of electrospray mass spectrometry and some drug delivery devices. The Washington University Center for Aerosol Science and Engineering conducts research spanning fundamental atomization physics, aerosol measurement methods, and applications in energy conversion and environmental remediation.

Atmospheric Aerosols and Measurement

Atmospheric aerosols influence Earth's radiation budget both directly, by scattering and absorbing solar radiation, and indirectly, by acting as cloud condensation nuclei (CCN) that affect cloud albedo and precipitation. The dominant natural sources are sea spray, mineral dust from arid regions, biogenic volatile organic compounds, and volcanic emissions. Anthropogenic contributions include fossil fuel combustion, industrial processes, and agricultural burning. Measurement techniques include optical particle counters, cascade impactors, condensation particle counters (CPC), and scanning mobility particle sizers (SMPS), each suited to different size ranges. The NOAA research programs on atmospheric aerosols provide long-term observational datasets that underpin climate model evaluations and air quality assessments.

Applications

Aerosols have applications across a wide range of engineering and scientific disciplines, including:

• Atmospheric science and climate research, where aerosol properties are a major source of uncertainty in climate projections
• Pharmaceutical inhalation therapy, including dry powder inhalers and nebulizers for respiratory drug delivery
• Industrial coating and spray painting, exploiting controlled droplet size for uniform film deposition
• Combustion engineering, where fuel atomization affects flame stability, efficiency, and emissions
• Semiconductor fabrication, where cleanroom aerosol monitoring protects yield-critical processes