5. Aerosol Measurement Techniques
A variety of instruments and methods are used to measure the size and concentration of aerosols. These techniques are based on the diverse physical and dynamic properties of the particles.
Overview of Measurement Methods
The following table summarizes key methods for aerosol particle size analysis, categorized by the underlying physical principle.
| Principle | Method / Instrument | Measured Quantity | Approx. Size Range (µm) |
| Inertial Motion | Impactor, Cascade Impactor | Mass, Number | ≥ 0.5 |
| Aerosol Beam Method (Low Pressure) | Number | ≥ 0.05 | |
| Particle Acceleration Method | Number | ≥ 0.5 | |
| Gravitational/Centrifugal Settling | Sedimentation Cell | Number | ≥ 1 |
| Spiral Centrifuge, Conifuge | Mass, Number | 0.05–1 | |
| Brownian Motion | Diffusion Battery with CNC | Mass, Number | 0.002–0.5 |
| Electrical Mobility | Electrical Mobility Analyzer (DMA, EAA) | Number (via current) | 0.002–0.5 |
| Optical Properties | Light Scattering Counter | Number | > 0.1 |
| Light Diffraction | Number | ≥ 1 | |
| Volume Displacement | Coulter Counter | Number | ≥ 0.3 |
| Microscopy | Electron Microscope | Length | ≥ 0.001 |
Detailed Methodologies
- Optical Methods: These instruments, such as optical particle counters, measure the intensity of light scattered by individual particles passing through a light beam (e.g., a laser). Based on Mie theory, the intensity of the scattered light is related to particle size. The output is typically an “optical equivalent diameter,” calibrated against standard spherical particles like polystyrene latex.
- Inertial Methods (Impactors): An impactor accelerates an aerosol stream toward a collection surface. Larger particles with greater inertia are unable to follow the curving streamlines of the gas and impact the surface, while smaller particles remain entrained in the flow. The collection efficiency is a function of the Stokes number (Stk). An impactor stage is characterized by a “cut-off size,” the diameter at which 50% of particles are collected. A cascade impactor uses multiple stages with progressively smaller cut-off sizes connected in series to obtain a size distribution.
- Sedimentation and Centrifuging Methods: These methods infer particle size by measuring their terminal settling velocity, either under gravity or in a centrifugal field. Centrifuges, such as the spiral centrifuge, can classify particles over a wide size range and allow for subsequent chemical analysis of the size-fractionated particles.
- Electrical Mobility Analyzers: These instruments first impart a known charge distribution onto the aerosol particles. The charged particles are then passed through an electric field (typically in a cylindrical capacitor geometry). Particles of different sizes have different electrical mobilities and are separated based on their trajectory in the field. By varying the electric field, a high-resolution size distribution can be measured. Two main types exist: integration and differential analyzers.