The quantitative measurement of aerosol properties such as size distribution and concentration is fundamental to both atmospheric science research and industrial process control. A wide array of measurement techniques has been developed, each designed to exploit one or more of the physical principles previously discussed—be it a particle’s inertial, electrical, or optical behavior. This section provides a survey of the primary classes of aerosol measurement instrumentation summarized in Table 4.

6.1 Optical Methods

Optical particle counters and sizers operate by measuring the light scattered by individual particles passing through an illuminated sensing volume. The operational principle, diagrammed in Figure 7, involves a light source (e.g., a laser) creating an incident beam and a detector (e.g., a photomultiplier tube) measuring the intensity of the light scattered by each particle at a specific angle.

It is crucial to understand that these instruments measure an optical equivalent diameter. The intensity of scattered light depends not only on size but also on the particle’s shape and refractive index. Therefore, instruments are calibrated using standard particles of a known size and refractive index, such as polystyrene latex (PSL) spheres. For particles with different shapes or optical properties, the measured optical diameter may not represent the true geometric or aerodynamic diameter.

6.2 Inertial Methods

Inertial measurement techniques separate particles based on their inertia—their tendency to resist changes in direction. The most common inertial instrument is the impactor. As illustrated in Figure 8, an impactor accelerates an aerosol stream through a nozzle directed at an impaction surface. Larger particles with greater inertia cannot follow the sharp turn in the gas streamlines and impact the surface, while smaller particles with less inertia remain entrained in the flow and pass around the plate.

The collection behavior is governed by the dimensionless Stokes number (Stk):

Stk = τ * u₀ / W (Eq. 33)

where τ is the particle’s relaxation time, u₀ is the jet velocity, and W is the nozzle width. An impactor stage exhibits a sharp collection efficiency curve as a function of Stk, resulting in a well-defined cut-off size—the diameter at which 50% of the particles are collected.

By connecting multiple impactor stages with progressively smaller cut-off sizes in series, a cascade impactor can be created. This device physically sorts an aerosol population into different size bins, allowing for the determination of a size distribution by measuring the mass collected on each stage. For accurate results, three practical challenges must be considered: (1) cross-sensitivity between stages, which can blur the sharpness of the size cuts; (2) particle rebound from the impaction surfaces, which can cause larger particles to be miscounted in a smaller size fraction; and (3) particle deposition on internal walls, which leads to sample loss.

6.3 Sedimentation and Centrifugal Methods

The sedimentation method is a direct application of Stokes’ law. By observing the terminal settling velocities of individual particles under gravity, it is possible to infer their Stokes diameter. This method is typically used for larger particles where settling velocities are significant enough to be measured over practical time scales.

Centrifugal methods enhance gravitational separation by subjecting particles to a strong centrifugal force field. Instruments like the spiral centrifuge, shown in Figure 10, spin to create an artificial gravitational field many times stronger than Earth’s. As clean air flows through the device, particles are introduced and deposited along a collection surface according to their aerodynamic size, allowing for high-resolution size separation.

6.4 Electrical Mobility Analysis

Electrical mobility analyzers are a cornerstone of modern aerosol measurement, particularly for submicron particles. These instruments function by separating charged particles based on their velocity in a controlled electric field, as governed by the principles of electrical mobility (ve = Be * E, from Eq. 20).

The process, diagrammed in Figure 11, involves three core steps:

1. Charging: The aerosol is passed through a charger (e.g., using a radioactive source to generate bipolar ions) to impart a known, equilibrium charge distribution onto the particles.
2. Classification: The charged aerosol then enters a classifier, typically a cylindrical capacitor with an applied voltage. Particles with a specific electrical mobility are drawn through an exit slit, while all others are removed. By systematically varying the voltage, the instrument can select particles of different sizes.
3. Detection: The classified, nearly monodisperse aerosol is passed to a detector, such as a Condensation Particle Counter (CNC), which counts the number concentration.

Two main operational types exist: the integration type measures the total current carried by all particles up to a certain mobility, while the differential type (such as the Differential Mobility Analyzer, or DMA) measures the concentration within discrete, narrow mobility (and thus size) windows.

From the fundamental definition of an aerosol and the complex theories governing its dynamic behavior to the practical instrumentation used for its characterization, this monograph has provided a comprehensive overview of the core tenets of aerosol science.