Aerosol sizing methods are generally based on exploiting the distinct dynamic and physical properties of particles, such as their interaction with light, their inertial motion in a moving fluid, or their response to an electric field. This section critically evaluates the principal categories of measurement techniques, detailing their operational principles, measurement mechanisms, and inherent limitations.

3.1 Optical Methods: Light Scattering

Principle of Operation Optical methods operate on the principle that when an incident beam of light strikes a particle, it scatters the light in a predictable pattern. According to Mie theory, the intensity and angular distribution of this scattered light are a function of the particle’s size, shape, and refractive index, as well as the wavelength of the incident light. For particles smaller than the wavelength of light—a regime known as Rayleigh scattering—this relationship is particularly sensitive, with scattered intensity being proportional to the sixth power of the particle diameter.

Measurement Mechanism In a typical optical particle counter, an aerosol stream is focused as it passes through a high-intensity light beam, often from a laser, creating an illuminated “sensing volume.” As a single particle traverses this volume, it scatters light in all directions. A detector, such as a photomultiplier tube positioned at a specific angle to the beam, captures this scattered light and converts it into an electrical pulse. The height of this pulse is proportional to the intensity of the scattered light and is therefore correlated with the particle’s size.

Measured Diameter This method measures the equivalent light scattering diameter. The instrument is calibrated using standard spherical particles of a known size and refractive index, such as Polystyrene Latex (PSL). The measured size of an unknown particle is reported as the diameter of the PSL sphere that would produce the same signal.

Key Considerations and Limitations The primary limitation of optical methods is their dependence on particle composition. The amount of light a particle scatters is strongly influenced by its refractive index. Consequently, unless the particles being measured are spheres of a known refractive index that matches the calibration aerosol, their real diameters cannot be evaluated from the measured optical equivalent diameters. This can lead to significant sizing errors when analyzing aerosols of mixed or unknown composition.

3.2 Inertial and Gravitational Methods

This category of techniques exploits a particle’s inertia—its resistance to a change in velocity—or its response to gravity. Both properties are strongly dependent on particle mass and size, allowing for physical separation of particles from a gas stream.

3.2.1 Impactor Method

Principle of Operation The operating principle of an impactor is inertial impaction. An aerosol stream is accelerated through a nozzle and directed at a solid impaction surface. Due to their inertia, larger particles are unable to follow the sharp turn in the gas streamlines and impact the surface. Smaller particles, having less inertia, remain entrained in the gas flow and are carried around the plate.

Measurement Mechanism The collection efficiency for particles of a given size is governed by the Stokes number (Stk), a dimensionless parameter representing the ratio of the particle’s relaxation time (its measure of inertia) to the characteristic time of the fluid flow around the impaction plate. Each impactor stage has a characteristic collection efficiency curve, and the “cut-off size” is defined as the particle size that is collected with 50% efficiency. To obtain a size distribution, cascade impactors are used, which consist of multiple stages connected in series. Each successive stage features smaller nozzle dimensions, which accelerates the aerosol jet to a higher velocity and thereby lowers the cut-off size, allowing for the collection of progressively smaller particles on each subsequent plate. The size distribution is determined by measuring the mass of particles collected on each stage.

Measured Diameter Because this method relies on a particle’s inertial behavior, it measures an inertia-dependent size, corresponding to either the Stokes diameter or the aerodynamic diameter.

Key Considerations and Limitations Achieving accurate results with a cascade impactor requires careful attention to several factors. Key considerations include:

• Particle Rebound: Solid particles can bounce off the impaction surface and be re-entrained into the aerosol stream, leading to their incorrect collection on a downstream stage.
• Internal Deposition: Particles may be lost to the internal walls of the device rather than being collected on the designated impaction surfaces.
• Data Reduction: The collection efficiency curves of adjacent stages can overlap, requiring data inversion techniques that account for this cross-sensitivity to obtain an accurate size distribution.

3.2.2 Other Inertial and Settling Methods

Several other techniques also leverage particle inertia or gravitational settling.

• Particle Acceleration Method: This method passes an aerosol through a converging nozzle, accelerating the gas flow. Larger particles, due to their inertia, do not accelerate as quickly as the surrounding air. A laser-based velocimeter at the nozzle outlet detects this velocity difference, which can be correlated to particle size.
• Sedimentation Method: This straightforward method infers particle size by directly observing their terminal settling velocities under gravity. The measured settling velocity is directly related to the Stokes diameter.
• Centrifuging Method: To analyze smaller particles with low settling velocities, this method enhances gravitational settling by using a strong centrifugal flow field. Particles are introduced into a rotating chamber and separate along a collection surface according to their size and density.

3.3 Electrical Mobility Analysis

Principle of Operation This technique operates on the fundamental principle that a charged particle’s terminal velocity in a static electric field is a direct function of its size and charge state. This relationship is defined by the particle’s electrical mobility, which is higher for smaller particles (for a given charge state).

Measurement Mechanism The measurement process involves two key steps.

1. Charging: The aerosol is first passed through a charging section to impart a predictable and well-defined electrical charge distribution onto the particles. This is typically achieved through diffusion charging, where the aerosol is exposed to a high concentration of bipolar ions (both positive and negative) generated by a radioactive source. This process brings the particles to an equilibrium charge state.
2. Classification: The now-charged aerosol enters a classification section, which is typically a cylindrical capacitor composed of a central high-voltage electrode and a grounded outer wall. Particles are carried axially down the annulus between the electrodes by a sheath of clean air, while the radial electric field pulls them outward. For a given voltage, only particles within a narrow range of electrical mobility possess the correct trajectory to exit through a small sampling slit at the bottom of the central electrode, while all others are deposited on one of the electrodes. By systematically scanning the voltage, a high-resolution size distribution can be measured.

Measured Diameter This method measures the electrical mobility equivalent diameter.

Key Considerations and Limitations The accuracy of electrical mobility analysis is critically dependent on achieving a known and predictable particle charge distribution. The underlying theory relies on the assumption that the charge on a particle of a given size is known. The goal is to achieve a bipolar equilibrium charge distribution, where the fraction of particles carrying a specific number of charges (0, +1, -1, +2, etc.) is a known function of particle size. Any deviation from this assumed charge distribution will result in sizing errors.

The diverse measurement principles reviewed—optical scattering, inertial impaction, and electrical mobility—each provide a different perspective on particle size. This highlights the importance of selecting a method that is not only suitable for the particle size range of interest but also measures the diameter most relevant to the research question.