Glossary of Key Terms
Glossary of Key Terms
| Term | Definition |
| Aerosol | A system of tiny particles suspended in a gas. Refers to any substance, except pure water, that exists as a liquid or solid in the atmosphere under normal conditions and is of microscopic or submicroscopic size but larger than molecular dimensions. |
| Aerodynamic Diameter (Dae) | The diameter of a sphere with unit specific gravity (1 g/cm³) that has the same gravitational settling velocity as the particle in question. |
| Brownian Coagulation | A fundamental mechanism, also known as thermal coagulation, where particles collide and adhere due to their random Brownian motion, causing a decrease in particle number concentration over time. |
| Brownian Motion | The random motion of a small particle resulting from incessant bombardment by surrounding gas molecules. It causes particles to diffuse in a manner analogous to gas molecules. |
| Cascade Impactor | A device for measuring particle size distribution by connecting several impactors with different cut-off sizes in series. |
| Coagulation Coefficient (K) | A parameter that describes the rate at which particles coagulate. For monodisperse aerosols, the rate of concentration decrease is proportional to the coagulation coefficient and the square of the number concentration. |
| Comminution | One of the two fundamental mechanisms of aerosol formation, involving the mechanical grinding or breaking up of solid or liquid matter (e.g., erosion, sea spray). It tends to produce particles with diameters exceeding 1 µm. |
| Cut-off Size | In an impactor, the particle size that is collected with 50 percent efficiency on the impaction surface. |
| Diffusion Charging | The charging of particles that arises from thermal, random collisions between particles and gaseous ions. It is the predominant charging mechanism for particles smaller than about 0.2 µm. |
| Drag Coefficient (CD) | A dimensionless quantity that relates the drag force on a particle to the fluid density, relative velocity, and the particle’s projected area. It is dependent on the Reynolds number. |
| Electrical Mobility (Be) | The velocity of a charged particle in an electric field of unit strength. It is dependent on the number of charges the particle carries, particle diameter, and the slip coefficient. |
| Electrical Mobility Analyzer | An instrument that separates and sizes particles based on their different velocities in an electric field according to their electrical mobility. |
| Equivalent Diameters | Diameters of hypothetical spheres that have the same specific property (e.g., volume, surface area, projection area, settling velocity) as the non-spherical particle being measured. |
| Extinction Coefficient (γ) | A measure of how strongly a medium absorbs and scatters light per unit distance. It is the sum of contributions from aerosol scattering, aerosol absorption, molecular scattering, and molecular absorption. |
| Field Charging | The charging of particles that occurs when ions drift along electric field lines and impinge upon the particle. |
| Geometric Mean Diameter (Dg) | For a log-normal distribution, the median diameter of the distribution based on particle count (equivalent to the Number Median Diameter). It corresponds to the 50th percentile on a cumulative log-probability plot. |
| Geometric Standard Deviation (σg) | A measure of the width or dispersion of a log-normal distribution. |
| Impactor | A device for sizing particles based on inertia, where an aerosol jet is directed at a surface, causing particles above a certain cut-off size to impact it. |
| Kelvin Effect | The phenomenon where the vapor pressure over a curved droplet surface is greater than that over a flat surface of the same liquid. The effect is more pronounced for smaller droplets. |
| Koschmieder Equation | An equation that relates the visual range (Lv) in the atmosphere to the light extinction coefficient (γ): Lv = 3.912 / γ. |
| Log-Normal Distribution | A continuous probability distribution of a random variable whose logarithm is normally distributed. It is frequently used to describe aerosol size distributions because it does not permit negative particle sizes. |
| Mass Median Diameter (MMD) | The median diameter of an aerosol size distribution when weighted by mass. It is the diameter at which 50% of the total particle mass is contributed by particles smaller than this size. |
| Mie Scattering | A light scattering regime that applies when the particle size is of a similar order of magnitude as the wavelength of light (typically when α is from 2 to 10). |
| Number Median Diameter (NMD) | The median diameter of an aerosol size distribution when weighted by number concentration. It is the diameter at which 50% of the total number of particles are smaller than this size. |
| Nucleation | One of the two fundamental mechanisms of aerosol formation, involving the condensation of vapor molecules (e.g., through photochemistry or combustion) to form new particles, typically with diameters smaller than 0.1 µm. |
| Phoretic Phenomena | Particle motion that occurs due to a difference in molecular collisions on different sides of the particle. Examples include thermophoresis (temperature gradient), photophoresis (light absorption), and diffusiophoresis (vapor gradient). |
| Rayleigh Scattering | A light scattering regime that applies when the particle is much smaller than the wavelength of light (typically when α is smaller than about 2). The scattered intensity is proportional to the sixth power of particle size. |
| Relaxation Time (τg) | A measure of a particle’s inertial motion, characterizing the time required for a particle to adjust its velocity in response to a change in external forces. |
| Reynolds Number (Re) | A dimensionless number that describes the ratio of inertial forces to viscous forces in a fluid flow around a particle. It determines the nature of the drag force. |
| Slip Coefficient (Cc) | A correction factor applied in calculations of particle dynamics (like drag force and diffusion) to account for the discontinuous nature of the gaseous medium when the particle size is comparable to the mean free path of gas molecules. |
| Stokes Diameter (Dst) | The diameter of a sphere having the same density and gravitational settling velocity as the particle in question. |
| Stokes Number (Stk) | A dimensionless number used in impactor analysis that represents the ratio of a particle’s relaxation time to a characteristic time of the fluid flow. It predicts whether a particle will follow fluid streamlines or impact a surface. |
| Thermophoresis | The movement of aerosol particles in a gas where a temperature gradient exists, with particles being driven from hotter regions to colder regions. |
| Visual Range (Lv) | The maximum distance at which an object can be just distinguished from its background, used to quantitatively evaluate visibility. |