Answer Key
- An aerosol is a system of tiny solid or liquid particles suspended in a gas; these substances are larger than molecular dimensions but microscopic or submicroscopic. The two fundamental formation mechanisms are nucleation from vapor molecules (e.g., combustion), which creates particles smaller than 0.1 µm, and comminution of solid or liquid matter (e.g., sea spray), which creates larger particles, often exceeding 1 µm.
- A geometric diameter, such as the Feret or Martin diameter, is a length scale characteristic of a particle’s silhouette, often measured under a microscope. In contrast, a diameter based on equivalent dynamic properties, like the Stokes diameter, defines the size of a hypothetical sphere that would have the same dynamic behavior (e.g., settling velocity) as the non-spherical particle.
- The log-normal distribution is a statistical distribution function used to describe particle sizes by applying a logarithmic transformation to the particle diameter. It is particularly useful for aerosols because, unlike a normal (Gaussian) distribution, it does not allow for negative particle sizes, which is a physical impossibility.
- A size distribution based on particle number concentration gives prominence to the smaller particles, which are typically more numerous. A distribution based on mass, however, emphasizes the larger particles, as mass is proportional to the cube of the diameter. As shown in Figure 1, this results in the mass median diameter (MMD) being significantly larger than the number median diameter (NMD) for the same aerosol population.
- A particle’s terminal settling velocity generally decreases as its size decreases. The slip coefficient (Cc) corrects for the non-continuum effects experienced by very small particles in a gaseous medium; it increases as particle size decreases, which results in a higher settling velocity than predicted by Stokes’ law alone.
- Brownian motion is the random movement of small particles caused by incessant bombardment from surrounding gas molecules. For very small particles (around 0.5 µm and smaller), the displacement due to Brownian motion can be comparable to or greater than the distance traveled due to gravitational settling, making it a dominant factor in their transport and deposition.
- The two primary mechanisms are diffusion charging and field charging. Diffusion charging results from random thermal collisions between ions and particles and is the dominant mechanism for particles smaller than about 0.2 µm. Field charging occurs when ions are driven along electric field lines and collide with a particle.
- Brownian coagulation is the process where aerosol particles collide and adhere due to their random Brownian motion. This process causes the total particle number concentration to decrease over time while the total particle volume remains constant, leading to a shift in the size distribution towards larger particles.
- The Kelvin effect describes the phenomenon where the vapor pressure over a curved droplet surface is higher than over a flat surface of the same liquid, with this effect increasing as the droplet diameter decreases. This means a smaller droplet requires a higher surrounding vapor saturation ratio to grow and will evaporate in conditions where a larger droplet would be stable or grow.
- An impactor sizes particles based on their inertia. An aerosol stream is accelerated through a nozzle and directed at an impaction surface, causing particles with sufficient inertia to deviate from the gas streamlines and collide with the surface while smaller particles remain in the flow. The “cut-off size” is the particle diameter at which 50 percent of the particles are collected on the impaction surface.