Aerosol Chemistry and Physics - Aerosols: Measurement, Dosimetry, and Health Effects - page 153

TABLE 6.6
Properties of Particles with Unit Density in a room with Volume 40 m 3 and 1 ACH at
Atmospheric Pressure and 20°C

Relaxation
Time b (s)

Terminal Settling
Velocity c (cm s −1 )

Diffusion
Coeficient d (cm 2 s −1 )

Ratio, Diffusion
to Settling e

Diameter ( μ m) Re a at 2.23 cm s −1

0.01 0.00015 6.8 × 10 −9 7.0 × 10 −6 5.2 × 10 −4 390

0.10 0.0015 8.8 × 10 −8 8.8 × 10 −5 6.7 × 10 −6 3.4

1.0 0.015 3.6 × 10 −6 3.5 × 10 −3 2.7 × 10 −7 1.7 × 10 −2

10 0.15 3.1 × 10 −4 0.29 2.4 × 10 −8 5.5 × 10 −5

100 1.5 3.2 × 10 −2 17 2.4 × 10 −9 2.2 × 10 −7

a Reynolds number, Re = 6.6 Vd, where V is the particle velocity and d its diameter, in cgs units. Hinds (1999, p. 28). Re is
shown for particle velocity and is equated to the linear low velocity in the example room, 2.23 cm s -1 .
b Hinds (1999, p. 112, Table 5.1).
c In still air, Hinds (1999, p. 51, Table 3.1).
d Hinds (1999, p. 153, Table 7.1).
e Cumulative deposition of particles over 100 s, Hinds (1999, p. 162, Table 7.5).

6.3.2.2 Particle Formation and Phase Partitioning
6.3.2.2.1 Nucleation
Nucleation describes the process of particle formation from gas-phase precursor compounds.
Homogeneous nucleation starts when the air becomes supersaturated with precursor gas molecules
that collide with each other. No preexisting particles are necessary. Supersaturation means that the
actual vapor pressure of a compound is higher than its equilibrium vapor pressure, as combustion
exhaust expands and cools, for example. After multiple collisions, molecular clusters or “embryos”
are formed. They grow by further collisions with gas molecules, and when they reach a critical
diameter that depends on the extent of supersaturation and gas molecular properties, some of them
become stable enough to form nanoparticles. Homogeneous nucleation requires supersaturation
ratios of 2-10 (ratios of actual vapor pressure to equilibrium vapor pressure (Hinds, 1999, p. 279).
Particles continue to grow by condensation as long as supersaturation continues.

6.3.2.2.2 Condensation
Heterogeneous nucleation or nucleated condensation occurs at much lower supersaturation when
preexisting nuclei are present to provide some initial surface area for adsorption of gas-phase mol-
ecules. Above the oceans water vapor condenses on soluble nuclei such as sodium chloride crystals
at low supersaturation. When mixtures of gases are present, UFP may appear at supersaturation
ratios somewhat lower than 1. SOA forms when concentrations of oxidized products of the gas-
phase reactions of unsaturated hydrocarbons with atmospheric oxidants reach supersaturation and
condense onto existing particles.

6.3.2.2.3 Coagulation
After particles are released into the indoor environment, they will bump into each other at a rate
that is proportional to the square of their number concentration. The number concentration will
decrease rapidly as the particles agglomerate, and their mass concentration will remain the same
until they become too large to remain suspended in the air. Hinds (1999) suggests that coagulation
can usually be neglected in industrial hygiene applications when the particle number concentration
is below 10 6 cm −3 .