Aerosol Chemistry and Physics - Aerosols: Measurement, Dosimetry, and Health Effects - page 152

urban and rural air. Sometimes trimodal distributions are seen very near sources of fresh aerosol,
when enough numbers of new tiny particles are present to contribute to the mass size distribution
(Seinfeld and Pandis, 2006).
Particle mass and size are key physical parameters that inluence the movement of particles in
air. Diffusion rate depends on particle mass, as do the effects of inertia and luid viscosity on par-
ticle behavior in moving air. During air movement through tubes (e.g., the respiratory system, sam-
pling tubing) the heavier the particle, the more likely the particle is to be caught in a bend because
it cannot keep up with the airlow. This property leads to particle deposition in bronchial tubes.
Size selective inlets, impactors, and virtual impactors take advantage of this property to separate
particles based on their mass.
Instruments based on particle mass : Particle mass size distributions are measured directly by
aerosol impactors. Mass size distributions can be integrated to yield total mass concentrations that
can be compared directly to mass concentrations determined gravimetrically from ilters. If particle
number and mass size distributions are known, the size dependence of particle density can be cal-
culated. Frequently, the particle density is estimated for spherical particles of the same aerodynamic
diameter as the measured distribution, and the calculated density is used to convert from mass to
number size distributions and vice versa. Geometric surface area estimates depend only on the aero-
dynamic diameter and can be derived from either the mass or number size distributions. However,
actual surface areas may be larger due to particle shape and porosity.
6.3.2 P article d ynaMics
The behavior of indoor particles depends on their size, the forces they experience, and their inter-
actions with surfaces in the indoor environment. These factors are embedded in the mass balance
equations used in Section 6.5 to describe their fate and transport. The objective of this section
is to illustrate how basic aerosol concepts apply in the indoor environment, starting with a room
with dimensions 3.65 m × 3.65 m × 3 m. This room has a total volume of 40 m 3 and an air exchange
rate of 1 h −1 .
6.3.2.1 Particle Motion
The low rate at one air exchange per hour through the example room is 11.1 cm 3 s −1 . A cube of air
with this volume has a linear velocity of 2.23 cm s −1 . A quick way to get a sense of the behavior of
particles in air at this velocity is to look at their Reynolds numbers (ratios of inertial to frictional
forces), relaxation times (for adjustment of particle velocity to applied force), and terminal settling
velocities in still air (Table 6.6). When particle Reynolds numbers are <1 the particles experience
laminar low. That is, they keep up with the streamlines, and gravity has little effect on them. In
this example, the particles move with the airlow unless they are large enough to be visible to the
human eye. Respirable particles (diameter <2.5 μm) would remain entrained in room air movement
even at much higher air exchange rates. The relaxation times show that they quickly adjust to the air
exchange rate, and the settling velocities show that respirable particles do not deposit onto the loor
quickly under the inluence of gravity.
Deposition : Airborne particles deposit to building materials and other indoor surfaces after col-
lision and adhesion. As outdoor air iniltrates through cracks in the building envelope, particles
below 0.1 μm in diameter are lost because of diffusion much more eficiently than larger particles.
However, larger particles deposit by impaction when they cannot keep up with the airlow around
turns and obstacles. Once inside the building, both diffusion and gravitational settling contribute
to particle loss. Table 6.6 shows that diffusion is responsible for more deposition to a horizontal
surface than gravitational settling, for particles smaller than 0.2 μm, over the 100 s period examined
by Hinds (1999, p. 162).