Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 62

3.7.1.2 Sedimentation
Reduced low velocities lead to increased particle residence times in airways. When this occurs
(e.g., when the peripheral airways of the tracheobronchial tree or alveolar region are being consid-
ered), particles may be deposited by gravitational forces (Figure 3.5A). This is called sedimentation,
and while particles as small as 0.1 μm may be affected, this mechanism increases in importance
with increasing particle size. 112,113 Particle deposition from sedimentation, like inertial impaction, is
directly dependent upon τ, the relaxation time of the aerosol particle. Sedimentation of particles has
been studied by Yu and Thiagarajan 114 and Gebhardt and Heyder. 115

3.7.1.2.1 Laminar Conditions
Particle deposition eficiency from sedimentation, P ( S ), under laminar plug low conditions can be
expressed by 24,101

2

⎣

1 2 +

⎦

2 − 1

P ( ) = π β −β ( 1 ) sin ( ) β (3.25)

The orientation of airways with respect to gravity must be considered, and, therefore, for upward
low,

L V
d V

cos( θ )

a TS b

β = (3.26)

[ ]

V sin( )

+ θ

a a TS b

and for downward low,

L V
d V

cos( θ )

a TS b

β = (3.27)

[ ]

V sin( )

− θ

a a TS b

These equations are suitable for calculating deposition in the peripheral airways of the tracheo-
bronchial network. For downstream regions where low is fully developed, and parabolic velocity
proiles exist, the formulation of Beeckmans 99 may be employed.

3.7.1.2.2 Turbulent Conditions
Under turbulent low conditions, deposition eficiency from sedimentation can be expressed as 24,99,101

− 4 β

⎡
⎢

⎤
⎥

P ( ) = − 1 exp (3.28)

π

where β is given by either Equation 3.26 or 3.27.

3.7.1.3 Diffusion
Deposition via diffusion (Figure 3.5A) occurs when particles exhibiting random Brownian motion
collide with the airway surface. While deposition of particles between 0.1 and 1 μm in size occurs
by both diffusion and sedimentation, 112,113 diffusion is the primary deposition mechanism for par-
ticles less than 0.1 μm in size, 116 since diffusion is governed by geometric diameter rather than
aerodynamic diameter. Many studies of varying emphases have been performed to quantify the
diffusion of aerosol particles. 104-106,117-124