Chemistry Reference
In-Depth Information
Fig. 3 Penetration efficiency as function of particle size and crack width [
10
]
The type of ventilation plays an important role as well. Chen and Zhao [
11
]
distinguish mechanical ventilation (artificial systems usually including filters),
natural ventilation (through open windows and doors), and infiltration (uncon-
trolled flow through cracks).
The second particle size-dependent parameter in the mass-balance equation is the
decay term (
k
). Decay of nonvolatile particles indoors is dominated by thermokinetic
and gravimetric deposition. Thermokinetic deposition affects UFP in the nanometer
size range, while particles larger than few hundreds of nanometers in diameter are
strongly affected by the gravimetric deposition. Between these two domains remains
a particle size range, which has the largest tendency of remaining suspended in the
indoor air. In the thermokinetic domain the deposition rate is strongly affected also by
friction velocity, i.e. turbulence of the air in indoor spaces, bringing the particles
effectively close to the deposition surfaces (Fig.
4
).
The deposition rate depends also on the availability of surfaces. While gravimetric
deposition takes place only on horizontal upward facing surfaces, thermokinetic
deposition occurs also on downward facing and vertical surfaces. The more such
surfaces are available for deposition, the faster the corresponding deposition rate is.
Relationship of the deposition surface availability and the deposition velocity in a
rectangular space is characterized by Lai and Nazaroff [
12
]as
b ¼
u
dv
A
v
þ u
du
A
u
þ u
dd
A
d
V
(5)
