Environmental Engineering Reference
In-Depth Information
C (d ,t ) C (d ,t ) P(d )C (d ,t )
=
+
λ (
t
)
Δ
t R
+
[
(d ) S(d )]/V
+
i
p
2
i
p
1
p
o
p
1
v
1
p
p
⎡
⎢
∑
⎤
⎥
−
C (d ,t )
λ (
t
)
+
k (d )
j
Δ
t
(6.11)
i
p
1
v
1
p
j
This example treats R and S as constant in time, but these could be restated to incorporate their time
variability by using the form R(d
p
, t
1
)Δt. The use of this forward-marching approach requires that
information/data are available for each parameter at each time step.
The key parameters determining aerosol transport and fate in buildings are discussed in the following.
6.5.2 a
irFlow
and
a
erosol
t
ransPort
tHrougH
P
enetrations
in
b
uilding
e
nveloPes
Airlow through building envelopes occurs as a result of temperature and wind-driven pressure
differences between the inside and outside of the building. Houses are often kept “closed” during
wintertime heating periods and in some regions, during summertime cooling periods, thus limiting
airlows to gaps, holes, or other inadvertent penetrations in the building shell, created as a result
of (poor) construction practices, settling, and/or aging of building components, etc. Since these
heating and cooling seasons often coincide with periods of high ambient aerosol concentrations,
transport of particles into the building during these periods will occur through building leaks.
Few measurements have been made to determine the nature and signiicance of most building
leaks. On average, the largest air leakage values are for leaks in the walls and loor (35%), in the
ceiling (15%), and around windows and doors (15%) (Diamond and Grimsrud, 1984). Not all of
these leaks will conduct aerosol into the building interior under most operating conditions. High
leaks, such as those in the ceiling or openings such as ireplace or furnace chimneys, are usually
locations for exiltration. The physical dimensions of many such penetrations are poorly character-
ized and the lows across the building envelope may proceed through tortuous pathways, making
a priori
prediction of aerosol penetration eficiency dificult.
Many studies have used indoor and outdoor mass measurements to provide broad categorical pen-
etration factors. In a summary paper (Wallace, 1996), penetration factors of ∼1 are derived for both
ine (PM
2.5
) and coarse (PM
10
) mode particles, based on a statistical analysis of data from the PTEAM
study. However, these estimates do not take into account the details in the size dependence of the
deposition rates nor can they account for any potential size-dependent effect of the penetration process
itself. Because these values are based on mass, the results will heavily depend upon the underlying
size distribution of the aerosols and, in particular, the populations of the largest particle-size fractions.
Similar results have been reported by Thatcher and Layton (1995), based on experimental mea-
surements of penetration; however, this study is limited to summertime measurements in one house
and to particles larger than 1 μm. In contrast, recently published results from a series of controlled
chamber experiments show a signiicant decline in the penetration factor as the test aerosol becomes
larger than 2 μm in diameter (Lewis, 1995). For particles larger than ∼6 μm, the penetration fraction
was measured to be essentially zero.
A set of experiments were performed in two residential buildings to examine both aerosol pene-
tration and deposition indoors (Thatcher et al., 2003). Continuous, size-resolved data were collected
during a three-step experimental procedure:
1. Artiicially enhancing indoor particle concentrations indoors and following the decay in
concentrations.
2. Rapidly reducing particle concentrations through induced exiltration by pressurizing the
dwellings with HEPA-iltered air.
3. Following the rebound in iniltrating particle concentrations when overpressurization was
stopped. Penetration factors as a function of particle size are shown in Figure 6.14.
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