Environmental Engineering Reference
In-Depth Information
where
m
and
b
are the same constants described earlier.
1
f (x
w
)
=
K
Lang
x
w
,
(4.109)
1
+
where
K
Lang
is the Langmuir adsorption constant for water on aerosols. Notice that as
x
w
→
1. As
x
w
increases,
f (x
w
)
decreases and therefore with increasing
relative humidity log
K
AP
increases.
0,
f (x
w
)
→
4.6 AIR-TO-VEGETATION PARTITION CONSTANT
A major portion of the land area (80%) on earth is covered by vegetation.As such the
surfaceareacoveredbyvegetationcannotbeoverlookedasanenvironmentalcompart-
ment. Plants also take up nutrients and organic compounds from the soil/sediment
environments. Thus plants participate in the cycling of both inorganic and organic
compounds in the environment. A large number of processes, both intra- and extra-
cellular, are identified near the root zones and leaves of plants. In the water-soil
environment a number of organic compounds are imbibed through the roots and
enzymatically degraded within the plants. In the air environment, a number of studies
have revealed that plant-air exchange of organic chemicals plays a major role in the
long-range transport and deposition of air pollutants.
Wax or lipid layers exist to prevent excessive evapo-transpiration from most
plant surfaces exposed to air. The combination of high surface area and the
presence of wax/lipid suggests the high partitioning of hydrophobic organic com-
pounds into vegetation. As in the case of the bioconcentration factor, we define a
vegetation-atmosphere partition coefficient,
K
VA
:
W
V
f
L
C
i
a
,
=
K
VA
(4.110)
where
W
v
is the concentration in vegetation (ng/g dry weight),
f
L
is the lipid content
of vegetation (mg/g dry weight), and
C
i
a
is the atmospheric concentration (ng/m
3
)
.
K
VA
has units of m
3
air/mg lipid.A dimensionless partition coefficient,
K
VA
, can also
be obtained if we use an air density of 1.19
10
6
mg/m
3
at 298 K.
Justas
K
AP
foraerosolswasrelatedtotemperature,
K
VA
hasthefunctionalrelation-
ship: ln
K
VA
×
B
. The intercept
B
is common for a given class of compounds.
For PAHs,
B
was
−
35.95 (Simonich and Hites, 1994). Values of
A
for several PAHs
are listed in Table 4.17.
Several investigators have shown that just as
K
AP
is correlated to
P
s
(
P
)
, so can we
relate
K
VA
to
P
s
(
P
)
. A better correlating parameter is
K
oa
, the octanol-air partition
constant. A few of these correlations are shown in Table 4.17 for PCBs.
Whereas Simonich and Hites (1994) attributed all of the seasonal variations in
K
VA
to the temperature variations, Komp and McLachlan (1997) have argued that this is
due more to a combination of effects such as growth dilution, decrease in dry matter
content of leaves in late autumn, and erosion of HOCs from the surface of vegetation.
These preliminary studies have shown the difficulties in understanding the complex
interactions of HOCs with the plant-air system.
=
A
/
T
+
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