Chemistry Reference
In-Depth Information
organic pollutants (log
K
ow
>
4) are taken up from the atmosphere via the cuticle
waxes or by the stomata of the leaf, where they tend to accumulate. Uptake of these
compounds from the soil via the roots is not significant; neither is POP transported
to other parts of the plant. Due to the large surface area of leaves, atmospheric
variability of POP concentrations and ambient temperatures affect directly the
concentrations found in the leaves of plant species [
56
]. Leaves can therefore be
useful as bioindicators for the atmospheric quality in terms of POP levels.
The significance of soot carbon for atmospheric transport and particulate-gas
partitioning of PAH has been assessed in several studies [
30
-
34
,
52
]. This exchange
is based on the particulate-gas partitioning coefficient
K
p
K
p
¼
C
p
=ð
C
g
TSP
Þ;
where
C
p
and
C
g
are the concentrations in the particulate and gas phase,
respectively.
Usually, there is a significant linear fit between
K
p
and the sub-cooled liquid
vapour pressure (
P
L
;[
58
]) of the PAHs. Organic matter plays an important role in
the particle/gas partitioning of these compounds ([
58
] and references therein),
which can be described by the following equation:
log
K
p
¼
log
K
oa
þ
log
f
OM
11
:
91
;
(1)
where
K
oa
is the temperature corrected octanol-air partitioning coefficient. log
K
oa
¼
log
K
ow
RT
/
H
,where
K
ow
is the octanol-water coefficient,
R
is the gas constant,
T
the mean air temperature and
H
the temperature corrected Henry's law constant.
However, when applying the fraction of organic matter (
f
OM
) to the model
calculations, there is often an underestimation of the predicted
K
p
, especially in
remote areas. This underestimation could only be corrected by using unrealistically
high
f
OM
values. Notwithstanding, a general agreement between predicted and
measured
K
p
is observed when both organic matter absorption and soot-carbon
adsorption are included in the model (see equation below):
f
OM
MW
OCT
g
OCT
r
OCT
MW
om
g
OCT
a
ec
a
ac
K
P
¼
10
12
K
oa
þ
f
EC
10
12
K
sa
;
f
OM
MW
OCT
g
OCT
r
OCT
MW
om
g
OCT
10
12
K
oa
is the relationship between
K
p
and
K
oa
as described in its simplified form in Eq. (
1
), assuming
g
OCT
/
g
OM
,MW
OCT
/
MW
OM
to be equal to 1 and
where the expression
r
OCT
to be 0.820 kg/L.
f
EC
is the fraction of elemental
carbon in atmospheric particles, which was estimated to be less than 5% in rural and
remote sites [
59
],
a
ac
is the surface area of active carbon,
a
ec
is the specific surface
of elemental carbon and 10
12
is a factor for unit correction. Assuming that the
ratios
g
OM
,MW
OCT
/MW
OM
and
a
ec
/
a
ac
are equal to 1, it is possible to predict
K
p
values from
f
OM
and
f
EC
[
30
-
34
,
52
].
g
OCT
/
