Chemistry Reference
In-Depth Information
Fig. 8 log
K
ow
and log
K
aw
of different organic molecules
belonging to the group of
POPs. The properties of these
compounds make them
suitable for long-range
atmospheric transport
(adapted from [
60
])
In general, POPs are volatile enough to be present in the atmosphere at ambient
temperatures and travel over long distances. However, they are not too volatile, so
they can be absorbed to surfaces, particularly if those are organic-rich, and be
incorporated to other environmental compartments. Their semi-volatility is
expressed by the partitioning coefficient air-water,
K
aw
. Moreover, they have
higher affinity for organic matter than water, which is expressed by the partition
coefficient octanol-water,
K
ow
, and this delays the washout from the atmosphere by
rain. The
K
ow
of the different POPs versus their
K
aw
is shown in Fig.
8
. POPs are
found in an area of intermediate
K
aw
and
K
ow
. Any other compound with similar
K
aw
and
K
ow
values is a potential POP.
3.2 Atmospheric Deposition
Deposition of POPs may take place by (1) snow and rain scavenging of gases and
aerosols (wet deposition), (2) dry particle deposition and (3) gas exchange with
surfaces [
61
]. The distributions of POPs between the gas and particle phases depend
on their physical-chemical properties (Fig.
8
) as well as the environmental
conditions in the atmosphere, such as temperature, amounts and composition of
particles [
26
,
35
].
3.2.1 Wet Deposition
Wet deposition involves the adsorption of gases and aerosols on surfaces after
removal by rain, snowfall, etc. Water solubility and vapour pressure are determi-
nant for this process. Compounds with low
H
constants are washed out from the
atmosphere faster than those with high H. The efficiency in which rain (
C
rain
)or
