Chemistry Reference
In-Depth Information
snow removes a gaseous compound from the atmosphere (
C
air gas
) can be defined
from the washout ratio of the gas phase (
W
g
)[
61
]:
W
g
¼
C
rain
=
C
air gas
¼
RT
=
H
ð
mass/volume
Þ:
According to this equation, compounds with low
H
tend to be washed out faster
than compounds with high
H
.
The washout factor of the particulate phase (
W
p
) is important in rain and snow
scavenging of particle-bound POP (
C
air particulate
)[
61
]:
W
p
¼
C
rain
=
C
air part
ð
mass
=
volume
Þ:
The
W
p
is however very variable and depends on the size of the particle, its
structure and chemical composition and on the meteorological conditions.
3.2.2 Dry Deposition
The velocity of particle dry deposition, and that of the particulate-bounded
compounds, depends largely on the size and composition of the aerosol. For large
particles (
>
25
m
m), the deposition is mainly governed by gravity. Aerosols of this
size are deposited near the emission sources. Smaller aerosols between 0.1 and
10
m are suitable to long-range atmospheric transport, involving sometimes more
than hundreds of kilometres. Invasions of Saharan dust to the North Atlantic and
Europe mainly consist of aerosols of these sizes (mostly
m
<
2.5
m
m). Particles
smaller than 0.1
m depend on molecular diffusion [
61
]. POPs have higher affinity
to particles with high organic carbon content due to their relative high lipophilicity
(log
K
ow
>
m
4).
The particulate flux to a surface (
F
d part
) and the particle-bound atmospheric
concentration of a compound (
C
air part
) at a given height are related to the dry
deposition velocity (
V
d part
):
V
d part
¼
F
d part
=
C
air part
:
This velocity is very variable and depends, for example, on the meteorological
conditions, particle size and composition and surface properties.
The lower molecular weight compounds are in general more abundant in the wet
deposition than the heavier molecular weight compounds (Fig.
9
). The former also
occur predominantly in the atmospheric gas phase, while the latter are particle-
bound in larger proportion [
35
]. The transfer of gas phase POP to rain and/or
snowflakes is very effective [
62
-
64
]. Wet deposition accounts for the predominant
incorporation pathway of gas phase and particulate phase POPs. But the efficiency
depends on the quantity of precipitation, which may show high variability between
different sites and seasons.
