Environmental Engineering Reference
In-Depth Information
If the last two reactions are fast in comparison to the reaction of ozone with Z, then
the ratio [O
3
]/[O] will decrease.
Although a number of species can catalyze the dissociation of ozone in the strato-
sphere, a major concern at present is the influence of chlorinated compounds called
CFCs. These have been in widespread use as refrigerants and aerosol propellants
and are purely anthropogenic in origin. They comprise mainly CF
2
Cl
2
(Freon 12,
CFC-12) and CFCl
3
(Freon 11, CFC-11) and are quite inert in the troposphere. Once
released to the atmosphere they are slowly transported to the stratosphere where they
remain for a very long time. Prior to the signing of the Montreal Protocol in 1987,
several tons of these compounds were pumped into the atmosphere. For example, the
concentration of CFC-12 continually increased (see Table 6.15) at a rate of 17 pptv/y
with a mean concentration of
130
years. Molina and Rowland (1974) first alerted the scientific community to the fact
that CFCs can adversely affect the stratospheric ozone layer. Radiations with wave-
lengths of 180-220 nm in the stratosphere are absorbed by CFCs to produce the highly
reactive Cl and ClO species (represented generally as ClO
x
)
.
∼
484 pptv in 1990. Its atmospheric lifetime is
∼
h
ν
−→
CF
2
Cl
+
Cl
•
,
CCl
2
F
2
(6.182)
k
1
−→
O
•
CCl
2
F
2
+
CF
2
Cl
+
ClO.
The ClO
x
can react with ozone and O atoms as follows:
k
2
−→
Cl
•
O
3
ClO
•
+
O
2
,
(6.183)
k
3
−→
O
•
ClO
+
Cl
+
O
2
.
The net effect is O
2O
2
. The Cl atoms are subsequently trapped by hydro-
carbons (e.g., CH
4
)
to form HCl, which appears in the troposphere, and are washed
down by rain. From the above scheme, we have
+
O
3
→
d
[
ClO
]
d
t
=
O
•
]+
Cl
•
]−
O
•
]
k
1
[
CCl
2
F
2
][
k
2
[
O
3
][
k
3
[
ClO
][
.
(6.184)
Therefore, the steady-state concentration of ClO is given by
k
1
k
3
[
k
2
k
3
[
O
3
]
Cl
•
]
[
ClO
]
ss
=
CCl
2
F
2
]+
O
•
]
[
.
(6.185)
[
Thus, the steady-state concentration of ClO can be maintained in the atmosphere
as long as CCl
2
F
2
is added to the stratosphere, even if no CCl
2
F
2
is photolyzed. If
photolysis of CCl
2
F
2
does occur, the second term becomes significant. Cl
•
will be
finite and [ClO]
ss
will increase.Thus, the above chain will sustain high concentrations
of ClO at the expense of O
3
. In other words, CFCs act as catalysts to remove ozone
from the stratosphere.
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