Environmental Engineering Reference
In-Depth Information
To simplify the above expression, we should have some idea about the relative
magnitudes of the various rate constants. Generally, in the stratosphere
[
Z
]=[
N
2
]
10
24
molecules/m
3
,
J
1
∼
10
−
10
min
−
1
, and
J
3
∼
0.019 min
−
1
.
k
2
∼
∼
1.5
×
1.2
×
10
−
43
m
6
/molecule
2
min,
k
4
∼
10
−
20
m
3
/molecule min, [O
2
]∼
10
23.5
mole-
7.5
×
cules/m
3
. Hence,
)
1
/
2
10
18.8
molecules/m
3
and
[
O
3
]=[
O
2
]
((J
1
/J
3
)(k
2
/k
4
)
·[
N
2
]
≈
10
13.4
molecules/m
3
.
The steady-state concentration of [O
3
] given by the equation earlier can be
rearranged to obtain the expression
[
]=
O
k
2
[
O
2
][
Z
]
[
O
3
]=
(6.176)
k
4
(
1
+
(J
3
/k
4
[
O
]
))
10
−
20
10
13.4
)
10
4
. Hence,
with
J
3
/k
4
[
O
]≈
(
0.019
/
7.5
×
×
=
1
×
[
O
3
]≈
(k
2
/J
3
)
[
O
2
][
Z
][
O
]
. Thus we have the ratio
[
O
3
]
[
k
2
J
3
[
≈
O
2
][
Z
]
.
(6.177)
O
]
10
5
)
if [O
2
] remains constant. This ratio can vary if either [O] or [O
3
] varies due to other
reactions. Since the production of O atoms by photolysis of O
2
is a slow process
(
J
1
is very small), any competing reaction that decomposes O
3
faster will reduce the
ratio rapidly. These competing reactions involve trace species such as OH
•
, NO, and
CFCs. The general catalytic cycle follows the scheme
The steady-state concentration ratio [O
3
]/[O] should remain constant (
∼
2.5
×
X
+
O
3
−→
XO
+
O
2
,
(6.178)
XO
+
O
−→
O
2
+
X
with the net reaction
O
+
O
3
−→
2O
2
.
(6.179)
Note that the species X is neither formed nor removed from the system, and thus acts
as a catalyst.
ThereactionwithOH
•
isestimatedtoaccountforthedecompositionofonly
−
15%
of the stratospheric ozone. Excited ozone molecules (
<
310 nm) give rise to excited
oxygen atoms, which in turn decomposes H
2
OorCH
4
to provide OH
•
:
λ
ν
−→
h
O
∗
+
O
3
O
2
,
O
∗
+
HO
•
,
(6.180)
CH
4
−→
CH
3
+
O
∗
+
2HO
•
.
H
4
O
−→
The OH radicals react with ozone in a self-propagating series of reactions to give two
O
2
molecules:
HO
•
+
O
3
−→
O
2
+
HO
2
,
(6.181)
HO
•
.
HO
2
+
O
3
−→
O
2
+
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