Environmental Engineering Reference
In-Depth Information
[CFC]
[CFC]
in1997
=
e
−
0.0065
t
,
and is shown as curve 3. It is evident that even under these most optimistic conditions,
the concentration of CFC in the year 2087 will decrease only to one-half its value in
1987. This clearly shows how long the effect of CFCs will linger in the stratosphere
even if a concerted attempt is made in this century to eliminate its production.
In the lower troposphere, ozone performs a different function. It reacts with oxides
of nitrogen (NO
x
)
and atmospheric hydrocarbons and leads to the formation of
smog
.
This process accounts for
∼
60-70% of the ozone destroyed in the troposphere.
E
XAMPLE
6.25 K
INETICS OF
S
MOG
F
ORMATION IN
U
RBAN
A
REAS
The chemistry of smog, as we have discussed, is tied to the chemistry of NO
x
species
in the atmosphere. The basic reaction is the photochemical dissociation of NO
2
(
λ =
435 nm) that gives rise to NO and O. The O then rapidly reacts with O
2
in the presence
of a third body, Z (e.g., N
2
)
, to produce ozone. The cycle is completed when O
3
reacts
with NO to regenerate NO
2
.
NO
2
h
−
J
1
NO
2
+
O;
J
1
≈
0.6 min
−
1
,
O
+
O
2
+
Z
k
2
−→
O
3
+
Z
∗
;
k
2
≈
6.1
×
10
−
34
cm
6
/(
molecule
2
s
)
,
(6.189)
O
3
+
NO
k
3
−→
O
2
+
NO
2
;
k
3
≈
1.8
×
10
−
14
cm
3
/(
molecules
)
.
Therateconstantsgivenareat298 K.Asexplainedearlier,wedistinguishphotochemical
rate constants from chemical rate constants by utilizing the symbol
J
for the former.
The rates of formation of the different species are
=−
J
1
NO
2
+
k
3
O
3
[NO],
d
[
NO
2
]
d
t
d
t
=
J
1
NO
2
−
k
2
[O]
O
2
[Z],
d
O
3
d [O]
(6.190)
d
t
=
k
2
[O]
O
2
[Z]
−
k
3
[NO]
O
3
.
Applying the PSSA for the intermediate species [O], we obtain
J
1
k
2
[
NO
2
]
[
O
2
][
Z
]
[O]
ss
=
.
(6.191)
If the NO
2
formation and dissipation is at steady state, we have
O
3
=
J
1
k
3
[
NO
2
]
.
(6.192)
[
NO
]
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