Environmental Engineering Reference
In-Depth Information
This is an important result, since it states that the concentration of ozone is dependent
not only on the magnitude of NO
x
, but also on the ratio [NO
2
]/[NO]. As
J
1
→
0 (i.e.,
nighttime conditions), in the presence of excess of ozone the ratio becomes very large,
whereas the lowest ratios are observed in bright sunshine (high
J
1
∼
20 h
−
1
)
.
If we now start with a system with initial condition [O
3
]
0
=
0 and [NO]
0
=
0, the
stoichiometry states that [O
3
]=[
NO
2
]
0
−[
NO
2
]=[
NO
]
. Thus,
J
1
[
NO
2
]
[
NO
]
k
3
[
NO][NO
2
]
J
1
+
k
3
[
NO
]
=[
NO
2
]
0
−[
NO
2
]
,
[
NO
2
]=
.
Now, since [O
3
]=[
NO
2
]
0
−[
NO
2
], we obtain the following quadratic in [O
3
]:
k
3
O
3
2
+
J
1
O
3
−
J
1
NO
2
0
=
0.
(6.193)
Since [O
3
]
>
0, we keep only the positive root of the above quadratic, and obtain
⎡
⎣
⎤
⎦
.
J
1
k
3
1
/
2
2
O
3
=
k
3
NO
2
0
1
2
+
4
J
1
J
1
k
3
−
(6.194)
We know that
J
1
∼
0.6 min
−
1
.
k
3
, which is in cm
3
/molecule S, can be converted into
ppmv/min by multiplying with 1.47
×
10
15
. Hence
k
3
∼
26 ppmv/min and
J
1
/k
3
=
0.02 ppmv. The variation in [O
3
] with [NO
2
]
0
is shown in Figure 6.49. When
[NO
2
]
0
=
0, [O
3
]=
0 and the ozone concentration increases as more NO is converted
to NO
2
.
1
0.8
[NO
2
]
0.6
0.4
0.2
[O
3
]
0
0
0.2
0.4
0.6
0.8
1
1.2
[NO
2
]
0
/
ppmv
FIGURE 6.49
Ozone concentration as a function of initial nitrogen dioxide
concentration.
continued
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