Geology Reference
In-Depth Information
on Figure 11. For example, in region A the loss of ozone (L(O
3
)) is
greater than the production of ozone (P(O
3
)), hence the net product of
this process, i.e.
N(O
3
)
¼
P(O
3
)
L(O
3
)
(2.31)
leads to a net ozone loss. The photochemical loss of ozone can be
represented as
L(O
3
)
¼
(f
j
2.7
(O
1
D)
[O
3
])
þ
k
2.15
[HO
2
]
þ
k
2.16
[OH] (2.32)
where f is the fraction of O(
1
D) atoms that react with water vapour
(reaction (2.8)) rather than are deactivated to O(
3
P) (reactions (2.9) and
(2.10)). Evaluation of Equation (2.32) is effectively a lower limit for the
ozone loss rate as it neglects any other potential chemical loss processes
for ozone such as cloud chemistry,
17
NO
3
chemistry (see Section 2.6) or
halogen chemistry (see Section 2.9). The balance point, i.e. where N(O
3
)
¼
0 is often referred to, somewhat misleadingly, as the compensation
point and occurs at a critical concentration of NO
x
. Above the com-
pensation point P(O
3
)4L(O
3
) and therefore N(O
3
) is positive and the
system is forming ozone. The in-situ formation rate for ozone is ap-
proximately given by the rate at which the peroxy radicals (HO
2
and
RO
2
) oxidise NO to NO
2
. This is followed by the rapid photolysis of
NO
2
(reaction (2.4)) to yield the oxygen atom required to produce O
3
.
P(O
3
)
¼
[NO]
(k
2.19
[HO
2
]
þ
Sk
i
[RO
2
]
i
)
(2.33)
It is also worth noting that P(O
3
) can also be expressed in terms of the
concentrations of NO
x
, j
2.2
(NO
2
), O
3
and temperature by substitution of
Equation (2.27) into Equation (2.33) to give
P(O
3
)
¼
j
2.2
[NO
2
]
k
2.24
[NO][O
3
]
(2.34)
At some concentration of NO
x
the system reaches a maximum produc-
tion rate for ozone at dP(O
3
)/d(NO
x
)
¼
0 and even though P(O
3
) is still
significantly larger than L(O
3
) the net production rate begins to fall off
with increasing NO
x
. Until this maximum is reached the system is said to
be NO
x
limited with respect to the production of ozone. The turn-over,
i.e. dP(O
3
)/d(NO
x
)
¼
0 is caused by the increased competition for NO
x
by the reaction
OH
þ
NO
2
þ
M
-
HNO
3
þ
M
(2.23)
In reality, the situation is somewhat more complicated owing to the
presence at high concentrations of NO
x
of increased levels of non-
methane hydrocarbons (NMHCs), especially in places such as the urban
