Geology Reference
In-Depth Information
2.9.1 Tropospheric Halogens and Catalytic Destruction of Ozone
Potentially, the most important effect of reactive halogen species maybe
that their chemistry may lead to the catalytic destruction of ozone via
two distinct cycles
Cycle I:
XO
þ
YO
-
X
þ
Y
þ
O
2
(or XY
þ
O
2
)
(2.82)
X
þ
O
3
-
XO
þ
O
2
(2.83)
Net:
O
3
þ
O
3
-
3O
2
(2.84)
In Cycle I, the rate-limiting step involves reaction (2.82), the self- or
cross-reaction of the halogen monoxide radicals. Cycle I has been
identified to be the prime cause for polar boundary layer ozone destruc-
tion.
38
The second-ozone destruction cycle, which is more prevalent at
low halogen levels, has the form
Cycle II:
XO
þ
HO
2
-
HOX
þ
O
2
(2.85)
HOX
þ
hn
-
X
þ
OH
(2.86)
X
þ
O
3
-
XO
þ
O
2
(2.83)
OH
þ
CO
-
H
þ
CO
2
(
þ
M)
-
HO
2
(2.11, 2.12)
Net: O
3
þ
CO
þ
hn
CO
2
þ
O
2
-
(2.87)
The rate-determining step in this reaction sequence is reaction (reaction
(2.85)) making ozone destruction linearly dependent on [XO]. The
fraction of HOX that photolyses to give back OH depends critically
on the accommodation coefficient of HOX on aerosols. Currently, there
is a large uncertainty in this parameter. An important side effect of Cycle
II is the potential for the reduction of the [HO
2
]/[OH] ratio by con-
sumption of HO
2
. The inorganic halogen chemistry described is sum-
marised in Figure 24.
