Geology Reference
In-Depth Information
NO
2
þ
hn
NO
þ
O
-
(2.2)
Net O
3
þ
hn
-
O
2
þ
O
(2.89)
The products of reaction (2.2) recombining via reaction (2.20) to reform
ozone. This form of cycle (2.24, 2.2, 2.89) is often referred to as a null
cycle. The fraction of the XO
x
tied up in null cycles is ineffective as a
catalyst.
Another key feature with respect to the effectiveness of catalytic cycles
is the formation of reservoir species via holding cycles. At any given time
about 99% of active Cl is held as reservoir species.
Cl
þ
CH
4
-
HCl
þ
CH
3
(2.94)
Cl
þ
H
2
-
HCl
þ
H
(2.95)
ClO
þ
NO
2
M
ClONO
2
ð
2
:
96
Þ
These reservoirs are of great importance to the chemistry of the strat-
osphere as they act to divert potential catalytic species from active to
inactive forms, but they remain available to release the active catalysts
again. HCl is the longest-lived and most abundant Cl reservoir species
having a lifetime of about one month in the lower stratosphere. It is
returned to active Cl largely via reaction with OH
OH
þ
HCl
-
Cl
þ
H
2
O
(2.97)
ClONO
2
formed via reaction (2.96) is destroyed mainly by photolysis or
via reaction with O atoms, leading to regeneration of active Cl species.
ClONO
2
þ
hn
-
Cl
þ
NO
3
(2.98a)
ClONO
2
þ
hn
-
ClO
þ
NO
2
(2.98b)
O
þ
ClONO
2
-
Products
(2.99)
The lifetime of ClONO
2
is approximately 6 h in the lower stratosphere
(
o
30 km) decreasing to about an hour at 40 km owing to the increase in
UV light. Figure 25 shows the key chemical interconversions of the
chlorine chemistry in the stratosphere, delineating the molecules into
source, active and reservoir species.
Where do these catalytic species come from and what effect does man
have in increasing the levels of these catalysts? The catalytic families of
