Geology Reference
In-Depth Information
pertains. Thus, the point where there is both enough molecular oxygen
and UV light is where the maximum in the ozone concentrations are
going to be found. In the terrestrial atmosphere this point is at a height
of about 15-40 km, as shown in Figure 6. When the Chapman oxygen-
only mechanism is applied to real atmospheric measurements, it is
found that about five times as much ozone is predicted as is measured.
This therefore follows that there must be some other, faster, way of
removing ozone in the stratosphere. One idea put forward by Bates and
Nicolet
51
is that a catalytic process could be speeding up the destruction
of ozone. A catalyst is something that can promote a chemical reaction
without itself being consumed. In the stratosphere, a catalytic cycle
like
X
þ
O
3
-
XO
þ
O
2
(2.83)
O
þ
XO
-
X
þ
O
2
(2.91)
Net O
þ
O
3
-
O
2
þ
O
2
(2.90)
can lead to the net destruction of ozone (reaction (2.90)). In this case the
catalyst is the molecule X, which by destroying ozone is converted into
XO, but the XO then reacts with an oxygen reforming X. This, the so-
called chain process, can proceed many hundreds of thousands of times
before termination. As to the identity of X, it is molecules like chlorine
atoms (Cl), nitrogen oxide (NO) and the hydroxyl radical (OH) and
therefore the corresponding XO is ClO, NO
2
and HO
2
, i.e.
Cl
þ
O
3
-
ClO
þ
O
2
(2.92)
O
þ
ClO
-
Cl
þ
O
2
(2.93)
These cycles are often referred to as the HO
x
,NO
x
and ClO
x
cycles and
the groups of species as families.
On the surface there is now an apparent contradiction; with only the
Chapman cycle we produce too much ozone, whereas the catalytic cycles
could destroy all the ozone! There are a number of cycles that can
interconvert the catalytic cycles without odd oxygen removal these will
be in competition with the catalytic cycles, e.g.
NO
þ
O
3
-
NO
2
þ
O
2
(2.24)
