Geology Reference
In-Depth Information
[BrO] show a pulse in the first two hours after dawn ascribed to the
photolysis of inorganic bromine compounds produced either by the
bromine explosion mechanism
41
or the photolysis of mixed bromo/
iodo-organohalogens
46
built up overnight. Using measured concen-
trations of BrO, IO and HO
2
, the data in Table 7 show the ozone
depletion cycle (Cycle type I) involving the BrO and IO cross-reaction
is the most important with an O
3
depletion rate of 0.3 ppbv h
1
.
2.10 STRATOSPHERIC CHEMISTRY
The stratosphere is characterised by increasing temperatures with in-
creasing height (see Figure 1). The presence of ozone and oxygen that
absorb the UV light and emit infrared radiation heats this region.
Approximately, 90% of ozone molecules are found in the stratosphere
(see Figure 6). The chemistry of the stratosphere, as compared to that of
the troposphere, maybe described as being more chemically simple but
initiated by shorter wavelength (more energetic) light.
A British scientist Sydney Chapman
50
suggested the basic ideas of
stratospheric ozone in the 1930s, which have become known as the
Chapman cycle. Short wavelength UV (hn) can dissociate molecular
oxygen and the atomic oxygen fragments produced react with oxygen
molecules to make ozone.
O
2
þ
hn
-
O
þ
O
(2.88)
O
þ
O
2
M
O
3
ð
2
:
20
Þ
There are some further reactions that complete the picture, which are
involved in the interconversion and removal of ozone and atomic
oxygen
O
3
þ
hn
-
O
þ
O
2
(2.89)
O
þ
O
3
-
O
2
þ
O
2
(2.90)
As O and O
3
can rapidly interconvert (reactions (2.89 and 2.90)) they are
often referred to as odd oxygen.
Even from this simple chemistry, it is possible to see why there is an
ozone layer (see Figure 6). For the production of ozone, both UV
radiation (hn) and molecular oxygen are required (O
2
). High up in the
atmosphere there is a plentiful supply of short-wavelength UV radiation
but little oxygen, lower down in the atmosphere the opposite situation
