Geology Reference
In-Depth Information
are illustrative of the method and controlling quantities for ozone
photolysis.
The most commonly used form of Equation (2.4) becomes
j
ð
s
1
Þ¼
X
l
max
s
av
ð
l
Þ
f
av
ð
l
Þ
F
av
ð
l
Þ
l
min
The av denotes that average of wavelength intervals. Therefore, j
¼
3.14
10
4
s
1
.
2.4 TROPOSPHERIC CHEMISTRY
In general, tropospheric chemistry is analogous to a low-temperature
combustion system, the overall reaction is given by
CH
4
þ
2O
2
CO
2
þ
2H
2
O
-
(2.5)
Unlike combustion, this is not a thermally initiated process but a process
initiated and propagated by photochemistry. The chemistry that takes
place in the troposphere, and in particular, the photochemistry is
intrinsically linked to the chemistry of ozone. Tropospheric ozone acts
as initiator, reactant and product in much of the oxidation chemistry
that takes place in the troposphere and stratospheric ozone determines
the amount of short wavelength radiation available to initiate photo-
chemistry. Figure 6 shows a typical ozone profile through the atmos-
phere illustrating a number of interesting points. First, 90% of
atmospheric ozone can be found in the stratosphere (see Section 2.10);
on average about 10% can be found in the troposphere. Second, the
troposphere in the simplest sense consists of two regions. The lowest
kilometre or so contains the planetary boundary layer
6
and inversion
layers which can act as pre-concentrators for atmospheric emissions
from the surface and hinder exchange to the so-called free troposphere,
the larger part by volume, that sits above the boundary layer.
For a long time, transport from the stratosphere to the troposphere
was thought to be the dominant source of ozone in the troposphere.
7,8
Early in the 1970s, it was first suggested
9,10
that tropospheric ozone
originated mainly from production within the troposphere by photo-
chemical oxidation of CO and hydrocarbons catalysed by HO
x
and
NO
x
. These sources are balanced by in-situ photochemical destruction
of ozone and by dry deposition at the earth's surface. Many studies,
both experimental- and model-based have set about determining the
