Geoscience Reference
In-Depth Information
Plate 2.1
Dust plumes over the Red Sea from MODIS on 15 January 2009. Courtesy NASA
other oxygen molecules to create ozone, as
illustrated by the simple photochemical scheme:
A
Ozone formation
(The Chapman cycle)
UV
RADIATION
MOLECULAR
OXYGEN
O
2
O
2
O
2
+ O + M → O
3
+ M
where M represents the energy and momentum
balance provided by collision with a third atom
or molecule; this Chapman cycle is shown
schematically in
Figure 2.2A
. Such three-body
collisions are rare at 80 to 100km because of the
very low density of the atmosphere, while below
about 35km most of the incoming ultra-
violet radiation has already been absorbed at
higher levels. Therefore ozone is mainly formed
between 30 and 60km, where collisions between
O and O
2
are more likely. Ozone itself is unstable;
its abundance is determined by three different
photochemical interactions. Above 40km odd
oxygen is destroyed primarily by a cycle involving
molecular oxygen; between 20 and 40km
NO
x
cycles are dominant; while below 20km a
hydrogen-oxygen radical (HO
2
) is responsible.
ATOMIC
OXYGEN
O
OZONE
O
3
UV RADIATION
B
Ozone destruction
O
3
+
X
O
2
XO
Figure 2.2
Schematic illustrations of (A) the
Chapman cycle of ozone formation and (B) ozone
destruction. X is any ozone-destroying species (e.g.
H, OH, NO, CR, Br).
Source: After Hales (1996) from Bulletin of the American
Meteorological Society, by permission of the American
Meteorological Society.
