132 ATMOSPHERIC CHEMISTRY
Stratospheric Ozone BalanceIn the stratosphere, there is sufficient high-energy ultraviolet
radiation to photolyze molecular oxygen:O 2 h ( 240 nm) → 2O(^3 P) (43)This will be followed by the oxygen-atom reaction with
O 2 (2) forming ozone. These processes describe the ozone
production in the stratosphere. They are also the processes
responsible for the heating in the upper stratosphere. This
ozone production must be balanced by ozone-destruction
processes. If we consider only oxygen chemistry, ozone
destruction is initiated by ozone photolysis (22), forming an
oxygen atom. The oxygen atom can also react with ozone,
re-forming molecular oxygen:O(^3 P) O 3 → 2O 2 (44)Reactions (43), (2), (22), and (44) describe the formation and
destruction of stratospheric ozone with oxygen-only chemis-
try. This is commonly known as the Chapman mechanism.
Other chemical schemes also contribute to the chemistry
in the natural (unpolluted) stratosphere. Water can be photo-
lyzed, forming hydrogen atoms and hydroxyl radicals:H 2 O h ( 240 nm) → H OH (45)The OH radical may react with ozone to form HO 2 , which
may in turn react with an O atom to reform OH. The net
effect is the destruction of odd oxygen (O and/or O 3 ).OH O 3 → HO 2 O 2 (46)
HO 2 O → OH O 2 (47)
O O 3 → 2O 2 (Net)These reactions form a catalytic cycle that leads to the
destruction of ozone. An alternative cycle isH O 3 → OH O 2 (48)
OH O → H O 2 (49)
O O 3 → 2O 2 (Net)Other catalytic cycles involving HO x species (H, OH, and
HO 2 ) are possible. Analogous reactions may also occur
involving NO x species (NO and NO 2 ),NO O 3 → NO 2 O 2 (3)
NO 2 O → NO O 2 (50)
O O 3 → 2O 2 (Net)and ClO x species (Cl and ClO),Cl O 3 → ClO O 2 (51)
ClO O → Cl O 2 (52)
O O 3 → 2O 2 (Net)These processes are some of the ozone-destruction processes
of importance in the stratosphere. These types of processescontribute to the delicate balance between the stratospheric
ozone production and destruction, which provide the natural
control of stratospheric ozone, when the stratospheric HO x ,
NOx , and ClO x species are of natural origin.
Ozone plays an extremely important role in the strato-
sphere. It absorbs virtually all of the solar ultraviolet radi-
ation between 240 and 290 nm. This radiation is lethal to
single-cell organisms, and to the surface cells of higher
plants and animals. Stratospheric ozone also reduces the
solar ultraviolet radiation up to 320 nm, wavelengths that
are also biologically active. Prolonged exposure of the skin
to this radiation in susceptible individuals may lead to skin
cancer. In addition, stratospheric ozone is the major heat
source for the stratosphere, through the absorption of ultra-
violet, visible, and infrared radiation from the sun. Hence,
changes in the stratospheric ozone content could lead to sig-
nificant climatic effects.Stratospheric PollutionOver the past 30 years, there has been considerable interest
in understanding the ways in which man’s activities might
be depleting stratospheric ozone. Major concerns first arose
from considerations of flying a large fleet of supersonic air-
craft in the lower stratosphere. These aircraft were expected
to be a significant additional source of NO x in the strato-
sphere. This added NO x could destroy stratospheric O 3 by
the sequence of reactions (3) and (50) and other similar cata-
lytic cycles. The environmental concerns, along with eco-
nomic factors, were sufficient to limit the development of
such a fleet of aircraft.
More recently, environmental concern has turned to the
effects of chlorofluorocarbons on the stratospheric ozone.
These compounds were used extensively as aerosol propel-
lants and foam-blowing agents and in refrigeration systems.
The two most commonly used compounds were CFCl 3 (CFC-
11) and CF 2 Cl 2 (CFC-12). These compounds are very stable,
which allows them to remain in the atmosphere sufficiently
long that they may eventually diffuse to the stratosphere.
There they may be photolyzed by the high-energy ultraviolet
radiation:CFCL 3 h ( 190 nm) → CFCl 2 Cl (53)This reaction, and similar reactions for other chlorinated com-
pounds, leads to a source of chlorine atoms in the stratosphere.
These chlorine atoms may initiate the catalytic destruction of
ozone by a sequence of reactions, such as reactions (51) and
(52). Numerous other catalytic destruction cycles have been
proposed, including cycles involving combinations of ClO x ,
HOx , and NO x species.
In recent years, our ability to model stratospheric chem-
istry has increased considerably, which allows good compari-
sons between model results and stratospheric measurements.
Based upon our improved understanding of the stratosphere
and the continuing concern with CFCs, about 45 nations met
during the fall of 1987 to consider limitations on the produc-
tion and consumption of CFCs. This led to an agreementC001_008_r03.indd 132C001_008_r03.indd 132 11/18/2005 10:15:23 AM11/18/2005 10:15:23 AM