Introduction
The presence of the Earth’s ozone layer is essential
for life on Earth. The realization that through
human activities we had started to destroy it has
been a major science and environmental issue since
the late twentieth century. This chapter briefly discusses the key science and policy issues behind the
issue of global stratospheric ozone depletion.
Section “Stratospheric Ozone” describes the
chemistry of ozone layer and summarizes the natural and man-made substances, which can lead to
its depletion. Section “Ozone Depletion”
describes observations of past ozone depletion
and the policy measures which have been introduced to reverse this trend. Section “Ozone
Recovery and Chemistry–Climate Interactions”
discusses the likely future evolution of the ozone
layer and how this will depend on interactions
with climate change. Some future directions for
ozone layer research are outlined in section
“Future Directions”.
Stratospheric Ozone
The Stratosphere and the Natural Ozone Layer
Ozone is produced in the stratosphere naturally by
the action of short wavelength ultraviolet radiation (l 242 nm) dissociating oxygen molecules:
O 2 þ hv ! O þ O
ð1Þ
where hv is a photon of light with sufficient
energy to break the O–O bond. The O atoms produced can rapidly combine with another O 2 molecule to form ozone:
O þ O 2 þ M ! O 3 þ M
ð2Þ
where M is any third body. In fact, O and O 3 form
a tightly coupled chemical family and are normally considered together as “odd oxygen.” The
production and removal of O or O 3 is equivalent;
destruction of O leads to O 3 loss. O 3 can be
converted back to O via:
O 3 þ hv ! O þ O 2
ð3Þ
Ozone (or odd oxygen) is destroyed by conversion to O 2 via:
O þ O 3 ! 2O 2
ð4Þ
The first theory for the chemistry of the ozone
layer was put forward by Chapman in 1930
[1]. His scheme comprised of reactions (1)–(4)
and was therefore based on oxygen-only chemistry. The chemical loss of odd oxygen in the Chapman mechanism occurs via reaction (4), i.e., the
direct reaction of O and O 3 . The Chapman mechanism successfully and qualitatively predicts the
occurrence of an ozone layer in the stratosphere.
However, by the 1960s, when good observations
of the ozone layer became available and improved
laboratory measurements gave accurate values of
the relevant chemical rate constants, it became
clear that the Chapman scheme could not explain
the observations quantitatively. While the Chapman scheme does include the source of stratospheric ozone (reaction 1), the direct reaction (4)
accounts for only about 20% of the actual loss.
The Chapman mechanism ignores the very important destruction of ozone by other chemical
families.
Catalytic Ozone Loss
There are a number of cycles which can catalyze
the destruction of stratospheric ozone. Many
cycles are based on:
XO þ O ! X þ O 2
X þ O 3 ! XO þ O 2
Net : O þ O 3 ! 2O 2
In this scheme, the species X (in practice X ¼
OH, NO, Cl, Br) reacts with O 3 to form XO. XO
then reacts with O to reform X. Therefore, X is
acting catalytically and the net effect of these two
reactions is the same as reaction (4) of the Chapman mechanism. In this way a species with an
abundance much smaller than O 3 can cause its
destruction, i.e., species present at the part per
billion (1 in 10
9 ) or part per trillion (1 in 10
12 )
374
Stratospheric Pollution
The presence of the Earth’s ozone layer is essential
for life on Earth. The realization that through
human activities we had started to destroy it has
been a major science and environmental issue since
the late twentieth century. This chapter briefly discusses the key science and policy issues behind the
issue of global stratospheric ozone depletion.
Section “Stratospheric Ozone” describes the
chemistry of ozone layer and summarizes the natural and man-made substances, which can lead to
its depletion. Section “Ozone Depletion”
describes observations of past ozone depletion
and the policy measures which have been introduced to reverse this trend. Section “Ozone
Recovery and Chemistry–Climate Interactions”
discusses the likely future evolution of the ozone
layer and how this will depend on interactions
with climate change. Some future directions for
ozone layer research are outlined in section
“Future Directions”.
Stratospheric Ozone
The Stratosphere and the Natural Ozone Layer
Ozone is produced in the stratosphere naturally by
the action of short wavelength ultraviolet radiation (l 242 nm) dissociating oxygen molecules:
O 2 þ hv ! O þ O
ð1Þ
where hv is a photon of light with sufficient
energy to break the O–O bond. The O atoms produced can rapidly combine with another O 2 molecule to form ozone:
O þ O 2 þ M ! O 3 þ M
ð2Þ
where M is any third body. In fact, O and O 3 form
a tightly coupled chemical family and are normally considered together as “odd oxygen.” The
production and removal of O or O 3 is equivalent;
destruction of O leads to O 3 loss. O 3 can be
converted back to O via:
O 3 þ hv ! O þ O 2
ð3Þ
Ozone (or odd oxygen) is destroyed by conversion to O 2 via:
O þ O 3 ! 2O 2
ð4Þ
The first theory for the chemistry of the ozone
layer was put forward by Chapman in 1930
[1]. His scheme comprised of reactions (1)–(4)
and was therefore based on oxygen-only chemistry. The chemical loss of odd oxygen in the Chapman mechanism occurs via reaction (4), i.e., the
direct reaction of O and O 3 . The Chapman mechanism successfully and qualitatively predicts the
occurrence of an ozone layer in the stratosphere.
However, by the 1960s, when good observations
of the ozone layer became available and improved
laboratory measurements gave accurate values of
the relevant chemical rate constants, it became
clear that the Chapman scheme could not explain
the observations quantitatively. While the Chapman scheme does include the source of stratospheric ozone (reaction 1), the direct reaction (4)
accounts for only about 20% of the actual loss.
The Chapman mechanism ignores the very important destruction of ozone by other chemical
families.
Catalytic Ozone Loss
There are a number of cycles which can catalyze
the destruction of stratospheric ozone. Many
cycles are based on:
XO þ O ! X þ O 2
X þ O 3 ! XO þ O 2
Net : O þ O 3 ! 2O 2
In this scheme, the species X (in practice X ¼
OH, NO, Cl, Br) reacts with O 3 to form XO. XO
then reacts with O to reform X. Therefore, X is
acting catalytically and the net effect of these two
reactions is the same as reaction (4) of the Chapman mechanism. In this way a species with an
abundance much smaller than O 3 can cause its
destruction, i.e., species present at the part per
billion (1 in 10
9 ) or part per trillion (1 in 10
12 )
374
Stratospheric Pollution