DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Although significant decreases in ozone have
been observed over much of the globe since the
1970s, the mechanisms are presently only partly
understood. The appearance of the Antarctic
ozone hole in the early 1980s demonstrated that
polar ozone can be drastically reduced by manmade
substances, namely chlorofluorocarbons
(CFCs). These CFCs provide a source of chlorine
in the atmosphere. Atomic chlorine is produced
from Cl2 or from CFCs by photodissociation
in sunlight. Hydrogen chloride and chlorine
nitrate are the most important two stratospheric
chlorine reservoirs. Especially in polar regions,
stratospheric clouds can serve as the reaction
sites to produce molecular chlorine and nitric
acid from the reservoirs. Chlorine is extremely
damaging to ozone, because it acts catalytically:
Cl ∗ + O3 → ClO ∗ + O2
O 3 + hν → O ∗ + O2
ClO ∗ + O ∗ → Cl ∗ + O2 .
© 2001 by CRC Press LLC
ozonosphere
The net result is to reform the chlorine radical,
and to convert two ozone molecules to three O2
molecules. The Montreal Protocol, signed in
1987, and its subsequent amendments, provide
an international framework for the phasing-out
of CFCs, although the extent and timescale of
the recovery in the ozone layer are yet to be
determined.
ozonosphere The atmospheric layer with
higher concentrated ozone, which lies about 10
to 50 km altitude and thus is essentially synonymous
with the stratosphere. The maximum concentration
of ozone occurs at about 20 to 25 km.
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