HISTORICAL OVERVIEW OF OZONE TRENDS AND FUTURE SCENARIOS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 1

HISTORICAL OVERVIEW OF OZONE TRENDS AND FUTURE SCENARIOS
JANET F. BORNMAN
Department of Genetics and Biotechnology, Research Centre
Flakkebjerg, Danish Institute of Agricultural Sciences (DIAS), Slagelse,
Denmark
1. Ozone distribution
Stratospheric ozone (O 3 ) is created over low latitudes by the action of ultraviolet
radiation of wavelengths shorter than ca 240 nm. An oxygen molecule (O 2 ) reacts with
the high energy radiation and two oxygen atoms are formed in the reaction. A third
molecule (M), e.g. another oxygen or nitrogen, is required to remove the excess kinetic
energy in the following way:
O 2 + UV (< 240 nm) ĺ O + O
O 2 + O + M ĺ O 3 + M
The destruction of ozone results in its breakdown to molecular oxygen and
atomic oxygen. In equilibrium, these two events of synthesis and degradation have in
the past resulted in an average ozone content of ca 300 DU (Dobson unit, DU = 1 mm of
ozone at STP). However, with the loading of the atmosphere with halogen compounds
containing Cl and Br from industrial activities, the balance is no longer in place, since
Br, BrO, Cl and ClO take part in catalytic breakdown cycles involving ozone.
Most of the stratospheric ozone occurs between 10 and 30 km above the surface
of the earth, providing an effective filter against harmful ultraviolet radiation. This high
energy radiation can cause erythema (sunburn), skin cancers, cataracts, and changes in
immune response, etc. UV also modifies terrestrial and aquatic life forms, as well as
detrimentally affecting synthetic and natural materials. The filtering layer typically
removes 70-90% of the UV radiation. Furthermore, since ozone absorbs solar energy,
the ozone layer is an important controlling factor of upper stratospheric temperature.
Between 1979 and 1997, the annual global average temperature decreased by 0.6 Kelvin
per decade in the lower stratosphere, and by 3 K per decade in the upper stratosphere 1 .
2. Progression of changes in stratospheric ozone
Reductions in ozone have been recorded in the Antarctic, Arctic, and mid-
latitudes in both hemispheres. This thinning of the ozone is also not confined only to the
polar spring, which is the period of minimum ozone concentration. The ozone
measurements in the Antarctic started in the mid-1950s. Up to the 1970s, the apparently