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International Day for the Preservation of
the Ozone Layer
The
Ozone Layer
Ozone molecules (O3) consist of three oxygen
atoms. This poisonous gas is extremely rare in the atmosphere,
representing just three out of every 10 million molecules. Ninety
per cent of ozone exists in the upper atmosphere, or stratosphere,
between 10 and 50 km (6‑30 miles) above the earth. Ozone at
ground‑level, at the bottom of the troposphere, is a harmful
pollutant resulting from automobile exhausts and other sources.
Figure 1 - Ozone Distribution in the Atmosphere
The
ozone layer absorbs most of the harmful ultraviolet‑B radiation from
the sun. It also completely screens out lethal UV‑C radiation. The
ozone shield is thus essential to life as we know it. Depleting the
ozone layer allows more UV‑B to reach the earth. More UV‑B means
more melanoma and non‑melanoma skin cancers, more eye cataracts,
weakened immune systems, reduced plant yields, damage to ocean
eco‑systems and reduced fishing yields, adverse effects on animals,
and more damage to plastics.
Scientific concern started in 1970 when Prof. Paul Crutzen pointed
out the possibility that nitrogen oxides from fertilizers and
supersonic aircraft might deplete the ozone layer. In 1974,
Professors F. Sherwood Rowland and Mario J. Molina recognized that
when CFCs finally break apart in the atmosphere and release chlorine
atoms they cause ozone depletion. Bromine atoms released by halons
have the same effect. The three scientists received the Nobel Prize
for Chemistry in 1995 for their pioneering work.
The ozone layer over
the Antarctic has steadily weakened since measurements started in
the early 1980s. The problem is worst
over this part of the globe due to the extremely cold atmosphere and
the presence of polar stratospheric clouds. The land area under the
ozone‑depleted atmosphere increased steadily to more than 20 million
sq km in the early 1990s and has varied between 20 and 29 million
sq. km since then.
Figure 2 - Measurements of Ozone and Reactive
Chlorine from a Flight Into the Antarctic Ozone Hole, 1987
In 2000, the area of the ozone hole
reached a record 29 million sq. kilometers on 12 September 2000.
Although it was the largest and the deepest ozone hole on record for
the month of September, it dissipated early in October, the earliest
since 1991. The lowest value recorded at the South Pole was 86 DU on
12 October 1993. This year, the area of the ozone hole has been
about 25 million sq. km. While no hole has appeared elsewhere, the
Arctic spring has seen the ozone layer over the North Pole thin by
up to 30%, while the depletion over Europe and other high latitudes
varies between 5% and 30%
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