Geoscience Reference
In-Depth Information
Ozone
HUMAN IMPACT
Ozone is a rare gas made up of three atoms of oxygen. Its concentration rarely exceeds a few parts per billion and
yet it is a vital component of our atmosphere. In the stratosphere it is formed through the interaction of the shorter,
ultra-violet part of the sun's radiation and oxygen molecules, which consist of two atoms of oxygen. The reaction for
ozone formation is:
O
2
+ UV light
q
O + O; O
2
+ O
q
O
3
The result is the almost total exclusion of the harmful part of the ultra-violet rays of the sun.
In the troposphere ozone exists as a by-product of photochemical processes between sunlight and pollutants,
particularly nitrogen oxides from car exhausts. It is considered toxic above sixty parts per billion and has harmful
effects on plant growth as well as causing respiratory problems. In the lower part of the atmosphere its concentration
has been increasing as a result of higher car pollution, but in the stratosphere its level should remain constant,
destruction being balanced by creation.
However, in 1985 scientists working in Antarctica announced that ozone levels in the southern hemisphere
stratosphere had fallen by 40 per cent between 1977 and 1984. In the Antarctic spring (October) a hole of deficient
ozone levels the size of the United States and about 10 km deep had appeared. By 1995 springtime daily levels were
below 100 Dobson units (parts per billion), less than a third of the natural level, and low values continue to the present,
with a record ozone loss during 2006 over the south pole. The average area of the Antarctic ozone hole is shown in
Figure 3.4
for the period 1996-2005. For comparison the area of the hole on 25 September 2006 was 29.5 M km
2
.
Subsequent research has shown that the hole has been caused, in part, by the ozone being destroyed by chlorine
and bromine atoms released from molecules of artificial halocarbons broken down by the ultra-violet radiation from
the sun. The simplified reactions are:
O
3
+ O
q
O
2
+ O
2
O
3
+ NO
q
NO
2
+ O
2
(nitrous oxide acting as the catalyst)
O + NO
2
q
O
2
+ NO
O
3
+ Cl
q
O
2
+ ClO (with chlorine acting as the catalyst)
O + ClO
q
O
2
+ Cl
Other gases such as nitric oxide were also implicated. During the extreme cold of an Antarctic stratospheric winter,
the chlorine and bromine atoms are able to destroy ozone faster than it is being formed. The Antarctic stratospheric
circulation is effectively isolated from other parts of the atmosphere at this time, so there is little mixing with warmer
air richer in ozone.
Background ozone levels have fallen in the northern hemisphere also but only by about 15 per cent. The circulation
is less isolated there and the atmosphere less cold. However, the cooling of the stratosphere as a result of greenhouse
warming in the troposphere can lead to more frequent conditions of ozone depletion here. The area of depletion has
expanded into the Mediterranean and the southern United States. In their sunny climates more harmful ultra-violet
radiation would have been reaching the surface.
With such a serious decrease in levels of ozone and the consequent reduced protection from ultra-violet radiation,
scientists and politicians agreed that something had to be done. The first aim was to reduce the production of
chlorofluorcarbons, which appeared to be the main source of chlorine. An agreement was reached at Montreal in
1987 to phase out the production and use of CFCs as soon as possible; 'as soon as possible' meant a 50 per cent
reduction by 1999. Recognizing that this was too slow, a further agreement in London in 1990 recommended the
elimination of the use of CFCs by industrialized countries by 2000. Despite this, ozone levels have continued to decline,
though the rate of decrease does appear to have slowed down. Ironically the replacement gases will still contribute
