Other greenhousegases 45non-flammable, appear to be ideal for use in refrigerators, the manufac-
ture of insulation and aerosol spray cans. Since they are so chemically
unreactive, once they are released into the atmosphere they remain for
a long time – one or two hundred years – before being destroyed. As
their use increased rapidly through the 1980s their concentration in the
atmosphere has been building up so that they are now present (adding
together all the different CFCs) in about l ppb (part per thousand mil-
lion – or billion – by volume). This may not sound very much, but it is
quite enough to cause two serious environmental problems.
The first problem is that they destroy ozone.^12 Ozone (O 3 ), a
molecule consisting of three atoms of oxygen, is an extremely reactive
gas present in small quantities in the stratosphere (a region of the atmo-
sphere between about 10 km and 50 km in altitude). Ozone molecules
are formed through the action of ultraviolet radiation from the Sun on
molecules of oxygen. They are in turn destroyed by a natural process as
they absorb solar ultraviolet radiation at slightly longerwavelengths –
radiation which would otherwise be harmful to us and to other forms
of life at the Earth’s surface. The amount of ozone in the stratosphere
is determined by the balance between these two processes, one forming
ozone and one destroying it. What happens when CFC molecules move
into the stratosphere is that some of the chlorine atoms they contain are
stripped off, also by the action of ultraviolet sunlight.These chlorine
atoms readily react with ozone, reducing it back to oxygen and adding
to the rate of destruction of ozone. This occurs in a catalytic cycle – one
chlorine atom can destroy many molecules of ozone.
The problem of ozone destruction was brought to world attention
in 1985 when Joe Farman, Brian Gardiner and Jonathan Shanklin at
the British Antarctic Survey discovered a region of the atmosphere over
Antarctica where, during the southern spring, about half the ozone over-
head disappeared. The existence of the ‘ozone hole’ was a great sur-
prise to the scientists; it set off an intensive investigation intoits causes.
The chemistry and dynamics of its formation turned out to be complex.
They have now been unravelled, at least as far as their main features
are concerned, leaving no doubt that chlorine atoms introduced into the
atmosphere by human activities are largely responsible. Not only is there
depletion of ozone in the spring over Antarctica (and to a lesser extent
over the Arctic) but also substantial reduction, of the order of five per
cent, of the total column of ozone – the amount above one square me-
tre at a given point on the Earth’s surface – at mid latitudes in both
hemispheres.
Because of these serious consequences of the use of CFCs, inter-
national action has been taken. Many governments have signed the Mon-
treal Protocol set up in 1987 which, together with the Amendments