Stratospheric Ozone Depletion - The Changing Arctic Environment: The Arctic Messenger - page 102

is equivalent to a layer of pure ozone 0.01 millimetres thick at standard temperature and
pressure.
Back to the Farman paper and the NASA reanalysis. They caused quite a scientific
commotion. There could now be no doubt that stratospheric ozone depletion was real and
already far more extensive than formerly estimated. Why was it so intense above Antarc-
tica? The answer came from an atmospheric chemist and it provided the basis for tidying
up our fundamental understanding of what is going on when CFCs and other oxygen-de-
pleting substances meet in the stratosphere. Susan Solomon suspected the involvement of
polar stratospheric clouds. In winter, the Antarctic stratosphere cools down and descends
close to the surface. At the same time, strong westerly winds circle the pole as a vortex that
contains and traps the polar atmosphere. As temperatures in the lower stratosphere plunge
to below -78°C, polar stratospheric clouds are formed. They consist of minute ice particles,
along with nitric and sulphuric acid. They can only develop at extremely low temperatures
and are much more common in the Antarctic than the Arctic. Solomon suggested that the
ice particles create a surface or substrate on which the ozone molecules and chlorine from
the decay of CFCs can efficiently interact. In the absence of UV radiation during the po-
lar winter, an inventory of CFCs can build up on the ice surfaces. Supportive information
quickly arrived.
First, it was demonstrated in laboratory studies that ice could form a substrate as pro-
posed by Solomon. Meanwhile, measurements of chlorine monoxide in the stratosphere
were found to be 100 times larger than could be expected without the icy particle substrate
being involved. Furthermore, airborne instruments showed that whenever chlorine monox-
ide levels are high in the stratosphere, ozone levels are low and vice versa. This is exactly
as would be predicted if UV-induced breakup of chlorine-containing chemicals is respons-
ible for ozone depletion.
International political action was remarkably swift. Clearly, any regulation of CFCs
would only be effective if it could be implemented globally. In 1985, the Vienna Con-
vention for the Protection of the Ozone Layer (VCPOL) was negotiated. The convention
entered into force in 1988 and has now been ratified by 197 countries. It is another example