tion for the dramatic decrease in O 3 observed over the Antarctic continent (Fig.
3.6). Future modelling of O 3 depletion will have to allow increasingly for the het-
erogeneous aspect of its chemistry. It may well be that, in addition to solid sur-
faces for reactions, liquid droplets also provide an important medium for reaction.
In the 1990s it became clear that bromine-containing compounds (halons) also
played an important role in stratospheric chemistry. Bromoform (CHBr 3 ), a gas
naturally released from the oceans, played some role in this but there were also
significant emissions from human activities. For example, halons are used in some
fire extinguishers, being non-toxic and leaving no residue after evaporation. This
class of compounds is typified by the simple halon 1211 (CF 2 ClBr). These mater-
ials can supply both bromine and chlorine to the stratosphere, so they have also
been regulated under protocols to reduce human impact on the ozone layer.
3.10.3 Saving the ozone layer
The clear links between CFCs, depletion in stratospheric O 3 , increased UV radi-
ation reaching the Earth’s surface and possible increased incidence of skin cancer
in humans have not escaped the media, who have been able, at times during the
1970s and 1980s, to capture the imagination of the general public. It is probably
correct that the CFC issue aroused immediate concern because its cause was
apparently obvious—in the shape of the aerosol can! Although it is true that
aerosol propellant was only a contributor to CFC build up in the atmosphere
(refrigerant coolants and industrial uses being other important sources), there is
little doubt that the aerosol can became a late 20th-century ‘icon’ for environ-
mental activism. It is this public awareness that has made the CFC–stratospheric
O 3 story such a good example of how environmental chemistry research can lead
to major international legislation.
Against the odds, the anti-aerosol lobby took on the multimillion-dollar aerosol
industry and achieved real success. By the late 1970s, CFCs were at least partially
banned in deodorant and hair sprays in the USA; Canada imposed similar controls
in the early 1980s. It was, however, the discovery of the Antarctic O 3 hole that pro-
voked stronger action. In 1987 a meeting of the United Nations Environment Pro-
gramme in Montreal resulted in 31 countries agreeing to the so-called ‘Montreal
Protocol’, under which developed countries agreed to a 50% cut in CFCs. Fol-
lowing this agreement, further meetings in Helsinki (1989) and Copenhagen
(1992) made the conditions of the Montreal Protocol more stringent, resulting in
an agreement to ban production of CFCs in developed countries.
Response to the Montreal Protocol by industry was positive, with agreements
to phase out CFC production, resulting in a search for viable safe alternatives;
decline in some atmospheric CFCs is now evident (Fig. 3.7). In developed coun-
tries, hydrocarbons or alternative means of pressuring containers have largely
replaced CFCs in aerosol cans, hydrochlorofluorocarbons (HCFCs)—which are
95% less damaging to O 3 than CFCs—are used in the production of polystyrene
foams and as refrigerant coolants and a propane/butane mixture is being devel-
oped as an alternative refrigerant coolant. Even HCFCs are gradually being
phased out and replaced by substances that are less likely to cause ozone deple-
The Atmosphere 63