Environmental Engineering Reference
In-Depth Information
Molina and Rowland (1974) recognized that the
photochemical degradation of CFCs releases
chlorine, which, through catalytic action, has a
remarkable ability to destroy ozone. The
importance of the chlorine catalytic chain lies in
its efficiency; it is six times more efficient
catalytically than the NO cycle. The chain is only
broken when the Cl or ClO gains a hydrogen
atom from the odd hydrogen group, or from a
hydrocarbon such as methane (CH
4
), and is
converted into HCl, which diffuses into the lower
atmosphere, eventually to be washed out by rain
(Hammond and Maugh 1974). Similar
conclusions were reached independently at about
the same time by other researchers (Crutzen
1974; Cicerone
et al.
1974; Wofsy
et al.
1975),
and, with the knowledge that the use of CFCs
had been growing since the late 1950s, the stage
seemed set for an increasingly rapid thinning of
the earth's ozone shield, followed by a rise in the
level of ultraviolet radiation reaching the earth's
surface.
The world production of CFCs reached
700,000 tonnes in 1973 (Crutzen 1974), after
growing at an average rate of about 9 per cent
in the 1960s (Molina and Rowland 1974). The
total production of CFCs and halons amounted
to 1,260,000 tonnes in 1986, before falling to
870,000 tonnes in 1990 (Environment Canada
1992). The effects of such increases in
production were exacerbated by the stability of
the products, which allowed them to remain in
the atmosphere for periods of 40 to 150 years,
and measurements in the troposphere in the
early 1970s indicated that almost all of the
CFCs produced in the previous two decades
were still there (Molina and Rowland 1974).
This persistence means that, even after a
complete ban on the production of CFCs, the
effects on the ozone layer might continue to be
felt for a further 20 to 30 years and, under
certain circumstances, for as long as 200 years
after production ceased (Crutzen 1974; Wofsy
et al.
1975).
The predicted effects of all of this on the
ozone layer varied. Molina and Rowland
(1974) estimated that the destruction of ozone
by chlorine was already equivalent to that
produced by naturally occurring catalysts.
Crutzen (1974) predicted that a doubling of
CFC production would cause a corresponding
10 per cent reduction in ozone levels, whereas
Wofsy
et al.
(1975) estimated that, with a
growth rate of 10 per cent per year, CFCs could
bring about a 20 per cent reduction by the end
of the century. All indicated the preliminary
nature of their estimates and the inadequacy of
the existing knowledge of the photochemistry
of the stratosphere, but such cautions were
ignored as the topic took on a momentum of its
own, and the dire predictions made following
the SST studies were repeated. The spectre of
thousands of cases of skin cancer linked to a
seemingly innocuous product like hairspray or
deodorant was sufficiently different that it
excited the media and, through them, the
general public. Although CFCs were being
employed as refrigerants and used in the
production of insulation, the problem was
usually presented as one in which the
convenience of aerosol spray products was
being bought at the expense of the global
environment. In 1975 there was some
justification for this, since, at that time, 72 per
cent of CFCs were used as propellants in
aerosol spray cans (Webster 1988), and the
campaign against that product grew rapidly.
The multi-million dollar aerosol industry led
by major CFC producers such as DuPont reacted
strongly. Through advertising and participation
in US government hearings, they emphasized the
speculative nature of the Molina-Rowland
hypothesis, and the lack of hard scientific facts
to support it. The level of concern was high,
however, and the anti-aerosol forces met with
considerable success. Eventually manufacturers
were forced to replace CFCs with less hazardous
propellants (Dotto and Schiff 1978). A partial
ban on CFCs, covering their use in hair and
deodorant sprays was introduced in the United
States in 1978 and in Canada in 1980. CFC
aerosol spray use remained high in Europe where
there was no ban. In 1989, however, the
European Community agreed to eliminate the
