Environmental Engineering Reference
In-Depth Information
necessary to build an SST, yet possessing a
remarkably incomplete understanding of the
environment into which the aircraft was to be
introduced.
Like all aircraft, SSTs produce exhaust gases
which include water vapour, carbon dioxide,
carbon monoxide, oxides of nitrogen and some
unburned hydrocarbons. These are injected
directly into the ozone layer since SSTs
commonly cruise at about 20 km above the
surface—just below the zone of maximum
stratospheric ozone concentration. Much of the
initial concern over the effect of SSTs on ozone
centred on the impact of water vapour, which
was considered capable of reducing ozone levels
through the creation of the hydroxyl radical, a
known ozone-destroying catalyst. Later
observations, which indicated that a 35 per cent
increase in water vapour had been accompanied
by a 10 per cent increase in ozone, rather than
the expected decrease (Crutzen 1972), caused
the role of water vapour to be re-evaluated. It
was suggested that water vapour helped to
preserve the ozone layer through its interaction
with other potential catalysts. It converted NO
x
to nitric acid, for example, and therefore
nullified its ozone-destroying properties
(Crutzen 1972; Johnson 1972). By the time this
had been confirmed, in 1977, NO
x
had already
replaced HO
x
as the villain in SST operations
(Dotto and Schiff 1978).
In 1970, Crutzen drew attention to the role
of NO
x
in the destruction of ozone through
catalytic chain reactions, and in the following
year, just as the SST debate was beginning to take
off, Johnston (1971) warned that NO
x
emitted
in the exhaust gases of 500 SSTs could reduce
ozone levels by as much as 22-50 per cent. Later
predictions by (Crutzen 1972) suggested a 3-22
per cent reduction, while Hammond and Maugh
reported in 1974 that the net effect of the NO
x
emissions from a fleet of 500 SSTs would be a
16 per cent reduction in ozone in the northern
hemisphere and an 8 per cent reduction in the
southern hemisphere.
When all of this was under consideration in
the early 1970s, it was estimated that the world's
fleet of SSTs would grow to several hundred
aircraft by the end of the century and perhaps as
many as 5,000 by the year 2025 (Dotto and Schiff
1978). The NO
x
emissions from such a fleet were
considered capable of thinning the ozone layer
sufficiently to produce an additional 20,000 to
60,000 cases of skin cancer in the United States
alone (Hammond and Maugh 1974). Other
predicted environmental impacts included
damage to vegetation and changes in the nature
and growth of some species as a result of
mutation. The extent to which such threats
helped to kill SST development is difficult to
estimate. At the time, the environmental
arguments seemed strong, but the economic
conditions were not really right for development,
and that, as much as anything else, led to the
scrapping of the projected Boeing SST.
Development of the Soviet Tupolev-144 and the
Anglo-French Concorde went ahead, with the
latter being the more successful of the two in
terms of production numbers and commercial
route development. Less than 10 SSTs are
currently in operation, and the effects on the
ozone layer are generally considered to be
negligible.
Chlorofluorocarbons, halons and the ozone
layer
If there was some doubt about the impact of SST
exhaust emissions on the ozone layer, the effects
of some other chemicals seemed less uncertain.
Among these, the chlorofluorocarbon (CFC)
group and related bromofluorocarbons or halons
have been identified as potentially the most
dangerous (see Table 6.2). The CFCs, sometimes
referred to by their trade name,
Freon,
came to
prominence as a result of lifestyle changes which
have occurred since the 1930s. They are used in
refrigeration and air conditioning units, but, until
recently, their major use was as propellants in
aerosol spray cans containing deodorants, hair
spray, paint, insect repellant and a host of other
substances. When the energy crisis broke, they
were much in demand as foaming agents in the
production of polyurethane and polystyrene
