Environmental Engineering Reference
In-Depth Information
than the 40 per cent reduction initially
predicted, by the middle of the year the
Canadian Atmospheric Environment Service
had become so concerned about the destruction
of ozone in mid-latitudes that it began to
include ultraviolet radiation warnings along
with its regular weather forecasts, and in March
1993 it announced that the ozone layer above
Toronto and Edmonton—at 43°N and 53°N
respectively—was thinner than at any time
since records began (Environment Canada
1993).
There is as yet no one theory which can
explain adequately the creation of the Antarctic
ozone hole or the thinning of the ozone layer in
the northern hemisphere. The impact of CFCs is
considered the most likely cause by many,
however. There can be little doubt that the link
between the ozone hole and the CFCs, tenuous
as it may have seemed to some scientists, helped
to revive environmental concerns and
contributed to the speed with which the world's
major industrial nations agreed in Montreal in
1987, to take steps to protect the ozone layer.
energy levels cause it to be oxidized into NO,
leading to the destruction of ozone molecules.
This process was part of the earth/ atmosphere
system before human beings came on the scene,
and with no outside interference, natural ozone
levels adjust to the output of N
2
O from the soil
and the oceans.
One of society's greatest successes has been
the propogation of the human species, as a
glance at world population growth will show.
This has occurred for a number of reasons, but
would not have been possible without a
growing ability to supply more and more food
as population numbers grew. By the late 1940s
and 1950s, this ability was being challenged as
population began to outstrip food supply. In an
attempt to deal with the problem, new
agricultural techniques were introduced into
Third World countries, where the need was
greatest. A central element in the process was
the increased use of nitrogen fertilizers along
with genetically improved grains, which
together produced the necessary increase in
agricultural productivity. Since that time,
continued population growth has been
paralleled by the growth in the use of nitrogen-
based fertilizers (Dotto and Schiff 1978).
The nitrogen in the fertilizer used by the
plants eventually works its way through the
nitrogen cycle, and is released into the air as
N
2
O to initiate the sequence which ultimately
ends in the destruction of ozone. Thus, in
theory, the pursuit of greater agricultural
productivity through the increased application
of nitrogen fertilizers is a threat to the ozone
layer. There is, however, no proof that increased
fertilizer use has, or ever will, damage the ozone
layer, through the production of N
2
O. If proof
does emerge, it will create a situation not
uncommon in humankind's relationship with
the environment, in which a development
designed to combat one problem leads to
others, unforeseen and perhaps undiscovered
until major damage is done. As Schneider and
Mesirow (1976) point out, the dilemma lies in
the fact that nitrogen fertilizers are absolutely
essential to feed a growing world population
Agricultural fertilizers, nitrous oxide and the
ozone layer
When concern for the ozone layer was at its
height, compounds other than CFCs were
identified as potentially harmful. These included
nitrous oxide, carbon tetrachloride and methyl
chloroform. Methyl bromide has recently been
added to the group. It is used extensively as a
fumigant to kill pests in the fruit and vegetable
industry, and may be responsible for as much as
10 per cent of existing ozone depletion. However,
its actual impact is still a matter of dispute
(MacKenzie 1992). Nitrous oxide (N
2
O), as one
of the oxides of nitrogen group, known for their
ability to destroy ozone, has received most
attention. It is produced naturally in the
environment by denitrifying bacteria which cause
it to be released from the nitrites and nitrates in
the soil. It is an inert gas, not easily removed
from the troposphere. Over time, it gradually
diffuses into the stratosphere where the higher
