Environmental Engineering Reference
In-Depth Information
atom can destroy 100,000 ozone molecules, allowing much more UV
radiation to reach earth's surface.
The chlorine atoms released from CFCs are responsible for 96 percent
of the ozone molecules destroyed in the ozone layer. The remaining 4
percent are destroyed by bromine atoms released into the air from agri-
cultural pesticides. Bromine atoms are only 1 percent as abundant as
chlorine atoms in the ozone layer but forty-fi ve times more effective per
atom in destroying stratospheric ozone. However, the bromine lasts less
than one year in the atmosphere and disappears shortly after its use is
stopped.
Ozone loss is more severe in colder air, which is the reason the
loss of ozone is greatest over Antarctica even though CFCs were released
12,000 miles to the north in industrialized Northern Hemisphere
nations. An unaffected ozone layer in 1955 contained 380 Dobson
units of ozone, but by 1995 over Antarctica, it had dropped by 76
percent to 90 Dobson units. (Dobson units measure the volume of
ozone in a vertical column of air at a particular location). Lesser but
signifi cant decreases were found at lower latitudes. Because of the loss
of much of their ozone shield, New Zealand and southern Chile have
experienced markedly increased rates of sunburn and skin cancer. Skin
cancer was virtually nonexistent in Chile a few decades ago but since
1994 has soared 66 percent. Because UV disorders take decades to
appear, the total impact of the ozone hole will not be known for
many years.
The Arctic is second to the Antarctic in terms of atmospheric chilli-
ness, and so the loss of ozone there is no greater than 60 percent. In
midlatitudes the loss is less than 10 percent.
The Cure When the scientifi c community pointed out the decline and
its likely effects to the leaders of the world's nations, there was leg-
islative panic, a condition almost unheard of unless a shooting war
is involved. In 1987 representatives of more than thirty industrial
countries approved the Montreal Protocol to phase out the production
and use of CFCs. In 1990 they tightened the restrictions and in 1992
tightened them still more. Most producing nations ceased production
in 1996, and all nations are pledged to do so by 2010. The amount
of chlorine in the ozone layer started decreasing in the mid-1990s and
has been accompanied by a thickening of the layer. The minimum
number of Dobson units of ozone in Antarctica was 92 in 1994 but
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