Light - Agroecology: The Ecology of Sustainable Food Systems - page 44

OZONE DEPLETION
Only about 1% of the UV light entering the earth's outer atmosphere actually reaches the surface. The rest is
absorbed by a layer of ozone gas high in the atmosphere. Organisms today are completely dependent on the screening
effect of the ozone, because most have no means for protecting themselves against the harmful effects of UV, which
include burning, cancer, and lethal mutations.
When ultraviolet light strikes an ozone molecule (O 3 ), the ozone is split apart and the energy of the UV light is
absorbed. An oxygen molecule (O 2 ) and a free oxygen atom, called a free radical, are created. The oxygen free
radical is extremely reactive, however, and readily combines with an oxygen molecule to reform a molecule of ozone.
When this reaction occurs, energy is released in the form of heat. Thus, absorption of UV light in the ozone layer
involves the continual destruction and creation of ozone, and the transformation of UV light into heat energy (infrared
light). There are enough ozone molecules in the ozone layer to intercept nearly all the UV light passing through it.
The ozone layer lies in the outer stratosphere, beginning about 20 km above sea level and extending for another
30 km out toward space. The stratosphere is well above the thick, turbulent region of the atmosphere responsible
for our weather, far removed from most human activities and surface sources of pollution. Nevertheless, human
beings do have an effect on the ozone.
For many decades, we have produced artificial gases called chlorofluorocarbons (CFCs) to use as coolants for
refrigerators and air conditioners, as spray can propellants, and for making plastic foam. These gases have been
freely released into the atmosphere, and they have leaked from cooling systems. Once they enter the atmosphere,
they slowly migrate into the stratosphere.
In the stratosphere, UV light bombards the CFC molecules, eventually breaking a chlorine atom off each one
in the form of a chlorine free radical. The chlorine free radicals formed through this process of photodissociation
attack and destroy ozone molecules, forming chlorine oxide (ClO) and molecular oxygen (O 2 ).
Cl - + O 3 5 ClO + O 2
The chlorine oxide thus formed has the ability to react with and destroy ozone as well.
ClO + O 3 5 ClO 2 + O 2
Worse, each chlorine oxide molecule can also react with one of the oxygen free radicals constantly being
generated by the absorption of UV by ozone, preventing the oxygen free radical from reforming ozone, and
regenerating the chlorine free radical!
ClO + O - 5 Cl - + O 2
Because the chlorine free radical can be regenerated, a single one, according to estimates, can destroy up to 100,000
ozone molecules before it reacts with an ozone molecule to form the relatively inactive chlorine dioxide (ClO 2 ).
CFCs are not the only ozone-destroying compounds that humans release into the atmosphere. Besides other
chlorine-containing compounds such as carbon tetrachloride and methyl chloroform, there are bromine-containing
compounds such as methyl bromide, an agricultural chemical used to fumigate and sterilize soil before the planting
of certain crops, such as strawberries. All of these substances, collectively called halocarbons, affect ozone in much
the same way, although bromine free radicals are even more reactive than chlorine, and so more destructive.
In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer, now signed and ratified by nearly
all countries, called for most ozone-depleting chemicals to be phased out by 2000. Most countries have adhered to
its provisions, and the production and release of ozone-depleting chemicals has decreased. As a result, increases in
the amounts of these chemicals in the stratosphere appear to have leveled off. Nevertheless, significant depletion
of atmospheric ozone continues to be observed every year, and the latest measurements show a 6 to 14% increase
in UV irradiation since the early 1980s at a variety of sites around the world (WMO, 2003).
Atmospheric scientists have been measuring the ozone layer since the 1970s. Although the concentration of
ozone in the stratosphere varies naturally from year to year, a marked seasonal depletion has been observed since
at least 1984, when a summer “hole” in the ozone layer over Antarctica was first detected.