206 ENVIRONMENTAL SCIENCE
Closer to the ground, in the troposphere, ozone produced through a series of chemical
reactions involving hydrocarbons and nitrogen oxide emissions from vehicles and industrial
activity is an effective greenhouse gas. Thus, ozone plays two very different roles in global
environmental change: one in the stratosphere as a shield against harmful ultraviolet
radiation, and another nearer the ground in the troposphere as a greenhouse gas find a
health hazard.
The researchers hypothesized in 1974 that increasing concentrations of chlorofluoro-
carbons (CFCs), synthetic compounds that are chemically very stable in the lower atmosphere,
rise unchanged through the lowest atmospheric layer, the troposphere. Even though CFCs
are produced mostly in the industrialized countries of Europe and North America—where
they are used in a wide variety of applications such as for solvents and refrigerants.
The researchers surmised that upon reaching the stratosphere, the CFCs encounter
high-energy ultraviolet light’, which breaks them down, releasing their chlorine atoms. The
chlorine atoms can then engage with ozone in a catalytic reaction in which each chlorine
fragment can destroy up to 100,000 ozone molecules before other chemical processes remove
the chlorine from the atmosphere.
The Antarctic Ozone Hole
Now, many scientists describe the Antarctic ozone hole as the first unequivocal evidence
of ozone loss due to man-made chlorine and one of the first clearly definable effects of
human-induced global change. They found that the ozone levels dip at about the same
latitudes where levels of chlorine monoxide ascend. Scientists are convinced that the elevated
levels of chlorine and bromine account for much of the ozone depletion. The ozone molecules
are formed over the tropics and are delivered along with chlorine to the Antarctic, as well
as to the Arctic, via atmospheric motions. In Antarctica, a circulation pattern known as the
Antarctic polar vortex traps the ozone cover the South Pole for several months. It is within
this vortex that scientists have measured such shockingly low ozone concentrations during
the first two weeks of October shortly after the beginning of the Southern Hemisphere
spring. The chemical reactions that take place on these surfaces convert chlorine from forms
that do not react with ozone to other, less stable forms that readily break up in the presence
of sunlight and go on to destroy ozone. Both cold temperatures and sunlight arc critical to
the process leading to ozone depletion in the Antarctic. Antarctic ozone is depleted not
during the winter, when temperatures are coldest and the South Pole is immersed in darkness,
but in the southern spring, after sunlight returns but temperatures are still low.
Effect on Line
The ozone layer is essential to life because it shields it from damaging ultraviolet
radiation. Researchers are trying to learn how humans, vegetation, and aquatic ecosystems
each may be affected by ozone depletion. Direct exposure to ultraviolet radiation can damage
the human immune system, cause cataracts, and increase the incidence or skin cancer. The
EPA estimated in 1986 that the incidence of skin cancers would rise 2 percent for each 1
percent depletion of stratospheric ozone. As part of the effort to understand the effects on
vegetation and crops, researchers have tested more than 200 plant species, two-thirds of
which show sensitivity to increased ultraviolet exposure. Soybeans, one of civilization’s
staple food crops, are particularly susceptible to ozone damage, as are members of the bean