The Ozone Shield
Environmental Science
Spring 2011
 Explain
how the ozone layer shields the
Earth from much of the sun’s harmful
radiation
 Explain how chlorofluorocarbons damage the
ozone layer
 Explain the process by which the ozone hole
forms
 Describe the damaging effects of UV
radiation
 Explain why the threat to the ozone layer is
still continuing today
 Ozone
layer: area in the stratosphere where
ozone is highly concentrated
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Absorbs most of UV light from sun
UV light is harmful to organisms, it can damage
genetic material in living cells
 Ozone:
atoms
molecule made of three oxygen
 Chlorofluorocarbons
(CFCs): human made
chemicals, damaging to ozone layer
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Nonpoisonous, nonflamable, do not corrode
metals, used as coolants in refrigerators and air
conditioners
1970’s
Stable on Earth’s surface
Break apart high in stratosphere, where UV
radiation is absorbed
Once CFCs break apart, parts of CFC molecules
destroy protective ozone
 CFC
breaking down Ozone:
 In
1985 the ozone layer above the South pole
had thinned by 50 to 98 percent
 Ozone hole: thinning of stratospheric ozone
that occurs over the poles during spring
 Polar
stratospheric clouds: when
temperatures fall below -80 degrees C, high
altitude clouds made of water and nitric acid
form
 On surfaces of polar stratospheric clouds,
products of CFCs are converted to molecular
chlorine
 When
sunlight returns to South Pole in
spring, molecular chlorine is split into
chlorine atoms by UV radiation
 Chlorine atoms destroy ozone
 This causes thin spot (ozone hole)
 ~70% of ozone layer can be destroyed in
spring
 As
amount of ozone in stratosphere
decreases, more UV light is able to pass
through the atmosphere and reach Earth’s
surface
 UV light damages DNA
 Exposure to UV light makes the body more
susceptible to skin cancer, and may cause
certain other damaging effects to the human
body
 High
levels of UV light can kill single celled
organisms (phytoplankton)
 Loss of phytoplankton disrupts ocean food
chains and causes increase of carbon dioxide
in atmosphere
 Damaging
to amphibians, because they have
eggs lacking shells that they lay in shallow
waters
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UV light can damage unprotected DNA
Amphibians often used as indicator species
 Can
damage plants by interfering with
photosynthesis
 Montreal
Protocol: 1987, nations agreed to
sharply limit their production of CFCs
 In 1992, developed countries agreed to
eliminate most CFCs by 1995
 US pledged to ban all substances that pose a
significant danger to the ozone layer by 2000
 CFC companies developed CFC replacements
 Considered a success story