Environmental Science
Name: ______________________________
Atmosphere and Climate
Goal: The student will 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 ultraviolet radiation and explain why the threat to the ozone layer is continuing
today.
Vocabulary:
1. ozone layer –
2. chlorofluorocarbons (CFCs) –
3. ozone hole –
4. polar stratospheric clouds Notes: Chapter 7.7:
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The Ozone Shield is located in the stratosphere; ozone is a form of oxygen made up of 3 oxygen atoms
Ozone absorbs most of the UV (ultraviolet) rays from the sun
UV rays damage the genetic material in living cells
The ozone shield acts like a sunscreen for the Earth and the inhabitants
Chemicals That Cause Ozone Depletion
• During the 1970’sscientists believed Chlorofluorocarbons (CFC’s) might be damaging the ozone shield; CFC’s
are nonpoisonous, noncorrosive (do not corrode metals) and nonflammable, man-made chemicals (thought to be
the miracle chemical) used in refrigerators and air conditioners (coolants)
• Also used as a gassy “fizz” when making plastic foams (styrofoam) and as a propellant in spray cans (deodorants,
insecticides and paints)
• CFC’s are chemically stable at the Earth’s surface; break apart high in the stratosphere where UV radiation is
powerful enough to break down CFC molecules
• Over a period of 10 – 20 years, CFC molecules releases at the Earth’s surface make their way into the
stratosphere, each CFC molecule contain between one and four chlorine atoms; one single chlorine atom can
destroy 10,000 ozone molecules.
The Ozone Hole
• 1985, scientists reported a study about the ozone layer near Antarctica and how it had thinned by 50% to 98%;
this was the first news about the ‘ozone hole’ thinning the stratospheric ozone over the poles during the spring
• This “hole” fluctuates depending on the time of year; greater during the summer months when the UV light is
breaking apart the CFC molecules, less during winter when they are building up
• 1997, ozone over Canadian Arctic was down to 45% below normal
How Does the Ozone Hole Form?
 During the dark polar winter, strong circulating winds over Antarctica, called the polar vortex, isolate cold air
from surrounding warmer air. The air within the vortex grows extremely cold; when temperatures fall below
about -80 degrees C, high altitude clouds made of water and nitric acid begin to form (polar stratospheric clouds)
 On the surface of these clouds, the products of CFCs are converted to molecular chlorine; when sunlight returns to
S Pole in spring, molecular chlorine splits into two chlorine atoms by UV radiation; chlorine atoms destroy ozone
causing a thin spot, or ozone hole, which can last several months (up to 70% of ozone layer can be destroyed
during spring)
 Ozone is produced by air pollution so why doesn’t the ozone repair this hole? Because ozone is very chemically
reactive. Ozone produced by pollution breaks down or combines with other substances in the troposphere long
before it can reach the stratosphere to replace the ozone that is being destroyed
The Effects of Ozone Thinning on Humans
• As ozone decreases in the stratosphere, more UV light is able to pass through atmosphere to Earth’s surface
causing an increase in skin cancer, cataracts (damages DNA)
Effects of Ozone Thinning on Animals and Plants
 High levels of UV light can kill single-celled organisms called phytoplankton that live near the surface of the
ocean; disrupts the ocean food chain and reduces fish harvest
 The reduction of phytoplankton would also increase the amount of CO2 in the atmosphere
 Scientists believe the increased UV light will be especially damaging to amphibians (toads and salamanders); lay
eggs that lack shells in shallow water of ponds and streams
 UV light at natural levels already kill many eggs of some species by damaging the unprotected DNA; higher
levels might kill more eggs and put amphibian populations at risk
 Ecologists oftentimes use the health of amphibian populations as environmental indicators of environmental
changes (sensitive)
Protecting the Ozone Layer
• In 1987, A group of nations met to take action against ozone depletion (Montreal Protocol); decided to sharply
limit their production of CFC’s
• A second conference was held in Copenhagen, Denmark in 1992; developed countries decided to:
1. Eliminate most CFC’s by 1995; U.S. pledged to ban all substances by 2000
2. Set up a fund to help developing countries switch to substitutes for CFC’s
3. Ban other substances that harm the ozone
 After develop countries banned most uses of CFCs, chemical companies developed CFC replacements ( aerosol
can no longer use CFCs as propellants and air conditioners are becoming CFC-free)
• Battle to protect the ozone is not over, some countries still make and use CFC’s; CFC molecules stay active for
decades (for 60 -120 years) so it will take decades for the ozone to recover
Lesson Reflection:
Assessment:
1. How does the ozone layer shield the Earth from much of the sun’s harmful radiation?
2. Describe some of the damaging effects of UV radiation on humans and on animals.
Active Reading: The Ozone Shield
Lesson Extension (Technology/Application/Connection to Real World):
PBS video: The Thin Green Line