THE OZONE SHIELD

CHAPTER 13SECTION 2

Enduring Understanding

• The ozone layer in Earth’s Stratosphere absorbs most of the UV light from the Sun.

• Thinning of the ozone layer increases the amount of light that is able to reach the surface of the Earth.

Essential Questions

• What are the potential consequences of humanity with continued thinning of the ozone layer?

• What does the ozone layer do and what can we do to protect it?

• what chemicals are currently causing the majority of ozone depletion?

Objectives

• Explain how the ozone layer shields the Earth from much of the sun’s harmful radiation

• Explain how chloroflurocarbons damage the ozone layer

• Explain the process by which the ozone hoe forms• Describe the damaging effects of ultraviolet

radiation• Explain why threat to the ozone layer is still

continuing today.

Questions

• Have you ever had a severe sunburn?• What do you think caused the sunburn, and

how long did it take to get it?• How do you think life would be if it took only

minutes to get a sunburn?

Ozone • The ozone layer is an area in the stratosphere where ozone is highly

concentrated. Ozone is a molecule made of three oxygen atoms.• The ozone layer absorbs most of the ultraviolet (UV) light from the sun• Ultraviolet light is harmful to organisms, because it can damage the

genetic material in living cells.• By shielding the Earth’s surface from most of the sun’s ultraviolet light, the

ozone in the stratosphere acts like a sunscreen for the Earth’s inhabitants.

Chemicals that cause ozone depletion

• Chloroflurocarbons are used as coolants in refrigerators and air conditioners. They are also used as a gassy “fizz” in making plastic foams and used as a propellant in spray cans of everyday products such as deodorants, insecticides, and paint.

• At the Earth’s surface CFCs are chemically stable. So they do not combine with other chemicals or break down into other substances. But CFC molecule break apart high in the stratosphere, where UV radiation, is absorbed. Once CFC molecules break apart, parts of the CFC molecules destroy protective ozone.

How CFC can destroy ozone in the stratosphere?

A single chlorine atom in the CFC structure can destroy 100, 000 ozone molecule

Activity • Modeling ozone reaction• O (one sphere)• O2 (two spheres of the same size)• O3 (three spheres of the same size)• CFC (three different spheres (1 carbon, 2 chlorine, 2 florine)• UV photons (very tiny spheres)• Use the spheres to replicate the reactions that:• a. Describe the way ozone is produced in the atmosphere b. Describe the way ozone is

destroyed in the atmosphere• CFCs that drift into the stratosphere react with ultraviolet photons releasing atoms that attack

ozone. In the late 1970s, scientists reported this fact, and predicted a depletion of the amount of ozone if large amounts of CFCs drifted into the stratosphere. The chlorine is released and it reacts with ozone, destroying the ozone, and creating another molecule that can in turn react with oxygen to produce free chlorine which can attack another ozone molecule. Thus an individual CFC molecule can end up destroying many ozone molecules.

• Display your models, and prepare an explanation for other members of your class.

Strategies that you may use to learn this topic

• Develop a set of study cards about the ozone layer• On separate index cards write a definitions of

ozone, describe the composition of ozone, describe the composition of ozone, describe why ozone is important in our lives, and list he potential effects of ozone depletion.

• You may use the completed cards for studying or make a poster board to share your understanding to others in the class

The Ozone Hole

• The studies by scientists working in Antarctica revealed that the ozone layer above the South Pole had thinned to 50 to 98 percent and this was published in an article in the scientific journal Nature in the year 1985.

• This was the first news of the ozone hole, a thinning of stratospheric ozone that occurs over the poles during the spring.

• Ozone levels over the Arctic have decreased as well. In fact, March 1997 ozone levels over part of Canada were 45 percent below normal

Growth of the Ozone Hole

How Does the Ozone Hole Form?• During the polar winter, 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 -80 C, high altitude clouds made of water and nitric ̊acid called polar stratospheric clouds, begin to form.

• On the surface of polar stratospheric clouds, the products of CFCs are converted to molecular chlorine.

• When sunlight returns to the South Pole in spring, molecular chlorine is split into two atoms by ultraviolet radiation.

• The chlorine atoms rapidly destroy ozone.• The destruction of ozone causes a thin spot, or ozone hole, which

lasts for several months.

Things to Ponder

• You know ozone is produced as air pollution. Why this ozone does not repair the ozone hole in the stratosphere?

Ozone produced by pollution breaks down or combines with other substances in the troposphere long before it can reach the stratosphere to replace ozone that is being destroyed.

Imagine that you live in a world in which the ozone layer does not exist. Discuss with your partner and describe a typical day in your life.

Connections to Physics

• Can you find oust the division of the ultraviolet portion of the electromagnetic spectrum?

• Which part of UV rays is very harmful to human beings?

Effects of Ozone Thinning on Humans

Damaging Effects of UV Light

• Humans • the body more susceptible to skin cancer• Premature aging of the skin. loss of skin

elasticity. photosensitivity• Increased incidence of cataract• Weakened immune response. Enhanced

susceptibility to infection

Effects of Ozone Thinning on Animals and Plants

• Plants• high levels of UV light can kill phytoplankton that

live near the surface of water. this in turn can disrupt ocean food chains and reduce food harvests. this in turn can increase the amount of carbon dioxide in the atmosphere.

• Land Plants• Interference with photosynthesis• reduced crop yields

Animals

• Amphibians• amphibians lay eggs that lack shells in the shallow

water of ponds and streams. UV light at natural levels kills many eggs of some species by damaging unprotected DNA. Higher levels might kill more eggs and put amphibian populations at risk.

• death of eggs• genetic mutations among survivors• reduction of populations

Protecting the Ozone Layer

• in 1987, a group of nations met in Canada and agreed to take action against ozone depletion. under the agreement called Montreal Protocol, these nations agreed to sharply limit their production of CFCs.

• A second conference was held in Copenhagen, Denmark, in 1992.

• Developed countries agreed to eliminate most CFCs by 1995.

• The United States pledged to ban all substances that pose a significant damage to ozone layer by 2000.

International Environmental Success story

• after developed countries banned most uses of CFCs, chemical companies developed CFC replacements.

• aerosol cans no longer use CFCs as propellants, and air conditioners are becoming CFC free.

• CFC molecules remain active in the stratosphere for 60 to 120 years. CFC produced 30 years ago are still destroying ozone today, so it will be many years before the ozone layer completely recovers.

Reduction in the use of CFCs