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Ozone Depletion

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Ozone Depletion

Why can't the children in Punta Arenas go outside in the spring?

Children in Punta Arenas, Chile, the world's most southern city, look forward to spring as much as anyone who lives through a frigid, dark winter. But some years, the children are instructed not to go outside because the ozone hole has moved north and the UV radiation is too high.

Ozone Depletion

At this point you might be asking yourself, “Is ozone bad or is ozone good?” There is no simple answer to that question: It depends on where the ozone is located ( Figure below ).

  • In the troposphere, ozone is a pollutant.
  • In the ozone layer in the stratosphere, ozone screens out high energy ultraviolet radiation and makes Earth habitable.

(1) Solar energy breaks apart oxygen molecules into two oxygen atoms. (2) Ozone forms when oxygen atoms bond together as O 3 . UV rays break apart the ozone molecules into one oxygen molecule (O 2 ) and one oxygen atom (O). These processes convert UV radiation into heat, which is how the Sun heats the stratosphere. (3) Under natural circumstances, the amount of ozone created equals the amount destroyed. When O 3 interacts with chlorine or some other gases the O 3 breaks down into O 2 and O and so the ozone layer loses its ability to filter out UV.

How Ozone is Destroyed

Human-made chemicals are breaking ozone molecules in the ozone layer. Chlorofluorocarbons (CFCs) are the most common, but there are others, including halons, methyl bromide, carbon tetrachloride, and methyl chloroform. CFCs were once widely used because they are cheap, nontoxic, nonflammable, and non-reactive. They were used as spray-can propellants, refrigerants, and in many other products.

Once they are released into the air, CFCs float up to the stratosphere. Air currents move them toward the poles. In the winter, they freeze onto nitric acid molecules in polar stratospheric clouds (PSC) ( Figure below ). In the spring, the sun’s warmth starts the air moving, and ultraviolet light breaks the CFCs apart. The chlorine atom floats away and attaches to one of the oxygen atoms on an ozone molecule. The chlorine pulls the oxygen atom away, leaving behind an O 2 molecule, which provides no UV protection. The chlorine then releases the oxygen atom and moves on to destroy another ozone molecule. One CFC molecule can destroy as many as 100,000 ozone molecules.

PSCs form only where the stratosphere is coldest, and are most common above Antarctica in the wintertime. PSCs are needed for stratospheric ozone to be destroyed.

The Ozone Hole

Ozone destruction creates the ozone hole where the layer is dangerously thin ( Figure below ). As air circulates over Antarctica in the spring, the ozone hole expands northward over the southern continents, including Australia, New Zealand, southern South America, and southern Africa. UV levels may rise as much as 20% beneath the ozone hole. The hole was first measured in 1981 when it was 2 million square km (900,000 square miles). The 2006 hole was the largest ever observed at 28 million square km (11.4 million square miles). The size of the ozone hole each year depends on many factors, including whether conditions are right for the formation of PSCs.

The September 2006 ozone hole, the largest observed (through 2011). Blue and purple colors show particularly low levels of ozone.

Find out how the ozone hole forms and view the hole over time on this National Geographic video: http://news.nationalgeographic.com/news/2008/11/081103-ozone-video-vin.html .

Ozone Loss in the North

Ozone loss also occurs over the North Polar Region, but it is not enough for scientists to call it a hole. Why do you think there is less ozone loss over the North Pole area? The region of low ozone levels is small because the atmosphere is not as cold and PSCs do not form as readily. Still, springtime ozone levels are relatively low. This low moves south over some of the world’s most populated areas in Europe, North America, and Asia. At 40 o N, the latitude of New York City, UV-B has increased about 4% per decade since 1978. At 55 o N, the approximate latitude of Moscow and Copenhagen, the increase has been 6.8% per decade since 1978.

This video explains an importance of the stratospheric ozone layer to life on Earth (8c) : http://www.youtube.com/watch?v=I1wrEvc2URE&feature=related (1:52).

This NASA video discusses the ingredients of ozone depletion of Antarctica and the future of the ozone hole, including the effect of climate change (8c) : http://www.youtube.com/watch?v=qUfVMogIdr8&feature=related (2:20).

Effects of Ozone Loss

Ozone losses on human health and environment include:

  • Increases in sunburns, cataracts (clouding of the lens of the eye), and skin cancers. A loss of ozone of only 1% is estimated to increase skin cancer cases by 5% to 6%.
  • Decreases in the human immune system’s ability to fight off infectious diseases.
  • Reduction in crop yields because many plants are sensitive to ultraviolet light.
  • Decreases in phytoplankton productivity. A decrease of 6% to 12% has been measured around Antarctica, which may be at least partly related to the ozone hole. The effects of excess UV on other organisms is not known.
  • Whales in the Gulf of California have been found to have sunburned cells in their lowest skin layers, indicating very severe sunburns. The problem is greatest with light colored species or species that spend more time near the sea surface.

When the problem with ozone depletion was recognized, world leaders took action. CFCs were banned in spray cans in some nations in 1978. The greatest production of CFCs was in 1986, but it has declined since then. This will be discussed more in the next lesson.

Summary

  • CFCs float up into the stratosphere where they break apart. The chlorine pulls an oxygen ion off of an ozone molecule and destroys it.
  • The ozone hole is where there is less ozone than normal at that altitude. It forms in the spring.
  • Ozone loss increases the amount of high-energy ultraviolet radiation that can strike Earth, causing ecological and health problems.

Practice

Use these resources to answer the questions that follow.

http://www.youtube.com/watch?v=xDOFJ5xoGmw

1. What is the purpose of the ozone?

2. Why is the ozone layer so fragile?

3. How much did ozone decrease between 1979 and 1993?

4. Which satellite was launched to study the ozone? When?

http://www.youtube.com/watch?v=NvtUYQ_eAwY

5. What caused this ozone loss?

Review

1. How do CFCs destroy ozone?

2. What is the ozone hole and where is it found? Is there an equivalent hole in the Northern Hemisphere?

3. What are some of the consequences of ozone loss that have been identified?

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