ozone depletion

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 1.2). In the spring, (1) Solar energy breaks apart oxygen molecules into two oxygen atoms. (2) Ozone forms when oxygen atoms bond together as O3 . UV rays break apart the ozone molecules into one oxygen molecule (O2 ) and one oxygen atom (O). These processes convert UV radiation into heat, which is how the Sun heats the stratosphere. (3) Under natural cir- cumstances, the amount of ozone cre- ated equals the amount destroyed. When O3 interacts with chlorine or some other gases the O3 breaks down into O2 and O and so the ozone layer loses its ability to filter out UV. the Suns 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 O2 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 de- stroyed.

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the ozone hole

Ozone destruction creates the ozone hole where the layer is dangerously thin (Figure 1.3). 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 2013). Blue and purple colors show particularly low levels of ozone.

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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 1.1). 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.

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 worlds most populated areas in Europe, North America, and Asia. At 40o N, the latitude of New York City, UV-B has increased about 4% per decade since 1978. At 55o N, the approximate latitude of Moscow and Copenhagen, the increase has been 6.8% per decade since 1978. Click image to the left or use the URL below. URL:

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 systems 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 concept.

instructional diagrams

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questions

in the troposphere, ozone is a pollutant.

-->  a) true

b) false

ozone in the stratosphere

a) is a pollutant.

-->  b) protects the surface from uvc from the sun.

c) is one of the main greenhouse gases.

d) all of these.

cfc molecules destroy ozone because

-->  a) when they break apart, the chlorine pulls an oxygen atom off an ozone molecule.

b) the ozone freezes to the cfc and in the spring they both break apart.

c) fluorine attaches to an ozone molecule, pulls an oxygen molecule off and forms flo2.

d) none of these

which of used to contain cfcs?

a) spray-can propellants

b) refrigerants

c) paints

-->  d) all of the above

the ozone hole is at its biggest each year in the _ hemisphere in the _ season.

a) northern hemisphere; spring.

b) southern hemisphere; fall.

-->  c) southern hemisphere; spring

d) northern hemisphere; fall.

once released into the air, cfcs float up to the stratosphere.

-->  a) true

b) false

antarctica is a good place for the ozone hole to form because

-->  a) its extremely cold and the cfcs freeze into the polar stratospheric clouds.

b) more people are in the southern hemisphere so more cfcs are there.

c) air circulates move vigorously in this region.

d) all of these.

how does ozone loss affect humans and the environment?

a) increase in skin cancer

b) decrease in phytoplankton productivity

c) whales have sunburns

-->  d) all of the above

over the north polar region, there is less ozone loss because

a) spring does not last as long and there is not time for the hole to form.

-->  b) the atmosphere is not as cold and there are not as many polar stratospheric clouds.

c) cfc use is more restricted in this region.

d) all of these.

cfcs were completely banned in 1978 and the ozone hole has since declined.

a) true

-->  b) false

diagram questions

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