1. Ozone

2. What is Ozone? Ozone is a colorless gas. Chemically very reactive
Near the Earth’s surface it will cause rubber to crack, hurt plant life, and damage people’s lung tissues.
The average amount of ozone in the atmosphere is roughly 300 Dobson Units
compressed into a single layer, it would be about 3 millimeters (0.12 inches) thick—the height of 2 pennies stacked together

3. Dobson Unit

4. Stratospheric Ozone
Stratospheric ozone absorbs ultraviolet (UV) light that can be dangerous to living things.
In the stratosphere, ozone blocks ultraviolet light that could cause skin cancers, cataracts, and other damage to animals and plants if it reached the surface.
In contrast, ozone that forms near the ground from air pollution is harmful. It can aggravate respiratory problems like asthma, bronchitis, and emphysema, and it can damage crops and other plants.

5. The figures below show the amount of ozone over North America
each March from

6. OZONE In 1985, a disturbing discovery was announced:
ozone levels in the stratosphere over the South Pole were dropping precipitously during September and October every year as the sun reappears at the end of the long polar winter.

7. The figures below show similar data for the South Pole, from 1979 - 1996.

8. Ozone Hole
The ozone hole is an area of the stratosphere where the concentration of ozone drops dramatically.
Any place where the concentration drops below 220 Dobson Units is considered part of the ozone hole.
Average ozone concentrations in the ozone hole are around 100 Dobson Units—about the height of a dime.

9. Hole does not persist year Round
It forms every spring in the Southern Hemisphere (August and September), when sunlight begins to return to Antarctica after the months of polar darkness.

10. Formation of the hole
During the winter (cold and dark), a vortex of winds encircles Antarctica preventing the stratosphere’s air from mixing with warmer air in troposphere.
Unusual clouds, called polar stratospheric clouds, form, containing chemically stable forms of chlorine and bromine (put into the atmosphere by people)
Sunlight returns in the spring and the ultraviolet light breaks the less-stable chlorine and bromine gases into free chlorine and bromine atoms.
These destroy the ozone layer by thinning the layer throughout August and into September or October.
As Southern Hemisphere spring progresses, temperatures climb, and the vortex of winds that isolated the Antarctic stratosphere weakens dispersing the free chlorine and bromine allowing the ozone layer to rebuild for the rest of the year.

11. Not the cause of global climate changes
The destruction of ozone has caused the Antarctic stratosphere to be cooler than it would normally be in the summer and fall.
Models and observations indicate that the cooling intensifies the vortex of winds.
Ozone-hole-related cooling may be the reason why interior Antarctica has not warmed up as fast as most of the rest of the planet due to global warming.

12. Ozone Destruction
Chlorine and bromine-containing molecules into the atmosphere
Chlorofluorocarbons—refrigerants
Methylbromides--pesticides
Halon gases—fire extinguisher
These molecules are so stable, they persist for decades or even centuries once released.
When they diffuse out into the stratosphere, the intense U.V. irradiation frees the chlorine and bromide atoms that destroy ozone.

15. The Ozone hole today

16. Ozone Destruction Continued
At a 1989 UN conference, eighty-one nations agreed to phase out CFC production by the end of the century.
Alternatives to CFCs exist including hydrochlorofluorocarbons (HCFCs) which release much less chlorine per molecule.
CFC production in industrialized countries has fallen nearly 80 percent since 1989.