1. Where does the Chlorine come from
Natural sources:
Ocean spray
Swimming pools
Volcanos
Most of this chlorine never reaches the stratosphere.
What about made made sources
First suggestion of man made sources of ozone depletion was made in 1970

2. Free radicals?
Free radicals: atoms, molecules or ions with unpaired electrons . These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions.
Hydroxyl (OH) and nitric oxide (NO) are present in the atmosphere and could catalyze the recombination reaction between ozone and oxygen that leads to a dissociation of ozone.
Nitrous oxide is released by bacteria in the soil and can live long enough in the atmosphere to be converted to NO (nitric oxide). In 1970, Paul Curtzen suggested that increased use of fertilizers would increase nitrous oxide production and thus nitric oxide in the stratosphere.

3. Chlorine
In 1970, researchers showed that chlorine was more effective than nitric oxide at catalyzing ozone destruction
1971, James Lovelock discovered large amounts of chloroflorocarbons (CFC) in the atmosphere.
After hearing about Lovelocks work, Rowland and Molina proposed that CFS could behave like nitrous oxide-reaching the stratosphere, being dissociated by UV light releasing chlorine which would then participate in the dissociation of ozone.
This became known as the Rowland-Molina hypothesis.

4. What is a CFC?
CFCs-Belonging to a group of chemical compounds called haloalkanes
These are alkanes (like methane) linked to halogens (like chlorine)
Created by substituting a hydrogen atom in the alkane with a halogen.
CFC are a compound of such substitutions substituting florine and chlorine atoms for the H atoms.
Freon is a trade name for a group of CFCs used as a refrigerant.
CFCs are also used for semiconductor device fabrication, foam blowing agents, solvents, aerosol spray propellants, fire extinguishing agents, and chemical reagents.

5. CFCs
Over the next 3 years, experimental data confirmed the RM hypothesis
Along with Crutzen, they were awarded the1995 Noble Prize in Chemistry for the work on Stratospheric Ozone
Beautiful example of the scientific method at work

6. Ozone Hole
1985, British scientists reported a sudden large depletion in the Ozone over the south pole
Simultaneous satellite measurements confirmed the decrease, but were rejected by the processing software-the decrease seen was so large it was rejected by the data quality control algorithms.
When the data were rerun through the processing programs without the data quality flags, the ozone hole had been seen all the way back to 1976!

7. Causes Unique set of conditions in the atmosphere at the south pole.
Polar Stratospheric Clouds (PSC) are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (50,000–80,000 ft).
Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk.
PSCs form at very low temperatures, below −78 °C.

8. PSCs and Ozone
Ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrochloric acid (HCl) and chlorine nitrate (ClONO2).
During the Antarctic winter and spring, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO).
The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO

9. What is being done
1976 Report by the National Academy of Science that there was credible scientific evidence for ozone depletion
US and several other countries banned the use of CFC in aerosol spray cans
1985 Vienna Convention for the Protection of the Ozone Layer signed
Established a framework for the negotiation of international agreements regulating ozone depleting substances
1987 Montreal Protocol is signed by 43 nations
Froze CFC production at 1986 levels and reduce it to 50% by 1999
Follow up meeting in London modified it to to phase out CFCs entirely by 2000
This date was moved up to in 1992
CFCs replaced now by HCFCs or HFC (which are greenhouse gases)
Most commonly used in the US is HFC-134a (R-134a) which has replaced CFCs in automobile air conditioners

10. Effect of these initiatives
Atmospheric concentrations have been declining
Antarctic Ozone hole will continue for decades
Expected to see Ozone concentrations increase by 5-10% in the lower stratosphere by 2020
A return to pre 1980 levels is expected by
Eventual complete recovery of the ozone will occur
Delays will be the result of developing countries which employ CFCs