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

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.

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.

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.

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

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!

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.

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

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 1996 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

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 2060-2075. Eventual complete recovery of the ozone will occur Delays will be the result of developing countries which employ CFCs