1. Changes in atmospheric ozone concentrations.

2. What is Ozone ?
Ozone is a pale blue, highly reactive toxic gas with a pungent odour. Stratosphere is the Ozone layer, the beneficial role is acts as primary UV radiation shield.

3. What is the information that indicates changes in atmospheric ozone concentrations?
Measurements show that the overall amounts of ozone in the atmosphere have been decreasing by around 3% every ten years since the 1960’s.
Holes in the ozone layer over the Antarctic in spring are more recent evidence of ozone depletion.
It was discovered, in the 1970s, that the CFCs were depleting the ozone layer in the stratosphere. 
Later investigations showed that halons were broken down by UV more readily than CFCs, releasing Br atoms that, like Cl atoms, catalysed decomposition of ozone. 

4. How CFCs depleting the ozone layer?
CFCs pass through the troposphere into the stratosphere where they photo dissociate to produce chloride radicals.
CCl3F(g) + UVlight  Cl۰(g) + CCl2F۰(g)
CCl2F۰(g) + UVlight  Cl۰(g) + CClF۰(g)

5.  The chlorine radical acts as catalyst to convert ozone to oxygen in two steps via chlorine monoxide.
Cl۰(g) + O3(g)  ClO۰(g) + O2(g)
 The chlorine radical is the reformed.
ClO۰(g) + O۰(g)  O2(g) + Cl۰(g)
Because the chlorine radical is reformed, then one CFC molecule has the potential to destroy thousands of ozone molecules before the chain broken.

6. What changes observed ?
‘total column ozone ’ levels are measured using satellites. The Total Ozone Measuring Spectrometer (TOMS) measures the total amount of ozone between the earth's surface and the top of the atmosphere. This data is usually presented using computer-generated colour.

8. The ozone depletion has reached significant levels since the 1980’s
The ozone depletion has reached significant levels since the 1980’s. Since 1979, and up to 2003, the ozone loss in spring has grow significantly.

9. There has been an estimated loss of about 3% of ozone in the stratosphere worldwide over the past 20 years, and much greater losses in spring over high southern latitudes. The reduction in ozone levels in the stratosphere results in higher levels of UV radiation reaching the earth's surface. Greater exposure to UV radiation leads to increases in sunburn and skin cancer, damage to plants, and possibly harmful effects on sensitive ecosystems such as those found in the Antarctic. It has been estimated that a l% decrease in ozone levels the stratosphere leads to an increase of 2% of harmful UV radiation reaching the earth's surface.

10. Thank you for listening.
by Vicky &Guangxin