3. §Ozone in the stratosphere undergo photodissociation by absorbing UV radiation UV light O 3(g) → O (g) + O 2(g) (1) §The free O atom further reacts with another molecule of ozone: O 3(g) +O (g) → O 2(g) + O 2(g) (2)

4. reduce the concentration of ozone in the stratosphere However,ozone is being made at the same time  Under natural equilibrium: rate producing O 3 = rate destroying O 3 §fairly constant concentration of ozone is maintained

5. §During past few decades: drastic change in concentration of ozone in the stratosphere §Ozone concentration has fallen over the Antarctica and Arctic. §Evidence shows: ozone depletion is related to the release of CFCs to the atmosphere.

6. §is a complex chemical process CCl 3 F (g) → CCl 2 F 2(g) → UV CCl 2 F ． (g) + Cl ． (g) (3) CClF 2 ． (g) + Cl ． (g) (4) ozone The Cl ． from reaction (3) & (4) readily depletes ozone via a sequence of reactions

7. Cl ． (g) + O3(g) → ClO ． (g) + O2(g) (5) ClO ． (g) + O(g) →Cl ． (g) + O2(g) (6) Net: O 3(g) + O (g) → 2O 2(g) depletion of ozone by reaction (5) is faster than that by reaction (2) O 3(g) +O (g) → O 2(g) + O 2(g) (2)

8. §reactive Cl consumed in (5) is regenerated in (6) disturb the balance in production and destruction of ozone one Cl many ozonepresence of one Cl can effectively destroy many ozone molecules. Cl ． (g) + O3(g) → ClO ． (g) + O2(g) (5) ClO ． (g) + O(g) →Cl ． (g) + O2(g) (6)

9. higher incidence of eye cataracthigher incidence of eye cataract reduce crop yieldreduce crop yield higher incidence of skin cancerhigher incidence of skin cancer develop suitable CFC substitutes minimize the use of CFCs

10. §24 countries signed the Montreal Protocol on Substances That Deplete the Ozone Layer CFC aerosol propellants were banned in some countries (eg.Canada and United States) purpose: cut back CFC production in stages

11. §Protocol was amended §93 countries pledged to stop all production of CFCs (other ozone depleting substances by 2000) Many industrialized nations have agreed to a complete phaseout

12. §very unreactive §diffuse very slowly into the stratosphere §scientists estimate ozone depletion over the Arctic and Antarctic will continue until at least the year 2000

13. §Hong Kong is a party to the Protocol §introduce the Ozone Layer Protection Ordinance (OLPO) in 1989 §enpowers the government to: l control the consumption of CFCs and other ozone depletion substances l through import/ export licensing control

14. §November 1992 §the Montreal Protocol was amended in Copenhange §an accelerated scheme to phase out ozone depleting substances §successfully implemented in Hong Kong

15. CFC import quota was completely banned Halogenated hydrocarbon imports were banned CFC import quota was cut to 25 per cent

17. §1. Hydrochlorofluorocarbons (HCFCs) such as CF 3 CHCl 2 low ozone depletion potential (OPD) Possible alternatives with low ozone depletion potential (OPD) for CFCs : break down more quickly in the atmosphere lower ozone depletion potential than CFCs because of lower percentage of chlorine damage ozone if overused

18. §2. Hydrofluorocarbons (HCFs) such as CF 3 CH 2 F safety question on toxicity is still unsolved have no chlorine 'ozone safe’ §3. Water and steam can replace some CFCs as solvents in cleaning effective for some cleaning applications

19. §4. Hydrocarbons such as butane and propane flammable and poisonous cheap and readily available contain no chlorine 'ozone safe’

20. * Ozone depletion potential (ODP) is a measure of the effectiveness of the compound in destroying stratosphere ozone. CCl 3 F is defined as having an ODP of 1.0