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Ozone in the stratosphere undergo photodissociation by absorbing UV radiation UV light

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).

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Ozone in the stratosphere undergo photodissociation by absorbing UV radiation UV light

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  1. Ozone in the stratosphere undergo photodissociation by absorbing UV radiationUV light O3(g) → O(g) + O2(g) (1) • The free O atom further reacts with another molecule of ozone: O3(g)+O (g) → O2(g) + O2(g) (2)

  2. reduce the concentration of ozone in the stratosphere Effect However,ozone is being made at the same time • Under natural equilibrium: rate producing O3 = rate destroying O3 • fairly constant concentration of ozone is maintained

  3. 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.

  4. Destruction of Ozone by CFCs • is a complex chemical process UV UV CCl3F(g) → CCl2F2(g) → CCl2F．(g) + Cl．(g) (3) CClF2．(g) + Cl．(g)(4) The Cl． from reaction (3) & (4) readily depletes ozone via a sequence of reactions

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

  6. disturb the balance in production and destruction of ozone Effect • reactive Cl consumed in (5) is regenerated in (6) Cl．(g) + O3(g) → ClO．(g) + O2(g) (5) ClO．(g) + O(g) →Cl．(g) + O2(g) (6) • presence of one Cl can effectively destroymany ozonemolecules.

  7. Effects on aquatic and terrestrial ecosystems • reduce crop yield • higher incidence of skin cancer • higher incidence of eye cataract Urgent Need • minimize the use of CFCs • develop suitable CFC substitutes

  8. Control of ozone depletion problem 1987 CFC aerosol propellants were banned in some countries (eg.Canada and United States) • 24 countries signed the Montreal Protocol on Substances That Deplete the Ozone Layer • purpose: cut back CFC production in stages

  9. Control of Ozone Depletion Problem 1990 • Protocol was amended • 93 countries pledged to stop all production of CFCs (other ozone depleting substances by 2000) 1995 Many industrialized nations have agreed to a complete phaseout

  10. CFCs • 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

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

  12. Control of Ozone Depletion Problem in Hong Kong • November 1992 • the Montreal Protocol was amended in Copenhange • an accelerated scheme to phase out ozone depleting substances • successfully implemented in Hong Kong

  13. Control of Ozone Depletion Problem in Hong Kong CFC import quota was completely banned Halogenated hydrocarbon imports were banned 1994 CFC import quota was cut to 25 per cent 1995 1996

  14. Phase-out of Ozone Depletion Subtances under Montreal Protocol

  15. Possible Alternative for CFCs Possible alternatives with low ozone depletion potential (OPD) for CFCs : • 1. Hydrochlorofluorocarbons (HCFCs) such as CF3CHCl2 • break down more quickly in the atmosphere • lower ozone depletion potential than CFCs because of lower percentage of chlorine • damage ozone if overused

  16. have no chlorine • 2. Hydrofluorocarbons (HCFs) such as CF3CH2F • 'ozone safe’ • safety question on toxicity is still unsolved • 3. Water and steam • effective for some cleaning applications • can replace some CFCs as solvents in cleaning

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

  18. More information on possible replacement cmpounds for CFCs * Ozone depletion potential (ODP) is a measure of the effectiveness of the compound in destroying stratosphere ozone. CCl3F is defined as having an ODP of 1.0

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