Ozone in the stratosphere undergo photodissociation by absorbing UV radiation

UV 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)

reduce the concentration of ozone in the stratosphere

However,ozone is being made at the same time

Under natural equilibrium: Under natural equilibrium:

rate producing Orate producing O33 = rate destroying O = rate destroying O33 fairly constant concentration of ozone is fairly constant concentration of ozone is

maintainedmaintained

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.

is a is a complex chemical processcomplex chemical process

CCl3F(g) →

CCl2F2(g) →

UV

UVCCl2F ． (g) + Cl ． (g) (3)CClF2 ． (g) + Cl ． (g) 　　(4) The Cl ． from reaction (3) & (4) readily de

pletes ozone ozone via a sequence of reactions

Cl ． (g) + O3(g) → ClO ． (g) + O2(g) (5)

ClO ． (g) + O(g) →Cl ． (g) + O2(g) (6)

Net:Net: O O3(g)3(g) + O + O(g)(g) → → 2O2O2(g)2(g)

Net:Net: O O3(g)3(g) + O + O(g)(g) → → 2O2O2(g)2(g)depletion of ozone by reaction (5) is faster than that depletion of ozone by reaction (5) is faster than that

by reaction (2)by reaction (2)

O3(g)+O (g) → O2(g) + O2(g) (2)

reactive Cl consumed in (5) is regenerated in (6)

disturb the balance in production and destruction of ozone

•presence of one Clone Cl can effectively destroy mamany ozoneny ozone molecules.

Cl ． (g) + O3(g) → ClO ． (g) + O2(g) (5)

ClO ． (g) + O(g) →Cl ． (g) + O2(g) (6)

•higher incidence of eye cataracthigher incidence of eye cataract

•reduce crop yieldreduce crop yield

•higher incidence of skin cancer higher incidence of skin cancer

•develop suitable CFC substitutes

•minimize the use of CFCs

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

Protocol was amended93 countries pledged to stop all production of CF

Cs (other ozone depleting substances by 2000)

Many industrialized nations have agreed to a complete phaseout

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

Hong Kong is a party to the ProtocolHong Kong is a party to the Protocol

introduce the Ozone Layer Protection introduce the Ozone Layer Protection Ordinance (OLPO) in 1989Ordinance (OLPO) in 1989

enpowers the government to: enpowers the government to: control the consumption of CFCs and other control the consumption of CFCs and other

ozone depletion substances ozone depletion substances through import/ export licensing controlthrough import/ export licensing control

November 1992November 1992

the Montreal Protocol was amended in Copenhange

an accelerated scheme to phase out ozone depleting substances

successfully implemented in Hong Kong

CFC import quota was completely banned

Halogenated hydrocarbon imports were banned

CFC import quota was cut to 25 pCFC import quota was cut to 25 per center cent

CFCs75% reduction of consumption by 1 January 1994100% elimination by 1 January 1996

Halons 100% elimination by 1 January 1994

Carbon Tetrachloride 85% reduction of consumption by 1 January 1995

1,1,1-trichloroethane(Methyl chloroform)

50% reduction of consumption by 1 January 1994100% elimination by 1 January 1996

HCFCsFreeze consumption at base level starting a January 1996100% elimination by 1 January 2030

HBFCs 100% elimination by 1 January 1996

Methyl BromideRestrict to guarantine and pre-shipment applicationstarting 1 January 1995

1. Hydrochlorofluorocarbons (HCFCs) s1. Hydrochlorofluorocarbons (HCFCs) such as CFuch as CF33CHClCHCl22

Possible alternatives with low ozone delow ozone depletion potential (OPD)pletion 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

2. Hydrofluorocarbons (HCFs) such as CF3CH2F

•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

4. Hydrocarbons such as butane and propane

•flammable and poisonous

•cheap and readily available

•contain no chlorine

•'ozone safe’

Compound ToxicityBoilingpoint/ K

FlammabilityOzone

depletionpotential*

CH3CH2CH3 Low 231 Yes 0

CH3CH2CH2CH3 Low 273 Yes 0

CH2Cl2 High 313 No <0.05

CH3OCH3 Low 249 Yes 0

CHClF2 Low 232 No 0.05

CH3CH2F Low? 247 No 0

CF3CCl2H Low? 302 No 0.02

CH3CCl2F Low? 305 Yes 0.12

CH3CHF2 Low 249 Yes 0

* 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