Air Pollution

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• 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)

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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

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• 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.

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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
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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)
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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 destroy many
  ozone molecules.

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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

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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

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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
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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

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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

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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

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

• CFC import quota was completely banned

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Phase-out of Ozone Depletion Subtances under
Montreal Protocol
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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

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• 2. Hydrofluorocarbons (HCFs) such as CF3CH2F

• have no chlorine

• '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

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• 4. Hydrocarbons such as butane and propane

• cheap and readily available

• contain no chlorine

• 'ozone safe

• flammable and poisonous

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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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