E 4. Ozone depletion in stratosphere
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Describe the formation and depletion of
ozone in the stratosphere by natural
processes.
List the ozone-depleting pollutants and
their sources.
Discuss the alternatives to CFCs in terms
of their properties.
Ozone
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Oxygen is present in two forms, O2 and O3
protect life on the Earth’s surface from
harmful ultraviolet (UV) radiation.
Ozone
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O3
very pale bluish gas
very powerful oxidising agent
pungent smelling odor
absorbs UV light
detection: [O3] in a sample of air can be
measured using UV spectroscopy; the more
UV is absorbed the higher [O3]
in upper stratosphere; 15 to 45 km
The bonds in oxygen and ozone are
broken by UV of different wavelengths
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The bond in oxygen and ozone are both
broken when they absorb UV radiation of
sufficient energy.
The double bond in O2 is stronger than the
1.5 bond in ozone and so is broken by
radiation of shorter wavelengths.
Worked Example
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The bond energy in ozone is 363 kJ mol-1. Calculate the
wavelength of UV radiation needed to break the bond.
Ozone depletion
Two functions
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absorbs UV – 290 – 320 nm; UV
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causes sunburn, skin cancer, eye cataracts (=clouding of
the eye – can lead to blindness)
reduces plant growth as O3 destroys apparatus for
photosynthesis
can cause genetic mutations
causes loss of plankton
Ozone production releases energy which produces an
increase in temperature in stratosphere which gives it
stability
The natural formation of ozone
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The temperature of the atmosphere
generally decreases with height
But at 12km above the Earth’s surface,
temperature starts to rise because the
ultraviolet radiation is absorbed in a
number of photo chemical reactions.
This part of the atmosphere is called the
stratosphere.
Ozone: natural cycle (stratosphere)
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formation of ozone:
O2 + uv  O +
O2 + O
 O3
O
(λ = 242 nm)
More energy needed to break double bond.
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natural depletion of ozone
O3
O3 + O
 2O2
+ uv  O2 + O (λ = 290 – 320 nm)
Less energy needed to break ‘1.5 bond’ (delocalized pi bond).
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rate of formation = equal to rate of depletion
= steady state
both types of reactions are slow
Ozone: man-made depletion
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nitrogen oxides: sources: combustion, airplanes, nitrogenous
fertilizers
Ozone depletion: equations
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catalytic depletion:
NO
+
O3  NO2
NO2 + O
 NO +
When added:
O3 + O
 2O2
+ O2
O2
Chlorofluorocarbons
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CFCs = chlorofluorocarbons
end up in stratosphere as they are not broken
down
C-Cl bond is weakest; easily broken by UV:
Cl free radical produced by uv photodissociation
Cl acts as catalyst in ozone depletion – catalytic
depletion
ChloroFluoroCarbons:
useful compounds
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chemically stable; long atmospheric life-time
low toxicity
low cost to manufacture
volatile liquids
good solvents
Insulating
fire-suppressant
coolant in ACs and fridges
Ozone depletion: equations
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photodissociation: C- Cl is weakest bond
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CCl2F2  CClF2  +
catalytic depletion:
Cl
+
O3  ClO
ClO + O
 Cl  +
Cl
+ O2
O2
Ozone: anthropogenic depletion
Ozone depletion: alternatives to CFCs
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hydrocarbons such as propane and 2- methyl
propane as refrigerant coolants: no halogens
fluorocarbons: stronger C-F bonds don’t break
hydrochlorofluorocarbons: hydrogen makes it
more stable; fewer halogen free radicals
released
hydrofluorocarbons: stronger C-F bonds don’t
break
Ozone depletion: alternatives to CFCs
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propane and 2- methyl propane as refrigerant
coolants: greenhouse gases/flammable; highly
hazardous
fluorocarbons: greenhouse gases but not
flammable; non-hazardous
Hydrofluorocarbons: greenhouse gas, nonflammable, low toxicity
hydrochlorofluorocarbons: still some depletion as
has Cl, and also greenhouse gases; hazardous
Ozone: evidence for depletion
Antartica, autumn 2003
ozone hole = area
having less than 220
Dobson units
(if 100 DU of ozone were brought to
the Earth's surface, it would form a
layer 1 millimeter thick)
Ozone: evidence of depletion
Ozone: depletion
http://www.epa.gov/ozone/science/hole/size.html
Ozone depletion at the poles
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Special mechanism causes esp high ozone
depletion around poles
During winter, poles become very cold,
with circular winds preventing warm air
from entering
Forms ice in clouds, which act as
heterogeneous catalysts for ozone depletion
HCl and ClNO3 combine to form Cl2,
which then disassociates in summer to