Theme: The Air We Breathe
Topic 5. Ozone depletion
A. Ozone

Ozone (O3) is found in both the troposphere and the stratosphere, see the following figure.

Exists only as a trace constituent but is very important to the chemistry of the atmosphere.

Ozone layer in the stratosphere is the effective shield against ultra violet (UV) radiation.
Formation and destruction of O3

In the presence of sunlight, oxygen (O2) is continually being changed into O3; O3 is
likewise converted back to O2.
UV radiation
O2 (g) +

O (g)
O3(g)
It would be incorrect to think that the natural process is O2 changed to O3, and O3 is
destroyed because of air pollution.
(E)-T2-5-1

Air pollutants only speed up the rate of loss of O3, so that the steady state concentration of
ozone decreases.

Ozone absorbs UV radiation in the range of 230-320 nm (nm = 1x10-9m).

The shielding property of O3 is precisely due to its ability to absorb radiation of these
wavelengths, which simultaneously converts it back to O2.
Protection from stratospheric ozone

The stratospheric ozone layer absorbs most of the sun’s UV radiation.

UV radiation - a type of solar radiation that has been linked to skin cancer, eye disease,
immune system disorders and damage to various marine and terrestrial ecosystems.
Suggested Student Activity：
Debate on the effect of ozone on human health.
Debate on the effect of overexposure to sunlight on health and the measure to avoid
exposure.
B.
Ozone depletion

Stratospheric ozone is constantly being created and destroyed through natural cycles.

Various ozone–depleting substances (ODS), however, accelerate the destruction
processes, resulting in lower than normal ozone levels.
— Include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and
tetrachloromethane, etc.
 They are generally very stable in the troposphere.
 Carried into stratosphere in a process that takes long time.
 Degraded under intense UV radiation there and lead to stratospheric ozone
depletion.
Chlorofluorocarbons (CFCs) and other ODS

CFCs contain chlorine, fluorine, and carbon
e.g. Trichlorofluromethane
(E)-T2-5-2
They are used mainly as refrigerant fluids, blowing agents for expanded foams, propellants
for aerosol sprays and solvents.

When CFCs reach the stratosphere, ultraviolet radiation from the sun causes them to:
— Break apart and release reactive chlorine atoms
 Chlorine atom reacts with ozone
 Starting chemical cycles of ozone destruction that deplete the ozone layer.
 One chlorine atom can break apart more than 100,000 ozone molecules.
— For example:
UV radiation
CFCl3 (g)
CFCl2  +
Cl 
ClO  + O2 (g)
+ O3 (g)
ClO  + O 
Net result: O3 (g)

Cl 
Cl  + O2 (g)
+O
2O2 (g)
When other ODS broken apart under UV radiation (such as tribromomethane and halons),
they release bromine atoms, which are 40 times more destructive to ozone molecules than
chlorine atoms.
Polar Ozone hole
Antarctic Ozone Levels in Fall 2000
(During the deepest ozone loss, the center of the hole
(the red area) can drop below 100 Dobson Units, DU)
(USEPA web site: http://www.epa.gov/ozone/).
(E)-T2-5-3

The ozone hole is defined as the area having less than 220 Dobson Units of ozone in
between the ground and space.
— The word "hole" means really a significant thinning, or reduction in ozone
concentrations, which results in the destruction of up to 70% of the ozone normally
found.
— The ozone hole is a well-defined, large-scale destruction of the ozone layer, e.g. over
Antarctica where air currents bring in the atmospheric pollutants and condense and
accumulates to cause ozone depletion.

Antaractica ozone hole is the most obvious effect of the release of ozone-depleting
substances into the atmosphere and also the most extreme example of ozone depletion.

Long-term downward trends in ozone levels over most of the earth also pose a serious
threat.
C. Consequences of ozone depletion
Increase dosage of high-energy (UV) radiation imposes adverse effects on many animals,
plants, and microorganisms and hence the whole ecosystem. For example,

Alters the productivity of oceanic phytoplankton and upsets the whole ocean ecosystem.
— UV radiation affects plant growth, leading to reduction in crop yield.
— UV radiation induces gene mutation of the plants resulting in lowering their
productivities.

Less protection from the stratospheric ozone layer increases the amount of UV radiation
reaching the ground. More UV provides the environmental conditions for the formation
of photochemical smog.

For humans, an increased intensity of high-energy radiation would lead to a high
incidence of skin cancer, cataracts and weakened immune systems.
— For example, a 5% decrease in the amount of stratospheric ozone has been predicted
to produce 20% more cases of skin cancer per year in US.
(E)-T2-5-4

Ozone depletion and climate change may be interconnected.
— Ozone depletion phenomenon and the greenhouse-warming phenomenon share many
common chemical and physical processes.

As the atmospheric CFCs decline because of the Montreal Protocol's provisions,
their global-warming contributions will decline.
D. Reduce ozone depletion
Montreal Protocol
 In 1987, the Montreal Protocol was signed and the signatory nations committed
themselves to a reduction in the use of CFCs and other ODS.
 Amended to ban CFC production after 1995 in the developed countries, and later in
developing countries.
 Today, over 160 countries have signed the treaty.
 In Hong Kong, the Government has set schedule to ban ODS
Halons
Import for local consumption banned by 1.1.1994
CFCs
Carbon Tetrachloride
Import for local consumption banned by 1.1.1996
1,1,1-Trichloroethane
Freeze consumption at base level starting 1.1.1996
HCFCs
Import for local consumption banned by 1.1.2030
CFCs replacement compounds

CFC replacement will probably be compounds with one or two carbon atoms and some
appropriate combination of H, F and Cl substituents, including hydroflurocarbons (HFCs)
and terpene-based solvents.

The replacement compounds (or called substitutes) have the following advantages:
— Do not contain or contain less halogen (F, Cl, Br, etc.)
— The presence of H in the molecule confers reactivity in the troposphere because it
allows destruction of the molecule.
(E)-T2-5-5
Individuals can help
Individuals could also do something to protect the ozone layer. For example:

Make sure refrigerants recovered and recycled during servicing.

Use non-ozone-depleting refrigerants and chemicals.

Make sure your service technician is authorized to check and repair appliances that use ODS.
References:
1. Jones, R. R. and T. Wigley (1989) Ozone Depletion: Health and Environmental
Consequences. Chichester, New York: Wiley.
2. Middlebrook, A. M. and M. A. Tolbert (2000) Stratospheric Ozone Depletion. Sausalito,
California: University Science Books.
3. Makhijani, A. and R. K. Gurney (1995) Mending the Ozone Hole: Science, Technology,
and Policy. Cambridge: MIT Press.
4. U.S. Environmental Protection Agency (USEPA) USEPA web site, Ozone depletion. Web
address: http://www.epa.gov/ozone/.
5. U.S. Environmental Protection Agency. Stratospheric Ozone Depletion: A Focus on
EPA’s Research. Office of Research and Development Washington, DC 20460.
(E)-T2-5-6