There is actually a thin layer of ozone in the stratosphere, about 20 – 50 km above the Earth. Here, the ozone acts as a kind shield, or giant sunscreen, filtering out some the harmful ultraviolet radiation
from the sun. It this radiation reaches the surface of the Earth, there could be a drastic increase in the number of cases of skin cancer, genetic mutations and eye damage e.g. cataracts being formed.
Ultraviolet radiation may also be harmful to marine life.
Since 1976, there has been an alarming decrease in the amount of ozone in the stratosphere over the South Pole. In recent years, a similar phenomenon has been occurring over the North Pole.
How is ozone different from oxygen? Ozone is an allotrope of oxygen. Unlike oxygen, which has two atoms in each molecule, ozone has
three atoms of oxygen per molecule. Therefore, its molecular formula is O
3
. Ozone is a pale blue, almost colorless gas with a characteristic odor. In small concentrations, it is non-toxic, but in
concentrations above 100 ppm parts per million, it is toxic. Breathing in air that contains high concentrations of ozone can be dangerous, especially for people with asthma.
What is causing the depletion of ozone in the stratosphere? Scientists have discovered that the depletion of the ozone layer is caused by chlorofluorocarbons.
Chlorofluorocarbons, commonly called CFCs, are compounds containing the elements carbons, fluorine, and chlorine. CFCs were widely used as propellants for aerosols and as coolants in
refrigerators and air conditioners. They were also used in the manufacture of packing foam.
Within the last decades, large amounts of CFCs have been released into the atmosphere. CFCs are very stable and can remain in the atmosphere for a very long time. Over the years, they slowly diffuse
through the air and react with ozone, destroying the ozone layer.
Is there a solution to the problem of ozone depletion? Many countries have now agreed to ban the use of CFCs. In 1992, an international agreement was
reached for a complete ban on the release of CFCs by 1996. Until now, most of the countries in the world have completely banned the use of CFCs. However, even if the use of CFCs is totally stopped
at once, the depletion of the ozone layer will continue for many years due to the CFCs already present in the atmosphere.
Key Ideas
1. The ozone layer filters out some of the harmful ultraviolet radiation from the sun. 2. Chlorofluorocarbons CFCs react with ozone in the stratosphere and cause the depletion of the
ozone layer. 3. The increase in ultraviolet radiation reaching the Earth could lead to increased case of skin cancer,
genetic mutations and eye damage.
3. Reducing Air Pollution
How is air pollution being controlled in Singapore?
The National Environment Agency NEA monitors and controls the problem of air and water pollution in the country. The following are some steps taken by the government to control air
pollution: Prohibition of the use of open fires for the disposal of domestic and industrial wastes. Using open
fires to burn domestic and industrial wastes can produce dust, smoke and a significant amount of air pollutants. The air pollutants released depend on the materials being burned. These pollutants
are toxic to humans and many cause irritation as well as skin and respiratory problems.
Introduction of unleaded petrol in 1991, and phasing out of leaded petrol by 1998. Reduction of the permissible level of sulphur in diesel since 1996 from 0.5 to 0.3 by mass.
Fitting of all petrol-driven vehicles with catalytic converters since 1994.
Reducing the Effects of Acid Rain
The major contributors of acid rain are sulphur dioxide and the oxide of nitrogen. Billions of dollars are lost each year to repair the effects of corrosion due to acid rain. Therefore, it is important to reduce
the quantity of these pollutants that is released into the environment. We shall discuss how this is done through the use of catalytic converters and flue gas desulphurization.
Catalytic Converters
What are the redox reactions in catalytic converters that help to remove pollutants that result from combustion?
Oxides of nitrogen and other undesirable gases such as carbon monoxide and various unburnt hydrocarbons emitted by car engines are a major source of air pollution. To reduce air pollution, most
cars are now manufactured with catalytic converters. Fig.3.shows the structure of a catalytic converters. A catalytic converters is attached to the exhaust
system of a car. It contains the catalysts platinum and rhodium. Fig. 3.catalytic converters
When the hot exhaust gases pass over the catalyst, redox reactions occur. The harmful pollutants are converted to harmless substances. For example, carbon monoxide is oxidized to carbon dioxide,
oxides of nitrogen are reduced to nitrogen, and unburnt hydrocarbons are oxidized to carbon dioxide and water.
nitric oxide + carbon monoxide nitrogen + carbon dioxide
2 NOg + 2 COg N
2
g + 2 CO
2
g octane + oxygen
carbon dioxide + water vapour 2 C
8
H
18
g + 25 O
2
g 16 CO
2
+ 18 H
2
O
What other measures can be used to reduce air pollution caused by motor vehicles? In the long run, the best way to control air pollution is to burn less fossil fuels. Future control
measures may include the use of alternative fuels, such as methanol and hydrogen, and the development of electric-powered motor vehicles.
Flue Gas Desulphurization
in living cells
There are two ways to minimizing the effects of sulphur dioxide pollution. The most direct approach is to remove sulphur from fossil fuels before they are brunt. However, this method is too expensive
and technologically difficult to accomplish. A cheaper way is to remove sulphur dioxide from the waste gases formed when fossil fuels undergo combustion. The waste gases are called flue gases. The
process of removing sulphur dioxide from flue gases is called desulphurization.
Fig. 5 shows a Flue Gas Desulphurization FGD plant. As sulphur dioxide passes through the plant, it reacts with an aqueous suspension of calcium carbonate to form solid calcium sulphite.
calcium carbonate + sulphur dioxide calcium sulphite + carbon dioxide
CaCO
3
s + SO
2
g CaSO
3
s + CO
2
g The calcium sulphite is further oxidized to calcium sulphate by atmospheric oxygen.
calcium sulphite + oxygen calcium sulphate
2 CaSO
3
s + O
2
g 2 CaSO
4
s Besides calcium carbonate, calcium oxide can also be used for deslphurisation.
calcium oxide + sulphur dioxide calcium sulphite
CaOs + SO
2
g CaSO
3
s
Key Ideas
1. Catalytic converters are used to convert the oxides of nitrogen, carbon monoxide and unburnt hydrocarbons in car exhaust fumes into harmless fumes.
2. Calcium carbonate is used to remove sulphur dioxide from flue gases.
4. The Carbon Cycle
