12 min readConcept sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B very short answer (2 marks), short answer (3 marks) or long answer (5 marks)

Global problems: ozone depletion, global warming, photochemical smog and acid rain

Some local pollution has worldwide effects: CFCs thin the ozone layer, greenhouse gases cause global warming, and vehicle and factory gases form smog and acid rain.

Ozone layerOzone (O₃) high in the atmosphere that stops the sun's harmful ultraviolet (UV) rays reaching the Earth

Ozone holeThinning of the ozone layer, mainly over the Antarctic (less over the Arctic), caused by chlorine from chlorofluorocarbons (CFCs)

Greenhouse gasesCarbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O): they trap heat and warm the Earth

Acid rainRain made acidic when nitrogen oxides, sulphur dioxide (SO₂) and particles react with water in the air; fish cannot survive below pH 4.5

The ozone hole

The ozone layer high in the atmosphere stops most of the sun's harmful ultraviolet (UV) rays. Chemicals called chlorofluorocarbons (CFCs) were widely used in refrigerators, air conditioners, cleaning solvents and aerosol spray cans (perfumes, insect sprays, medicines). Fire extinguishers used close relatives called halons, which also destroy ozone; your NIOS lesson lists fire extinguishers with CFCs. Chlorine from CFCs splits ozone (O₃) into oxygen (O₂). The layer thins, mainly over the Antarctic and less over the Arctic, and this thinning is called the ozone hole. More UV then causes sunburn, fast ageing of the skin, skin cancer and cataract (clouding of the eye lens). It can also cause other eye damage and genetic disorders, and lowers the productivity of seas and forests. To protect the layer, use CFC-free fridges and ACs, and avoid aerosol spray cans. Most uses of CFCs have now been phased out.

How the greenhouse effect warms the Earth

A greenhouse is a glass chamber where plants are kept warm, as in cold hill areas. Sunlight enters, but the heat is trapped inside.

1
Sunlight passes through the atmosphere and warms the Earth's surface.
2
The warm Earth gives out heat as infrared rays.
3★
Greenhouse gases, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), trap this heat and send it back down, like the glass of a greenhouse.
4
Industry, vehicles and deforestation add more CO₂; plant-eating animals such as cattle give out a lot of methane.
5★
More gas traps more heat, so the Earth's average temperature rises: this is global warming.

★ key step

Effects of global warming

Melting of snow caps and glaciers, and a rise in sea level.
Low coasts and islands, such as the Maldives in the Indian Ocean, are at risk of going under water.
Unpredictable weather patterns.
Crops ripen too early, with smaller grains and lower yields.
The hatching of eggs in some fish is disturbed.

Photochemical smog

Smog is a mix of smoke and fog, with fumes from factories, homes and vehicles. Ordinary smoky smog forms when soot and sulphur dioxide from burning coal and other fuels mix with fog. Photochemical means caused by chemical reactions in light. Photochemical smog forms when sunlight acts on nitrogen oxides and hydrocarbons, mainly from vehicle exhaust, in still, dry air. Your NIOS lesson also lists sulphur dioxide and soot; these mainly make the ordinary smoky kind. Photochemical smog contains ozone and PAN (peroxyacetyl nitrate), called photochemical oxidants, which are toxic to the lungs. Smog stays close to the ground because of temperature inversion. Normally warm air rises. Here, a layer of warm air sits on top and traps cool, polluted air below until wind sweeps it away. Smog causes breathing problems, bronchitis, sore throat, headache and red, irritated eyes, cuts visibility and damages crops.

Acid rain

Acid rain forms when nitrogen oxides, sulphur dioxide (SO₂) and particles in the air react with water to make acids. It harms life in water and on land: fish cannot survive in water below pH 4.5. It damages forest trees, changes the soil and lowers crop yields. In people, food, water or air in contact with acid deposits can cause asthma. Acid rain also corrodes metals and wears away buildings and monuments, especially those of marble.

Ozone high up vs ozone near the ground

High in the atmosphereUseful. The ozone layer absorbs harmful ultraviolet (UV) rays and protects life on Earth.

Near the groundHarmful. Ozone formed in photochemical smog is a toxic gas that irritates the lungs.

In real life

Fridges and ACs sold today use CFC-free gases to protect the ozone layer. On still winter mornings, smog over north Indian cities makes eyes sting and breathing hard, so people with asthma are advised to stay indoors.

Exam angle

1 marks: “Name one harmful effect on humans of more ultraviolet rays reaching the Earth.” Skin cancer (or sunburn, fast skin ageing, cataract).

2 marks: “What is temperature inversion, and how does it make smog worse?” A layer of warm air lying over cool air near the ground. It stops the cool, polluted air from rising, so smog stays low until wind clears it.

5 marks: “Low islands such as the Maldives and farmers in north India both have reason to fear a warmer Earth. Explain how heat gets trapped near the Earth, name three gases that trap it, and give three effects of the warming.” Sunlight warms the Earth; the Earth gives out heat as infrared; CO₂, CH₄ and N₂O trap it; more gas from industry, vehicles, deforestation and cattle raises the average temperature. Effects: melting glaciers and rising seas (Maldives at risk), unpredictable weather, early-ripening crops with low yields, fish eggs failing to hatch.

Remember it

CFC eats the shield, CO₂ thickens the blanket. Sunlight + NOₓ + hydrocarbons = smog; SO₂ + NOₓ + water = acid rain.