Air pollution, the greenhouse effect and acid rain

On a still morning in Freetown, the air at a busy junction carries exhaust from poda-podas, smoke from charcoal stoves and fumes from generators. Chemistry explains what these pollutants are, where they come from and what they do: from a headache caused by carbon(II) oxide to the global problems of climate change, acid rain and the ozone hole.

Learning objectives

By the end of this lesson you should be able to:

• define pollution and name the main air pollutants and their sources
• describe the effects of carbon(II) oxide, sulphur(IV) oxide, nitrogen oxides, particulates and lead
• explain the greenhouse effect and its link to global warming
• describe how acid rain forms, its effects and how it can be reduced
• explain how CFCs damage the ozone layer
• describe ways of controlling air pollution

Pollution and pollutants

The greenhouse effect and global warming

Figure 1. Greenhouse gases trap outgoing heat. WASSCE Tutor diagram.

1. Sunlight (mainly visible and short-wave radiation) passes through the atmosphere and warms the Earth's surface.
2. The warm surface gives out infrared (long-wave) radiation.
3. Greenhouse gases, mainly CO₂, methane, water vapour, nitrogen(I) oxide and CFCs, absorb much of this infrared and re-emit it, some back towards the Earth.
4. This natural greenhouse effect keeps the Earth about 33 °C warmer than it would be otherwise; without it, the Earth would be frozen.

Global warming: burning fossil fuels and cutting down forests have raised the CO₂ concentration of the air from about 280 ppm before 1800 to over 420 ppm today. More heat is trapped, and the average temperature of the Earth is rising.

Effects of global warming:

• rising sea levels (ice melts and warm water expands), flooding coastal towns
• more extreme weather: heavier rains, floods, droughts, storms
• changing rainfall patterns that damage farming and food supply
• spread of diseases such as malaria to new areas; loss of species

Reducing it: use renewable energy (solar, hydroelectric, wind), use fuel efficiently, plant trees and stop deforestation, and cut methane from rubbish dumps.

Acid rain

Figure 2. How acid rain forms and what it damages. WASSCE Tutor diagram.

Normal rain is slightly acidic (pH about 5.6) because CO₂ dissolves in it. Acid rain has a lower pH because of sulphur and nitrogen oxides:

$$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3$$

Effects:

• lakes and rivers become too acidic for fish and other animals
• trees are damaged; nutrients are washed out of the soil and poisonous aluminium ions are released
• buildings and statues made of limestone or marble are worn away: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
• metal structures (bridges, roofs) corrode faster

Reducing acid rain:

• remove sulphur from fuels at the refinery
• flue-gas desulphurisation: pass power-station waste gases through powdered limestone or lime, which absorbs SO₂: CaO + SO₂ → CaSO₃
• catalytic converters in car exhausts: 2CO + 2NO → 2CO₂ + N₂
• add lime to acidic lakes and soils (a temporary fix)

The ozone layer

Ozone, O₃, in the upper atmosphere (stratosphere) absorbs most of the Sun's harmful ultraviolet (UV) radiation.

CFCs (chlorofluorocarbons, such as CCl₂F₂) are very stable, so they drift up to the stratosphere. There, UV light breaks them down to release chlorine atoms, which destroy ozone in a chain reaction; one chlorine atom can destroy thousands of ozone molecules:

$$\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$$

Effects of ozone depletion: more UV reaches the ground, causing skin cancer, eye cataracts and damage to crops and plankton.

Action: the Montreal Protocol (1987) phased out CFCs worldwide; they have been replaced by less harmful gases. The ozone layer is slowly recovering.

Key points

• CO (incomplete combustion) stops blood carrying oxygen; SO₂ and NOₓ cause acid rain and lung problems; particulates cause lung disease; lead harms the nervous system.
• Greenhouse gases (CO₂, CH₄, H₂O, N₂O, CFCs) trap infrared; extra CO₂ from fossil fuels and deforestation causes global warming.
• Acid rain (H₂SO₃, H₂SO₄, HNO₃) harms lakes, trees, buildings and metals; reduce it by removing sulphur, desulphurising flue gases with lime and catalytic converters.
• CFCs release chlorine atoms that destroy ozone, letting in harmful UV.

WASSCE examination focus

Quick check

1. Why is carbon(II) oxide poisonous?
2. Name two greenhouse gases.
3. Write an equation for the formation of sulphuric(VI) acid in acid rain.
4. How do CFCs damage the ozone layer?

Answers:

1. It combines strongly with haemoglobin, so the blood cannot carry oxygen.
2. Any two: carbon(IV) oxide, methane, water vapour, nitrogen(I) oxide, CFCs.
3. 2SO₂ + O₂ + 2H₂O → 2H₂SO₄.
4. In the stratosphere, UV light breaks CFCs down to chlorine atoms, which destroy ozone molecules in a chain reaction.