Air pollution
| Pollutant | Source | Harm |
|---|---|---|
| Carbon monoxide, CO | Incomplete burning of fuels in vehicles and charcoal stoves | Binds to haemoglobin and stops it carrying oxygen; poisonous |
| Sulphur dioxide, SO₂ | Burning fuels containing sulphur; smelting sulphide ores | Acid rain; breathing problems |
| Nitrogen oxides, NOₓ | Hot engines and lightning | Acid rain, smog, breathing problems |
| Particulates (soot) | Burning wood, diesel, waste | Lung disease |
| Lead compounds | Leaded petrol (now phased out) | Damage to the brain, especially in children |
| Volatile organic compounds | Solvents, paint | Smog, health effects |
Acid rain
Rain is naturally slightly acidic (about pH 5.6) because of dissolved carbon dioxide. Acid rain has a pH below 5.6. Sulphur dioxide and nitrogen oxides dissolve in rain: SO₂ + H₂O → H₂SO₃, then oxidation to H₂SO₄ and 2NO₂ + H₂O → HNO₃ + HNO₂. Acid rain damages trees, makes lakes too acidic for fish, corrodes buildings (CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂) and metals, and washes nutrients out of soil. It can be reduced by removing sulphur from fuels, using flue gas desulphurisation (calcium carbonate or lime reacts with SO₂) and catalytic converters.
Choose the pH of rain. Normal rain is about pH 5.6. Each pH unit is a ten-fold change in hydrogen ion concentration.
The greenhouse effect and climate change
Greenhouse gases (carbon dioxide, methane, water vapour, nitrous oxide, CFCs) absorb infrared radiation from the Earth and send some back. This natural effect keeps the Earth about 33 °C warmer. Burning fossil fuels, cutting down forests, and farming (methane from cattle and rice fields) add extra greenhouse gases, so the average temperature rises. Effects include melting ice, rising sea levels, droughts and floods, and changes in crop yields.
The ozone layer
The ozone layer (O₃) in the stratosphere absorbs harmful ultraviolet radiation. Chlorofluorocarbons (CFCs), once used in refrigerators and aerosols, release chlorine atoms in sunlight: Cl· + O₃ → ClO· + O₂ and ClO· + O → Cl· + O₂. One chlorine atom can destroy thousands of ozone molecules. CFCs are now banned (the Montreal Protocol).
Water pollution and treatment
| Pollutant | Source | Effect |
|---|---|---|
| Sewage and animal waste | Households, farms | Disease (cholera, typhoid); oxygen depletion |
| Fertiliser (nitrates and phosphates) | Run-off from farms | Eutrophication: algae grow, then die and use up oxygen |
| Heavy metals (mercury from gold mining, lead) | Mining, industry | Poisoning of people and fish |
| Oil | Spills | Coats birds and fish |
Key terms
- Acid rain
- Rain with a pH below 5.6.
- Greenhouse gas
- A gas that absorbs infrared radiation and warms the atmosphere.
- Eutrophication
- Excess nutrients in water causing algal growth and loss of oxygen.
- Ozone layer
- A layer of the stratosphere that absorbs ultraviolet radiation.
Summary
- Pollutants: CO, SO₂, NOₓ, particulates and lead each have sources and effects.
- Acid rain comes from SO₂ and NOₓ; the greenhouse effect from CO₂, CH₄ and others.
- CFCs destroy ozone and are banned.
- Water is polluted by sewage, fertilisers, metals and oil and made safe by coagulation, filtration and chlorination.