18.3 Air Pollution

A free GCSE Biology lesson for AQA (8461) from OBA School: acid rain from sulfur dioxide and nitrogen oxides, smoke and particulates, and how to reduce them. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain how burning fossil fuels releases acidic gases that form acid rain
• Describe the effects of acid rain on trees, lakes and the organisms living in them
• Describe how smoke particles affect human health and reduce the light reaching the ground
• Describe ways of reducing air pollution
• Analyse data on pollution and biodiversity, including percentage change

Key words

• Sulfur dioxide: SO₂. An acidic gas made when fuels containing sulfur impurities (such as coal) are burned.
• Nitrogen oxides: NOx. Acidic gases made when nitrogen and oxygen from the air react at the high temperatures inside engines and furnaces.
• Acid rain: Rain, snow or mist with a low pH, made when acidic gases dissolve in water droplets in clouds.
• Particulates: Tiny solid particles of carbon (soot) and unburnt fuel in smoke.
• Global dimming: A fall in the amount of sunlight reaching the Earth’s surface, caused by particles in the air reflecting light.

Where air pollution comes from

Burning fossil fuels in power stations, factories, homes and vehicles releases smoke and acidic gases into the air. As the population grows and uses more energy, more of these pollutants are released.

• Sulfur dioxide; Main source Burning coal and oil that contain sulfur, e.g. in power stations; Main effect Acid rain
• Nitrogen oxides; Main source Car, lorry and aircraft engines; power stations; Main effect Acid rain; breathing problems
• Smoke / particulates; Main source Incomplete burning of fuels, e.g. diesel engines, wood and coal fires; Main effect Lung disease; global dimming; dirty buildings

Acid rain

• Trees: acid rain damages leaves and lowers the pH of the soil. Minerals are washed out and roots are damaged, so trees grow badly and may die.
• Lakes and rivers: as the water becomes more acidic, many organisms (fish eggs, insect larvae, snails) cannot survive. The whole food web is affected, so biodiversity falls.
• Buildings: acid rain wears away limestone and damages metal.

Exam tip: Clouds can be carried hundreds of kilometres by the wind, so acid rain often falls in a different area, or even a different country, from where the gases were made.

Acid rain and biodiversity: using data

Worked example: percentage decrease in species

At pH 7.0 there are 12 fish species. At pH 5.0 there are 3. Calculate the percentage decrease.
Decrease = 12 − 3 = 9 species
Percentage decrease = 9 ÷ 12 × 100 = 75%

Answer: 75% fewer fish species

This is a correlation: as pH falls, the number of species falls. To be sure the acid is the cause, scientists keep other factors, such as lake size and temperature, as similar as possible.

Smoke pollution and reducing air pollution

Smoke contains tiny particulates. When breathed in, they can reach deep into the lungs and make asthma and other lung and heart diseases worse. Particles in the air also reflect sunlight, causing global dimming; in still weather smoke can mix with fog to form smog.

• Low-sulfur fuels; How it helps Less sulfur dioxide is made when the fuel burns.
• Scrubbers (filters) in power station chimneys; How it helps Remove sulfur dioxide and particles before the waste gases leave the chimney.
• Catalytic converters on cars; How it helps Change nitrogen oxides into harmless nitrogen.
• Renewable energy (wind, solar), electric vehicles, public transport; How it helps Less fuel is burned, so fewer acidic gases and less smoke are produced.
• Smokeless zones and clean-air laws; How it helps Limit what can be burned in towns and cities.

Worked example: cleaner city air

The mean level of particulates in a city centre was 48 µg/m³. After a low-emission zone was introduced it fell to 30 µg/m³.
Percentage decrease = (48 − 30) ÷ 48 × 100 = 18 ÷ 48 × 100 = 37.5%

Answer: 37.5% decrease

Common mistakes

• Saying carbon dioxide causes acid rain. At GCSE, acid rain is caused by sulfur dioxide and nitrogen oxides; carbon dioxide is linked to global warming.
• Thinking acid rain only affects the area where the gases are released. Wind carries polluted clouds a long way.
• Writing that acid rain “kills fish” with no link to biodiversity. Say that fewer species can survive, so biodiversity falls.
• Mixing up global dimming (less light reaching the ground) with global warming (higher average temperatures).

Revision questions

Try these, then check your answers below each one.

Question 1. Which two gases are the main causes of acid rain? (Choose from: Sulfur dioxide and nitrogen oxides, Carbon dioxide and methane, Oxygen and nitrogen, Water vapour and argon.)

Answer: Sulfur dioxide and nitrogen oxides

Working: Sulfur dioxide and nitrogen oxides dissolve in water droplets in clouds to form acid rain.

Question 2. True or false? Acid rain always falls close to the power station that released the gases.

Answer: False

Working: False. Polluted clouds can be carried hundreds of kilometres by wind, even to other countries.

Question 3. A lake at pH 7.0 has 15 species of insect larvae. A lake at pH 5.0 has 6. Calculate the percentage decrease in the number of species.

Answer: 60 %

Working: 15 − 6 = 9. 9 ÷ 15 × 100 = 60%.

Related lessons

• Previous lesson: 18.2 Land and Water Pollution
• Next lesson: 18.4 Deforestation and Peat Destruction
• In this chapter: 18.1 The Human Population Explosion
• In this chapter: 18.5 Global Warming
• In this chapter: 18.6 The Impact of Change

Continue in the full course

This lesson is section 18.3 (AGCBH1803) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.