Key Idea: Most marks in topic 8.3 come from reducing urban air pollution and the impacts of acid deposition. Then learn primary and secondary pollutants and how to manage acid rain.
1. Reducing urban air pollution (most marks)
- Three levels: stop the pollutant being made, limit its release, clean up after it.
- London: a congestion charge (2003) cut traffic into the zone by about a fifth; a ULEZ since 2019.
- Delhi: a metro, buses and auto-rickshaws on CNG since about 2002, and a smog tower (2021).
2. Acid deposition
- SO₂ and NOₓ dissolve in rain to make sulfuric and nitric acid.
- Acid leaches nutrients and frees aluminium: trees die and fish gills are damaged.
- Manage it: scrubbers, less coal, liming lakes. The UK's last coal power station closed in 2024.
3. Primary and secondary pollutants
- Primary pollutants are released directly (SO₂, NOₓ, soot); secondary ones form in the air (ozone).
- In the 2020 COVID-19 lockdowns, NOₓ fell sharply because traffic stopped.
- Ozone near the ground is secondary: it forms from NOₓ and VOCs on sunny days.
How it is examined:
| Question | How to score |
|---|---|
| [1] to [2] Read a pollution graph | Name the peak, month or trend, with values and units. |
| [1] Identify or outline a primary pollutant | Released directly from a source, such as NOₓ from car exhausts. |
| [4] Distinguish primary from secondary pollutants | Define each and give a named example of each. |
| [2] to [4] Outline ways to cut urban air pollution | One strategy per mark, with how it works. |
| [3] to [7] Evaluate a strategy | Strengths, limits, a named city, then a judgement. |
| [7] Explain the impacts of acid deposition | Acid, leaching, aluminium, the harm; soils, lakes, buildings, health. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Primary pollutant | a pollutant released directly from a source, such as SO₂ from a chimney |
| Secondary pollutant | a pollutant formed in the air when primary pollutants react, such as ozone |
| PM2.5 | particles smaller than 2.5 micrometres, which reach deep into the lungs |
| Acid deposition | acids from SO₂ and NOₓ falling as rain, snow or dry particles |
| Scrubber | equipment that removes SO₂ from power station chimney gases |
Last-minute check: answer, then tap to see.
Is ozone near the ground primary or secondary? Secondary: it forms from NOₓ and VOCs in sunlight.
Why did NOₓ fall in early 2020? COVID-19 lockdowns stopped most traffic and factories.
How does acid rain kill fish? Acid frees aluminium from the soil, which damages fish gills.
Name one way to restore an acidified lake. Liming: adding crushed limestone to neutralise the acid.
A city plans a low emission zone, where older, more polluting vehicles must pay to enter. Evaluate a low emission zone as a way to reduce urban air pollution.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- Say which level a strategy works at: stop, limit or clean up.
- Name a real city for every strategy: London, Delhi, Mexico City.
- For acid deposition, link each step: acid, then leaching or aluminium, then the harm.
- For primary v secondary, define both and give an example of each.