Key Idea: Most marks in topic 8.3 come from reducing urban air pollution, acid deposition and primary and secondary pollutants. At HL, also learn photochemical smog, what traps it and the harm ozone does.
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.
- Adirondacks, USA: about 24% of lakes surveyed (1984-87) had no fish. China's SO₂ fell about 75% (2007-2016).
3. Pollutants and smog (HL)
- Primary pollutants are released directly (SO₂, NOₓ, soot); secondary ones form in the air (ozone).
- Photochemical smog: NOₓ and VOCs in sunlight make ozone, worst in the early afternoon.
- Thermal inversions and valleys trap smog: Los Angeles, Mexico City, Santiago.
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. |
| [2] to [7] Explain how photochemical smog forms | NOₓ and VOCs plus sunlight make ozone; inversions trap it; the daily pattern. |
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 |
| Photochemical smog | a haze of ozone and other gases made when sunlight acts on NOₓ and VOCs |
| Thermal inversion | a warm layer of air above cooler air that traps pollution near the ground |
Last-minute check: answer, then tap to see.
Why does ozone peak in the early afternoon? Morning traffic releases NOₓ and VOCs; strong sunlight turns them into ozone hours later.
What is a thermal inversion? Warm air above cool air, which traps pollution near the ground.
Give one health impact of ozone. It inflames the airways and worsens asthma; about 490,000 deaths were linked to it in 2021.
Give one impact of ozone on crops. It damages leaves: it cuts world soybean yields by about 12%.
In March 2016 ozone in Mexico City reached its highest level in over a decade. Explain how a city can reduce photochemical smog at different levels of pollution management.
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 smog, give the ingredients, sunlight and the trap: an inversion or a valley.