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IB DemystifiedMYP Sciences

Atmosphere and air pollution

Every breath you take is mostly nitrogen, about one-fifth oxygen, and, in many cities, a dose of invisible particles and gases that shorten lives. Chemistry explains where these pollutants come from, what they do, and how we can remove them.

Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

JFMAMJJASONDWHO 24-hour guideline (15 µg/m³)050100150200250PM2.5 / µg per m³month
Figure 1. Fine particle pollution peaks in winter in many South Asian cities.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Pollutants at a glance
  6. Air pollution in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

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

  • describe the composition of the atmosphere and how it formed
  • identify the main air pollutants and their sources
  • explain the effects of pollutants, including acid rain and smog
  • explain methods of reducing pollution, such as catalytic converters and scrubbing
  • analyse air-quality and acid-rain data
  • discuss policies to improve air quality

Before you start

You will use these skills. If any feel shaky, review them first.

  • combustion of fuels (see Hydrocarbons and fuels)
  • acids, alkalis and pH
  • balancing equations

Key vocabulary

Pollutant
A substance that harms living things or the environment.
Particulate matter (PM2.5)
Tiny solid or liquid particles less than 2.5 micrometres across.
Acid rain
Rain with a pH below about 5.6, caused mainly by sulfur dioxide and nitrogen oxides.
Smog
A mixture of smoke, particles and gases trapped in the air near the ground.
Catalytic converter
A device in a car exhaust that converts CO, NOx and hydrocarbons into less harmful gases.
Flue gas desulfurisation
Removing sulfur dioxide from power-station waste gases using an alkali such as calcium carbonate.

Understanding the ideas

  1. What is it?

    Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide, plus small amounts of other gases and variable water vapour. Earth's early atmosphere was mostly carbon dioxide; photosynthesis and the formation of carbonate rocks changed it over billions of years.

  2. Why does it happen?

    Burning fuels releases pollutants: carbon monoxide and soot from incomplete combustion, sulfur dioxide from sulfur impurities, and nitrogen oxides formed when nitrogen and oxygen from the air react at high temperatures. These cause breathing problems, acid rain and smog.

  3. How do we know?

    Air-quality monitors measure PM2.5, nitrogen dioxide, sulfur dioxide and ozone continuously, and satellites track smoke from fires. The World Health Organization set guideline limits in 2021, far below the levels recorded in many large cities.

  4. Why does it matter?

    Air pollution is one of the world's largest environmental health risks. Catalytic converters, cleaner fuels, scrubbers on power stations, renewable energy and better public transport have all been used to improve air quality.

  5. What does it connect to?

    This topic links to combustion and acids in chemistry, to lungs and health in biology, and to weather, climate and sustainable energy in Earth science.

Pollutants at a glance

  • Carbon monoxide (CO): incomplete combustion; toxic, reduces oxygen carried by the blood.
  • Particulates (PM2.5, soot): burning fuels, dust; reach deep into the lungs and blood.
  • Sulfur dioxide (SO₂): sulfur in coal and oil; breathing problems and acid rain.
  • Nitrogen oxides (NOx): nitrogen and oxygen react in hot engines; breathing problems, smog and acid rain.
  • Ground-level ozone: formed from NOx in sunlight; lung irritation, crop damage.

Air pollution in the real world

Lahore, Delhi and other cities in the Indo-Gangetic Plain regularly face severe winter smog, leading to school closures and health warnings. Air-quality apps now let people check PM2.5 levels hour by hour and plan outdoor activities.

Worked examples

Example 1: percentage composition

Carbon dioxide is 420 ppm of the air. Convert to a percentage.

  1. 1% = 10 000 ppm.
  2. 420 ÷ 10 000 = 0.042%.

Example 2: SO₂ from coal

200 t of coal contains 1.5% sulfur. What mass of SO₂ forms? (S = 32, SO₂ = 64)

  1. Sulfur = 1.5% of 200 t = 3 t.
  2. SO₂ is twice the mass of S: 3 × 64 ÷ 32 = 6 t.

In the eAssessment

Air pollution questions often use real monitoring data and ask you to link pollutants to sources, effects and solutions. Expect:

  • Identify pollutants and explain how they form.
  • Process air-quality data and compare them with guidelines.
  • Explain how pollution-control methods work chemically.
  • Discuss policies such as bans, standards and alternatives.

Common ways to lose marks: saying nitrogen oxides come from the fuel; confusing ozone-layer damage with acid rain or climate change; saying catalytic converters remove CO₂; and confusing correlation with cause when interpreting data.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: lichens as air-quality indicators

Partially guided investigation · about 60 minutes · groups, outdoors

Research question
How does the number of different lichen types growing on tree trunks differ between sites near busy roads and sites far from traffic?
Scientific background
Lichens absorb water and dissolved substances from the air and are sensitive to sulfur dioxide and other pollutants. Cleaner air usually supports more types of lichen, especially leafy and bushy forms.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Quadrat (10 × 10 cm grid on clear plastic), hand lens, lichen identification chart, tape measure, map of the school grounds and nearby streets.
Method
  1. Choose at least five trees of the same species at each site, near and far from traffic.
  2. Place the quadrat on the trunk at the same height (1.5 m) and on the same side (for example north).
  3. Count the number of different lichen types and estimate the percentage cover.
  4. Compare the mean number of lichen types at each site.

Safety. Work in groups with teacher supervision, stay on pavements away from traffic, wash hands after touching bark and lichens, and do not climb trees.

Then evaluate: apart from pollution, what else could affect lichen growth at each site?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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