🎯 Context

Last lesson ended with a line about air: a bigger city burns more fuel for traffic and heating, and that changes the air its people breathe. This lesson starts there.

On Wednesday 13 August 2025 some families in Geneva could not drive into the city centre. After several days of high pollution, the canton switched on Stick'AIR, its emergency traffic plan, for only the second time since it began in 2020. Cars without the right sticker were kept out: about 11% of Geneva's vehicles, which together give out 30% of the nitrogen oxides from all of them [Source].

In this lesson you will find out what is in a city's air, where it comes from, and what cities can do about it. You will compare two cities: Geneva, where the air is close to the cleanest since measuring began but still has bad days, and Lagos in Nigeria, where air pollution causes thousands of early deaths every year.

Map of western Europe and West Africa locating Geneva in Switzerland and Lagos in Nigeria, with the Sahara between them

Map: Natural Earth (public domain).

[25 January 2023]

Task 0 Watch the clip from the California Air Resources Board.

According to the clip, why are very small particles more harmful than larger ones?
Identify two groups of people the clip says are most at risk from air pollution.

Phase 1: What goes into a city's air

8.3.1 Urban air pollution is caused by inputs from human activities to atmospheric systems, including nitrogen oxides (NOx), sulfur dioxide, carbon monoxide and particulate matter.

Particulate matter is categorized according to size of particle, with PM2.5 being fine particulate matter with a diameter of 2.5 micrometres or less and PM10 being air pollution that is made of larger particulate matter with a diameter of 10 micrometres.

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Think of the air over a city as a system, as you did in Topic 1. Human activities add inputs to it: gases and particles that would not be there, or not in such large amounts, without us. The IB names four.

Nitrogen oxides (NOx), mainly nitric oxide (NO) and nitrogen dioxide (NO2). When fuel burns at a high temperature, in a car engine or a boiler, the nitrogen and oxygen in the air itself combine. Road traffic is the biggest source in cities.
Sulfur dioxide (SO2) is released when fuels that contain sulfur are burned: coal, heavy oil and high-sulfur diesel.
Carbon monoxide (CO) forms when something burns without enough oxygen, in an engine, a generator, a stove or a fire.
Particulate matter (PM) is tiny solid or liquid particles in the air: soot from engines and fires, dust from roads and building sites, smoke. It is grouped by size. PM10 is particles 10 micrometres (µm) across or smaller, and PM2.5 is particles 2.5 µm or smaller. A micrometre is a thousandth of a millimetre, so PM2.5 is part of PM10. The smaller the particle, the deeper into your lungs it can travel.

To-scale diagram comparing a human hair, about 70 micrometres across, with a PM10 particle of 10 micrometres and a PM2.5 particle of 2.5 micrometres

Source: US Environmental Protection Agency, Particulate Matter (PM) Basics.

Measuring Geneva's air. The canton measures its air at four fixed stations, run by its air monitoring network (ROPAG): Necker in the city centre, Foron and Meyrin in the suburbs, and Passeiry in the countryside near the French border. In 2024 Geneva recorded its lowest annual levels of NO2 and fine particles since measuring began [Source]. In 2025 every station stayed below the Swiss limit for NO2, an annual mean of 30 µg/m³ (micrograms per cubic metre of air) [Source].

Bar chart of annual mean nitrogen dioxide in 2025 at four Geneva stations: Necker 26.4, Foron 12.2, Meyrin 12.1 and Passeiry 8.5 micrograms per cubic metre, all below the Swiss limit of 30

Source: Canton of Geneva, ROPAG air quality report 2025, Table 1.

Task 1.1 Complete the table.

Pollutant	A human activity that releases it
Nitrogen oxides (NOx)
Sulfur dioxide
Carbon monoxide
Particulate matter

Task 1.2

Distinguish between PM10 and PM2.5.
Describe the pattern of NO2 across Geneva's four stations in 2025. Use figures.
Suggest why NO2 at Necker is more than three times higher than at Passeiry.

Phase 2: Where it comes from

8.3.2 Sources of primary pollutants are both natural and anthropogenic.

Primary pollutants are directly active at the point of emission. Include forest fires, dust and volcanic eruptions as natural sources. Include burning for agricultural and forest clearance, burning of fossil fuels and biomass for energy production, and dust from construction/roads as anthropogenic sources.

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8.3.3 Most common air pollutants in the urban environment are either derived directly or indirectly from combustion of fossil fuels.

PM2.5, PM10, carbon monoxide and sulfur dioxide are primary pollutants, and tropospheric ozone is a secondary pollutant.

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Primary and secondary pollutants. A primary pollutant is harmful as soon as it leaves its source: the CO and soot coming out of an exhaust pipe are already doing damage. A secondary pollutant is not released at all. It forms in the air when primary pollutants react with each other or with other gases. The guide's example is tropospheric ozone: ozone at ground level, which forms from NOx and other pollutants. (This is not the ozone layer high in the stratosphere, which you will meet in Topic 6.)

Natural sources. Not all primary pollutants come from people. Volcanic eruptions release sulfur dioxide and ash. Forest fires started by lightning release smoke, soot and carbon monoxide. Dust is lifted by wind from deserts and dry ground. Geneva gets dust from the Sahara: in 2025 about 15 days of Saharan dust were detected over the city, and the strongest episode, from 12 to 15 November, clearly raised the level of fine particles [Source].

Anthropogenic sources. Most of the pollution in a city is anthropogenic, caused by human activity: burning fossil fuels in vehicles, boilers and power stations; burning biomass (wood, charcoal, crop waste) for heating and cooking; burning land to clear forest or crop stubble for farming; and dust thrown up by traffic on roads and by building sites.

Directly or indirectly. Burning fossil fuels gives a city its air pollution in two ways. Directly, it releases primary pollutants straight from the exhaust pipe or chimney: CO, NOx, SO2 and particles. Indirectly, the NOx it releases then helps form tropospheric ozone in the air, a secondary pollutant that no exhaust pipe gives out. That is why Geneva's Stick'AIR plan targets the cars that release the most NOx, even on days when the problem is ozone.

flowchart TD
  A["Fossil fuel<br>burned"] --> B["Primary pollutants<br>CO, NOx, SO2, PM"]
  B --> C["Harm at once<br>(direct)"]
  B --> D["NOx reacts<br>in the air"]
  D --> E["Tropospheric ozone<br>(secondary)"]

Task 2.1 Complete the table. Two rows are done for you.

Source	Natural or anthropogenic?	Primary pollutant(s) it releases
Saharan dust over Geneva	Natural	Particulate matter
Volcanic eruption	Natural	Sulfur dioxide, particulate matter (ash)
Traffic jam in the city centre
Coal-fired power station
Building site
Forest burned to clear land for farming

Task 2.2

Explain, with one example of each, how burning fossil fuels pollutes a city's air directly and indirectly.
Suggest why Geneva cannot prevent every day of high particulate matter, however strict its traffic rules are.

🎯 Context

According to the clip, why are very small particles more harmful than larger ones?

Identify two groups of people the clip says are most at risk from air pollution.

Phase 1: What goes into a city's air

Distinguish between PM10 and PM2.5.

Describe the pattern of NO2 across Geneva's four stations in 2025. Use figures.

Suggest why NO2 at Necker is more than three times higher than at Passeiry.

Phase 2: Where it comes from

Explain, with one example of each, how burning fossil fuels pollutes a city's air directly and indirectly.

Suggest why Geneva cannot prevent every day of high particulate matter, however strict its traffic rules are.

Phase 3: A city with a bigger problem

Where does Nigeria sit?

Case study: Lagos, Nigeria