IB ESS HLTopic 8 — Urban Air PollutionPaper 1 & 2Core idea~10 min read
Where Urban Air Pollution Comes From
Nearly all urban air pollution comes down to one thing: burning fuel. What varies is what is burned, where, and what the sunlight does to the fumes afterwards. Get the difference between primary and secondary pollutants clear now — almost everything in this topic depends on it.
📚 What you need to know
Primary pollutants are emitted directly from a source and are active as soon as they enter the air.
Secondary pollutants are not emitted at all. They form in the atmosphere when primary pollutants react.
The main primary pollutants are nitrogen oxides (NOx), sulphur dioxide (SO2), carbon monoxide (CO) and particulate matter (PM).
PM2.5 is 2.5 micrometres across or smaller; PM10 is 10 micrometres or smaller. Smaller means more dangerous.
Sources are natural (fires, dust storms, volcanoes) or anthropogenic (traffic, power stations, industry, construction, agricultural burning).
Tropospheric ozone (O3) is the key secondary pollutant and a major part of urban smog.
Named examples: Beijing for PM2.5 from coal, Los Angeles for ozone, New Delhi for crop burning and traffic.
Primary and secondary
A primary pollutant leaves an exhaust pipe or a chimney already harmful. A secondary pollutant is manufactured in the air itself, usually with sunlight providing the energy. That distinction matters practically: you can filter a primary pollutant at the source, but you cannot filter something that has not been made yet.
Note that natural sources feed the same primary pollutant pool. A volcano and a power station both release sulphur dioxide; only one of them can be regulated.
The four primary pollutants
Pollutant
Main urban source
Why it matters
Nitrogen oxides (NOx)
Vehicle engines and power stations, where high temperatures make nitrogen and oxygen in the air combine
Irritates airways; forms nitric acid and, in sunlight, tropospheric ozone
Sulphur dioxide (SO2)
Burning coal and heavy oil, which contain sulphur impurities
Irritates the lungs; forms sulphuric acid and acid deposition
Carbon monoxide (CO)
Incomplete combustion in engines and stoves, where oxygen is limited
Binds to haemoglobin far more strongly than oxygen, so it starves tissues of oxygen
Particulate matter (PM)
Exhaust soot, brake and tyre wear, construction dust, and smoke
Penetrates deep into the lungs; the pollutant most strongly linked to early deaths
Notice where NOx comes from. It is not an impurity in the fuel, like sulphur is — it is made from the nitrogen already in the air, because engines run hot enough to force nitrogen and oxygen to react. That is why cleaner fuel alone does not fix NOx, and why the fix has to happen in the exhaust.
Particle size is the whole story
Particulate matter is classified by diameter in micrometres. PM10 means particles 10 micrometres across or smaller; PM2.5 means 2.5 micrometres or smaller. The smaller ones matter far more, because size decides how far into the body a particle can travel. Larger particles are caught in the nose and throat. PM2.5 reaches the deepest parts of the lungs, and the finest fraction crosses into the bloodstream.
Smog is visible because there are billions of these particles scattering light, not because any one of them is big.
WORKED EXAMPLE
A PM10 particle has four times the diameter of a PM2.5 particle. Assuming both are spheres, calculate how many PM2.5 particles have the same total volume as one PM10 particle, and explain why measuring pollution by mass can be misleading.
Step 1: the diameter ratio10 ÷ 2.5 = 4Step 2: volume scales with the cube of the diameter43 = 6464 PM2.5 particles equal one PM10 by volumeStep 3: why mass misleads
Air quality is often reported in micrograms per cubic metre, so a single large particle counts as much as 64 small ones. But the 64 small ones do far more harm, because they reach much deeper into the lungs.
This is exactly why PM2.5 is now measured and reported separately from PM10.
Sources
Natural
Forest fires release smoke, ash and huge quantities of particulate matter.
Dust storms lift material from dry ground and carry it into cities hundreds of kilometres away.
Volcanic eruptions produce large amounts of sulphur dioxide and ash.
Anthropogenic
Burning fossil fuels in vehicles, power stations and factories, producing NOx, SO2, CO and PM. This is the dominant source in almost every city.
Agricultural burning and deforestation, releasing smoke and dust in large seasonal pulses.
Construction sites and roads, generating dust and particulates from machinery, demolition and tyre and brake wear.
Industrial processes, releasing NOx and PM from chimneys and chemical processing.
Not all vehicle particulates come from the exhaust. Brake dust, tyre wear and dust lifted from the road surface are now a large share of traffic PM in many cities — and electric vehicles produce all three. Mentioning this shows you understand why cleaner engines alone will not solve particulate pollution.
Three cities, three problems
Beijing, China. High PM2.5, driven by coal burning for energy and heating and by heavy industry, worst in winter.
Los Angeles, USA. Ozone pollution, caused by a very large number of vehicles combined with strong sunshine that drives the reactions forming ozone.
New Delhi, India. Severe episodes from crop burning on the surrounding plains, plus traffic and industry, trapped by still winter air.
WORKED EXAMPLE
Classify each of the following as a primary or secondary pollutant and justify your choice: (a) sulphur dioxide from a coal power station; (b) tropospheric ozone over Los Angeles; (c) soot from a diesel engine; (d) nitric acid in rainfall.
(a) Sulphur dioxidePrimary. Released directly from the chimney and harmful immediately.
(b) Tropospheric ozoneSecondary. Nothing emits it. It forms in the air when nitrogen oxides react in sunlight.
(c) Diesel sootPrimary. Particulate matter emitted straight from the exhaust.
(d) Nitric acid in rainSecondary. Formed when nitrogen dioxide reacts with water and oxygen in the atmosphere.
Ask one question: was it emitted, or was it made?Sulphur dioxide is primary, but the sulphuric acid it becomes is secondary. The same atoms can appear in both categories.
💡 Exam tip
Define primary as directly emitted and secondary as formed in the atmosphere. Those exact phrases score.
Learn the four primary pollutants and one source for each. It covers most short-answer questions.
Give PM sizes in micrometres. “PM2.5 is 2.5 micrometres or less” is precise and quick.
Say why smaller particles are worse: they get further into the lungs.
Split sources into natural and anthropogenic if a question asks about causes.
Attach a city to each pollutant type: Beijing for PM2.5, Los Angeles for ozone, New Delhi for burning.
⚠ Common mix-ups
Calling ozone a primary pollutant. Nothing emits tropospheric ozone; it is always secondary.
Thinking PM10 excludes PM2.5. PM10 means 10 micrometres or smaller, so it includes the fine fraction.
Confusing carbon monoxide with carbon dioxide. CO is an acutely toxic pollutant; CO2 is a greenhouse gas.
Assuming NOx comes from the fuel. It is made from nitrogen in the air by the heat of combustion.
Ignoring natural sources. Volcanoes and dust storms can dominate air quality for weeks.
Believing electric cars end particulate pollution. Brake, tyre and road dust remain.
Up next: Cleaning Up Urban Air — what actually works, and the three levels at which any pollution problem can be tackled.
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