IB ESS SL8.3 Urban Air PollutionPaper 1 & 2~12 min read
Where Urban Air Pollution Comes From
Nearly all urban air pollution traces back to one thing: something is being burned. Fuel in an engine, coal in a power station, crop waste in a field. Get that idea fixed and the list of pollutants stops being a list to memorise and starts making sense.
📘 What you need to know
Urban air pollution is caused by human activities releasing harmful substances into the atmosphere.
The common pollutants are nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO) and particulate matter (PM).
PM2.5 is 2.5 micrometres across or smaller; PM10 is 10 micrometres or smaller.
Primary pollutants are emitted directly from a source and are immediately active in the atmosphere.
Primary pollutants come from natural sources (forest fires, dust storms, volcanic eruptions) and anthropogenic ones (fossil fuels, agricultural burning, construction, industry).
Secondary pollutants are not emitted directly; they form in the atmosphere when primary pollutants react.
Tropospheric ozone (O3) forms when NOx reacts with sunlight, and is a major part of urban smog.
Named examples: Beijing (PM2.5 from coal), Los Angeles (ozone), New Delhi (crop burning, industry, traffic).
What urban air pollution is
Urban air pollution happens when human activities release harmful substances into the atmosphere. Those pollutants come from many different sources, and they damage both human health and the environment.
Pollutant
What it is
Where it mainly comes from
Nitrogen oxides (NOx)
A group of gases formed when fuel burns at high temperature
Vehicle emissions and power plants
Sulfur dioxide (SO2)
A gas released when fuels containing sulfur are burned
Mainly burning coal and oil
Carbon monoxide (CO)
A gas produced by incomplete burning of fuel
Cars and industrial processes that burn fuels
Particulate matter (PM)
Tiny solid particles or liquid droplets suspended in the air
Dust, soot, smoke and vehicle exhaust
PM2.5 and PM10
Particulate matter is classified by size, and the number in the name is the diameter in micrometres.
PM2.5: fine particles, 2.5 micrometres or smaller.
PM10: larger particles, 10 micrometres or smaller.
The dashed ring is only there so you can find the PM2.5 dot. At this scale it is almost invisible, which is exactly the problem.
If a question asks why PM2.5 is more dangerous than PM10, the answer is size, not toxicity. Bigger particles get caught in the nose and throat. Fine particles travel all the way into the deepest parts of the lung and can cross into the bloodstream.
Primary pollutants
Primary pollutants are harmful substances that are directly emitted from a source and are immediately active in the atmosphere. They enter the air through activities such as burning fossil fuels and industrial processes, or through natural events such as volcanic eruptions.
Natural sources
Some air pollutants come from natural events with no human involvement at all.
Forest fires release smoke, ash and particulate matter.
Dust storms: strong winds lift dust from dry areas and carry it into cities.
Volcanic eruptions produce large amounts of SO2 and ash.
Anthropogenic sources
Most pollutants in urban areas come from human activities, especially anything involving the burning of fuels.
Construction is the odd one out: its dust comes from moving machinery and disturbed ground rather than from burning fuel, although the machinery burns fuel too.
Human source
What it releases
Burning fossil fuels
Emissions from vehicles, power plants and factories produce NOx, SO2, CO and PM
Agricultural burning and deforestation
Large quantities of smoke, dust and other pollutants
Construction sites and roads
Dust and PM from the movement of machinery and vehicles
Industrial processes
Pollutants such as NOx and PM from smokestacks and chemical processing
Secondary pollutants
Secondary pollutants are not emitted directly. They form in the atmosphere when primary pollutants react with other chemicals.
The one you must know is tropospheric ozone (O3), which forms when nitrogen oxides react with sunlight. It is a major component of urban smog.
No chimney anywhere emits ozone. It is assembled in the air out of things that were emitted, which is what makes it secondary.
Named examples
City
Main problem
Why
Beijing, China
High levels of PM2.5
Mainly coal burning for energy, plus industrial activity
Los Angeles, USA
Ozone pollution
A high number of vehicles combined with sunny weather, which speeds up the reaction that forms ozone
New Delhi, India
Severe general air pollution
Burning of crops, industrial activity and vehicle emissions together
Notice the pattern. Beijing’s problem is what gets emitted. Los Angeles has fewer heavy industries but far more cars and much more sunshine, so its problem is what gets made in the air. Same category of pollution, two completely different causes and two different solutions.
Worked examples
WE 1
Primary and secondary pollutants
Distinguish between primary and secondary pollutants, using an example of each. (4 marks)
Primary pollutants
These are emitted directly from a source and are immediately active in the atmosphere. Nitrogen oxides from vehicle exhausts are an example.
Secondary pollutants
These are not emitted directly. They form in the atmosphere when primary pollutants react with other chemicals.
The example
Tropospheric ozone forms when nitrogen oxides react with sunlight, and it is a major component of urban smog.
Primary comes out of the pipe; secondary is made in the air afterwardsthe word “directly” is what earns the mark — use it
WE 2
Comparing two cities
Suggest why Los Angeles suffers from ozone pollution while Beijing suffers mainly from PM2.5. (4 marks)
Point 1: Los Angeles emissions
Los Angeles has a very high number of vehicles, which release large quantities of nitrogen oxides.
Point 2: the climate does the rest
Its sunny weather speeds up the reaction between NOx and sunlight that forms tropospheric ozone, so a secondary pollutant dominates.
Point 3: Beijing emissions
Beijing relies heavily on coal burning for energy and has extensive industrial activity, both of which release particulate matter directly.
Point 4: the contrast
So Beijing’s problem is a primary pollutant emitted straight into the air, while Los Angeles has a secondary pollutant formed in the atmosphere.
Different fuels and different weather give different pollutantssuggest means apply the theory to the situation, so link climate to the reaction
WE 3
Particulate matter
Outline what is meant by PM2.5 and explain why it is a concern for human health. (3 marks)
Step 1: the definition
PM2.5 is particulate matter with a diameter of 2.5 micrometres or smaller — tiny solid particles or liquid droplets suspended in the air.
Step 2: where it comes from
It comes from exhaust fumes, industrial activity, construction dust, soot and smoke.
Step 3: why it matters
Because the particles are so small they are not filtered out by the nose and throat, so they reach deep into the lungs, causing tissue damage and inflammation.
Fine enough to get past the body’s defences and into the lungsgive the number with its unit — “2.5 micrometres”, not just “very small”
💡 Exam tips
Learn the four common pollutants by name and formula: NOx, SO2, CO, PM.
Match each pollutant to its main source. SO2 means coal and oil; NOx means vehicles and power plants.
Give the size when defining PM2.5 and PM10, in micrometres.
Use the word directly when defining a primary pollutant, and forms in the atmosphere for a secondary one.
Remember that pollution has natural sources too — forest fires, dust storms, volcanoes.
Have the three cities ready: Beijing, Los Angeles, New Delhi.
⚠ Common mistakes
Calling ozone a primary pollutant. Nothing emits tropospheric ozone; it is formed in the air.
Confusing tropospheric ozone with the ozone layer. Down here it is a pollutant; up in the stratosphere it protects us.
Saying PM10 is worse than PM2.5 because it is bigger. The smaller particles travel further into the lungs.
Writing “smog” as if it were a single pollutant. It is a mixture, with ozone as a major component.
Forgetting natural sources. Volcanoes and dust storms pollute without any human involvement.
Mixing up CO and CO2. Carbon monoxide is the toxic one from incomplete combustion.
Up next: Cleaning Up Urban Air. Now that you can name the pollutants and their sources, the obvious question is which of those sources a city can realistically do something about.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
