Primary air pollutants
Big idea: Primary pollutants are released directly into the atmosphere from urban sources — mainly vehicles, industry, and domestic heating.
Major primary pollutants
- Particulate matter (PM): Dust, soot, smoke. PM2.5 is most dangerous — penetrates deep into lungs. Sources: vehicles, industry, construction, burning.
- Carbon monoxide (CO): Colourless, odourless, toxic. From incomplete combustion, mainly vehicle exhausts.
- Nitrogen oxides (NOₓ): NO and NO₂. From high-temperature combustion in vehicles and power plants. Contributes to smog and acid rain.
- Sulfur dioxide (SO₂): From burning coal and oil. Causes acid rain and respiratory problems. Less common in modern cities with clean fuels.
- Volatile organic compounds (VOCs): Evaporate easily. From vehicles, solvents, paints, industry. React to form secondary pollutants.
Sources by sector
- Transport: Largest source in most cities — CO, NOₓ, PM, VOCs from vehicle exhausts
- Industry: Power plants, factories — SO₂, NOₓ, PM, heavy metals
- Domestic: Heating and cooking — PM, CO (especially with wood/coal burning)
- Construction: Dust, PM from building and demolition
- Natural: Dust, pollen, volcanic emissions (less significant in cities)
Exam tip: Questions often ask you to identify pollutants from sources or link sources to pollutants. Know which activities produce which pollutants.
Secondary pollutants and smog
Big idea: Secondary pollutants form when primary pollutants react in the atmosphere. Photochemical smog is a major urban air quality problem.
Secondary pollutant formation
- Ground-level ozone (O₃): NOT emitted directly — forms when NOₓ and VOCs react in sunlight. Main component of photochemical smog.
- Photochemical smog: Brown haze containing ozone, PAN, and other oxidants. Common in sunny cities with heavy traffic (Los Angeles, Beijing, Mexico City).
- Secondary PM: Particles formed from gas reactions — sulfates from SO₂, nitrates from NOₓ
Conditions for smog formation
- Sunlight: UV radiation drives photochemical reactions
- Primary pollutants: NOₓ and VOCs from vehicles and industry
- Temperature inversions: Trap pollutants near ground level
- Low wind: Prevents dispersion of pollutants
- Geographic factors: Valleys and basins trap pollution (LA, Mexico City, Santiago)
Ground-level ozone is BAD (respiratory irritant, smog component). Stratospheric ozone is GOOD (UV protection). Same molecule, different location, different effect!
Temperature inversions
Normally, warm air near the surface rises and disperses pollutants. In a temperature inversion, a warm layer sits above cooler surface air, acting like a lid that traps pollution.
Exam tip: Understand the difference between primary and secondary pollutants. Ground-level ozone (O₃) is a common exam topic — its secondary, formed from NOₓ + VOCs + sunlight.
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
IB-style question — Sources of urban air pollution [2]
Mardelo is a fast-growing city in a mountain valley. In winter, many homes burn wood and coal for heating, and old diesel minibuses crowd the main roads.
Outline two human sources of urban air pollution in Mardelo. [2]
How to answer it, step by step
- Name a source + its pollutant
• Combustion source: diesel traffic, factories, biomass/coal burning for heat or cooking
• Each gives a pollutant — e.g. traffic → NOₓ and particulates; coal → SO₂ and PM - Make the two sources distinct
• Don't give two versions of 'traffic' — pick from different categories
• Here: diesel minibuses (transport) AND wood/coal heating (domestic combustion)
Final answer
Examiner tip: 'Outline two' = two clearly different sources. Pair each source with the actual pollutant it releases (e.g. 'vehicle exhaust → nitrogen oxides') to secure both marks.