The most common urban air pollutants are nitrogen oxides, sulfur dioxide, carbon monoxide, particulate matter, volatile organic compounds and tropospheric ozone. Most arise directly or indirectly from fossil-fuel combustion in transport, electricity generation and industry.
Subtopic 8.3 Urban air pollution: SL and HL core; photochemical smog formation is HL only.
Urban pollutants are classified as primary pollutants, emitted directly from a source, or secondary pollutants, formed through chemical transformations in the atmosphere.
| Pollutant | Sources or formation | Major impacts |
|---|---|---|
| Nitrogen oxides, | High-temperature combustion in engines and power stations | Respiratory irritation; contributes to acid rain and photochemical smog |
| Sulfur dioxide, | Burning sulfur-containing coal and oil; metal smelting | Respiratory damage; forms sulfuric acid and acid rain |
| Carbon monoxide, | Incomplete combustion in vehicles and heating systems | Reduces the blood's capacity to transport oxygen |
| Particulate matter, including | Diesel exhaust, construction, industry and fires | Enters deep into lungs; increases respiratory and cardiovascular disease |
| Volatile organic compounds | Fuel evaporation, solvents and vehicle exhaust | Some are toxic; react with nitrogen oxides in sunlight |
| Tropospheric ozone, | Secondary pollutant formed from nitrogen oxides and volatile organic compounds in sunlight | Damages lungs, crops and vegetation; forms photochemical smog |
A common misconception is that all ozone is harmful. Stratospheric ozone protects organisms from ultraviolet radiation, whereas tropospheric ozone is a damaging pollutant. Acid rain is not principally caused by carbon dioxide; its main urban precursors are nitrogen oxides and sulfur dioxide.
Delhi, India, provides a named example: traffic, industry, construction dust and regional biomass burning combine with atmospheric conditions to produce severe particulate pollution. This is also an environmental justice issue because lower-income communities may face greater exposure and have less access to healthcare or cleaner housing.
In an IB response, identify and classify the pollutant, link it to a specific source, and explain one impact. For HL photochemical-smog questions, explain the sunlight-driven transformation and role of tropospheric ozone.
