Urban air pollution is mainly caused by emissions from fossil-fuel combustion in transport, electricity generation, industry and buildings. Natural sources also contribute, while weather and topography can restrict dispersion and increase pollutant concentrations.
Level: SL and HL core; photochemical smog formation and its intensification by meteorological and topographical factors are HL-only content. Subtopic: 8.3 Urban air pollution.
The Reasoning and Mechanism
A primary pollutant enters the atmosphere directly from a source. Important urban examples include nitrogen oxides, sulfur dioxide, carbon monoxide and particulate matter. Most common urban pollutants are directly or indirectly associated with fossil-fuel combustion, although natural sources also exist.
| Cause or factor | Mechanism |
|---|---|
| Road transport | Engines release nitrogen oxides, carbon monoxide and particles; tyre and brake wear adds particles. |
| Industry and power generation | Fuel combustion releases sulfur dioxide, nitrogen oxides and particles. |
| Heating and cooking | Burning coal, oil, gas or biomass releases pollutants near homes. |
| Construction and waste burning | Construction creates dust; open burning releases smoke and toxic pollutants. |
| Natural sources | Wildfires, dust and volcanic activity add pollutants. |
| Weather and topography | Low winds, valleys and thermal inversions trap pollutants. |
Pollutants may undergo transformations. Nitrogen oxides and sulfur dioxide react with water and oxygen to form nitric and sulfuric acids, causing acid rain. At HL, sunlight acts on primary pollutants such as nitrogen oxides and volatile organic compounds to produce secondary pollutants, including tropospheric ozone, forming photochemical smog.
The 1952 Great Smog of London shows how coal combustion combined with stagnant inversion conditions to produce pollution. Exposure is also an environmental justice issue because lower-income communities may live closer to major roads or industrial sites.
Exam Technique
For an explain question, link each source to a named pollutant and then explain why its concentration rises. Avoid the misconception that weather creates pollution: inversions and low winds trap existing pollution.
