6.3 Photochemical smog

Significant ideas

  • The combustion of fossil fuels produces primary pollutants that may generate secondary pollutants and lead to photochemical smog, the levels of which can vary by topography, population density and climate.
  • Photochemical smog has significant impacts on societies and living systems.
  • Photochemical smog can be reduced by decreasing human reliance on fossil fuels.

Knowledge and understanding

  1. Primary pollutants from the combustion of fossil fuels include carbon monoxide, carbon dioxide, black carbon or soot, unburned hydrocarbons, oxides of nitrogen, and oxides of sulfur.
  2. In the presence of sunlight, secondary pollutants are formed when primary pollutants undergo a variety of reactions with other chemicals already present in the atmosphere.
  3. Tropospheric ozone is an example of a secondary pollutant, formed when oxygen molecules react with oxygen atoms that are released from nitrogen dioxide in the presence of sunlight.
  4. Tropospheric ozone is highly reactive and damages plants (crops and forests), irritates eyes, creates respiratory illnesses and damages fabrics and rubber materials. Smog is a complex mixture of primary and secondary pollutants, of which tropospheric ozone is the main pollutant.
  5. The frequency and severity of smog in an area depends on local topography, climate, population density, and fossil fuel use.
  6. Thermal inversions occur due to a lack of air movement when a layer of dense, cool air is trapped beneath a layer of less dense, warm air. This causes concentrations of air pollutants to build up near the ground instead of being dissipated by “normal” air movements.
  7. Deforestation and burning, may also contribute to smog.
  8. Economic losses caused by urban air pollution can be significant.
  9. Pollution management strategies include:
    • altering human activity to consume less fossil fuels—example activities include the purchase of energy-efficient technologies, the use of public or shared transit, and walking or cycling
    • regulating and reducing pollutants at the point of emission through government regulation or taxation
    • using catalytic converters to clean the exhaust of primary pollutants from car exhaust
    • regulating fuel quality by governments
    • adopting clean-up measures such as reforestation, regreening, and conservation of areas to sequester carbon dioxide.

Applications and skills

  • Evaluate pollution management strategies for reducing photochemical smog.


  • The global rise of urbanization and industrialization has led to an increase in urban air pollution.

Theory of knowledge

  • Environmental problems are often emotive—under what circumstances should we maintain a detached relationship with the subject matter under investigation?


Photochemical smog and urban air pollution are directly connected to these other ESS topics: