HL only; Subtopic 8.3 Urban air pollution. Photochemical smog is a brownish mixture of secondary pollutants formed when sunlight drives chemical reactions involving primary pollutants, especially nitrogen oxides and volatile organic compounds. Its most important component is harmful tropospheric ozone.
How photochemical smog forms
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Vehicles, power stations and industry release primary pollutants directly into the atmosphere. These include nitrogen oxides, especially nitrogen monoxide and nitrogen dioxide, and volatile organic compounds (VOCs).
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Sunlight breaks down nitrogen dioxide:
- The free oxygen atom combines with molecular oxygen to form ozone:
- Reactions involving VOCs convert nitrogen monoxide back into nitrogen dioxide without removing as much ozone. This allows ozone and other secondary pollutants, including peroxyacetyl nitrates, to accumulate near ground level.
| Factor | Effect on smog formation |
|---|---|
| Strong sunlight and high temperature | Increase the rate of photochemical reactions |
| Heavy traffic or industrial emissions | Supply nitrogen oxides and VOCs |
| Low wind speed | Prevents pollutants from dispersing |
| Temperature inversion | Traps polluted air near the ground |
| Basin topography | Restricts air movement, as in Los Angeles |
Tropospheric ozone damages lung tissue, irritates eyes, reduces photosynthesis and lowers crop productivity. Exposure is also an environmental justice issue because communities near major roads and industrial areas may experience greater health risks.
A common misconception is that photochemical smog is emitted directly. It is not: its major components are produced in the atmosphere from primary pollutants. Also, harmful tropospheric ozone must not be confused with protective stratospheric ozone.
Exam technique
For an explain question, give the causal sequence: primary emissions, sunlight-driven transformations, secondary pollutant formation and accumulation under suitable meteorological or topographical conditions. Use precise terms and support the explanation with a named example such as Los Angeles.
