HL only; Subtopic 8.3 Urban air pollution. Strong sunlight, high temperatures, weak winds, temperature inversions and dry, stable weather intensify photochemical smog. Basin or valley topography surrounded by mountains traps pollutants and prevents their dispersal.
Photochemical smog forms when sunlight drives reactions involving primary pollutants, particularly nitrogen oxides and volatile organic compounds. These reactions produce secondary pollutants, including tropospheric ozone, which can damage respiratory systems, plants and materials.
Meteorological and topographical conditions do not necessarily create the original emissions. Instead, they accelerate photochemical reactions, reduce pollutant removal or restrict atmospheric mixing.
| Factor | How it intensifies photochemical smog |
|---|---|
| Strong sunlight | Supplies the energy required for photochemical reactions, increasing secondary pollutant formation. |
| High temperature | Accelerates chemical reactions and increases evaporation of volatile organic compounds from fuels and solvents. |
| Weak winds | Reduce horizontal dispersion, allowing pollutants to accumulate above an urban area. |
| Temperature inversion | A layer of warm air above cooler surface air prevents vertical convection, trapping pollutants near ground level. |
| Stable high-pressure conditions | Produce sinking air, clear skies and weak winds, creating conditions favourable for pollutant accumulation and intense sunlight. |
| Low rainfall | Prevents pollutants from being removed through wet deposition. |
| Basin or valley topography | Surrounding mountains restrict airflow and concentrate pollutants within the urban basin. |
| Coastal mountains and sea breezes | Sea breezes may move polluted air inland, where mountains block it or return it toward the city, causing recirculation. |