7.2 Photochemical Smog

How photochemical smog forms

Photochemical smog forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) from vehicle exhaust and fossil fuel combustion react in sunlight. UV radiation drives these reactions, producing a brownish haze and secondary pollutants like ground-level ozone (O3). It's most common in cities with lots of traffic, sunlight, and geographic features (like valleys) that trap air.

Ozone: the key player

Ground-level ozone is the main harmful component of photochemical smog. Unlike stratospheric ozone (which protects us from UV), ground-level ozone is a pollutant that irritates lungs, worsens asthma, and damages plants. It forms through a cycle: NOx + VOCs + sunlight → ozone + other secondary pollutants like PANs (peroxyacyl nitrates).

Conditions that worsen smog

Smog worsens with temperature inversions, where a layer of warm air traps cooler air (and pollutants) near the ground, preventing dispersal. Sunny, warm, calm days with heavy traffic maximize smog formation. Geography like mountain-ringed basins (e.g., Los Angeles) traps pollutants, while wind and rain help disperse them.

Key terms

Photochemical smog

Air pollution formed when NOx and VOCs react with sunlight, producing ozone and other harmful compounds.

Ground-level ozone

A secondary pollutant and main harmful component of smog; irritates lungs and damages plants (different from protective stratospheric ozone).

VOCs (volatile organic compounds)

Carbon-based chemicals that evaporate easily, released from vehicles, solvents, and industry; react with NOx to form smog.

Temperature inversion

Atmospheric condition where warm air traps cooler air (and pollutants) near the ground, worsening smog.

PANs (peroxyacyl nitrates)

Secondary pollutants formed in photochemical smog reactions; eye/respiratory irritants.

Try an AP-style question

Which ingredients produce photochemical smog?

1. CFCs, ice, and wind
2. SO₂, water, and darkness
3. CO₂, rain, and cold air
4. NOx, VOCs, and sunlight