Smog & Thermal Inversions

You’ll be able to

Distinguish photochemical smog from industrial (gray) smog
Trace the formation of photochemical smog from its precursors
Explain how a thermal inversion traps pollutants and worsens smog

Two kinds of smog

The word "smog" covers two different problems. Photochemical smog (Los Angeles–type, "brown smog") forms on hot, sunny days when sunlight drives reactions among vehicle-emitted nitrogen oxides (NOₓ) and volatile organic compounds (VOCs), producing ground-level ozone and other irritants. Industrial smog (London-type, "gray smog") comes from burning coal and other fossil fuels, producing sulfur dioxide and particulates; it is worst in cold, wet conditions and older industrial cities. Photochemical smog is the dominant modern urban problem.

How photochemical smog forms

Photochemical smog is a chain of reactions powered by sunlight. Morning traffic releases NOₓ and VOCs (primary pollutants). As the sun climbs, its energy splits nitrogen dioxide (NO₂), freeing oxygen atoms that combine with O₂ to build ground-level ozone (O₃) — a harmful secondary pollutant. The reactions also form other irritants such as PANs (peroxyacyl nitrates). This is why photochemical smog and its ozone peak in the afternoon of hot, sunny days in traffic-heavy cities, and are worst in summer.

Thermal inversions trap the pollution

Normally, air cools with altitude, so warm surface air rises and carries pollutants away. A thermal inversion flips this: a layer of warm air sits atop cooler air near the ground, acting like a lid. The cool, polluted surface air cannot rise through the warm cap, so pollutants accumulate near the ground for days. Cities in valleys or surrounded by mountains (Los Angeles, Mexico City, Beijing) are especially prone to inversions, which turn ordinary smog into a dangerous, concentrated haze. The deadly 1952 London Smog was an inversion event.

Photochemical smog formation

NOₓ + VOCs + sunlight → ground-level ozone (O₃) + other secondary pollutants

The precursors (NOₓ and VOCs) come mostly from vehicle exhaust. Sunlight is essential, which is why photochemical smog peaks on hot, sunny afternoons.

Worked example

A car-dependent city in a mountain-ringed valley reports its worst smog and ozone levels on hot, sunny summer afternoons, and the haze lingers for days during calm weather. Explain the two factors driving this pattern.

1.Identify the smog type: ozone plus hot, sunny conditions and heavy traffic point to photochemical smog.
2.Explain its formation: morning traffic emits NOₓ and VOCs, and afternoon sunlight drives their reaction into ground-level ozone — hence the afternoon peak.
3.Explain why it lingers: the valley promotes a thermal inversion, a warm-air lid over cooler surface air that traps pollutants near the ground.
4.Combine: abundant precursors plus strong sunlight create the smog, and the inversion prevents it from dispersing for days.

Answer: The heat and sunlight drive photochemical smog formation from vehicle NOₓ and VOCs (peaking in the afternoon), while the valley’s thermal inversion caps the air and traps the pollution, so the haze persists.

Checkpoint

Photochemical smog is most severe under which conditions?

Tip

Tell the two smogs apart by weather and fuel. Photochemical = hot + sunny + cars + ozone (brown). Industrial = cold + wet + coal + sulfur/particulates (gray). The presence of ozone and sunshine almost always signals photochemical smog.

Checkpoint

During a thermal inversion, why do air pollutants become trapped near the ground?

On the exam

A classic free-response asks you to explain why a valley city has persistent smog. Hit both factors: (1) the source and formation of the smog (vehicles + sunlight → photochemical ozone), and (2) the thermal inversion that traps it. Naming both earns full credit.

Answer the 2 checkpoints as you read.