IB ESS HLTopic 8 — Urban Air PollutionPaper 1 & 2HL only~11 min read
How Photochemical Smog Forms
Photochemical smog needs three ingredients: traffic fumes, sunshine, and air that is not going anywhere. Take away any one and the smog does not form. That is why two cities with identical traffic can have completely different air — the difference is usually the weather and the shape of the land.
📚 What you need to know
Photochemical smog forms when sunlight triggers reactions between primary pollutants, producing secondary pollutants. It appears as a brown or grey haze.
The primary ingredients are nitrogen oxides (NOx) and volatile organic compounds (VOCs).
The main secondary products are tropospheric ozone (O3) and peroxyacyl nitrates (PANs).
Meteorological factors: strong insolation, low wind, and temperature inversions.
A temperature inversion is a layer of warm air sitting above cooler air, which stops pollutants rising and dispersing.
Topographical factors: mountains blocking wind, and urban canyons between high-rise buildings.
Named examples: Los Angeles and Mexico City for ozone in a basin, New Delhi for winter episodes, Hong Kong for dense high-rise development.
The ingredients
Primary pollutants that start it
Nitrogen oxides (NOx) — released from combustion in vehicles and power plants. Includes nitric oxide (NO) and nitrogen dioxide (NO2).
Volatile organic compounds (VOCs) — hydrocarbons such as benzene, emitted from vehicle exhausts, industrial processes, chemical solvents, paints and fuel evaporation.
Secondary pollutants that result
Tropospheric ozone (O3) — the main component of the smog. Causes respiratory problems and damages crops.
Peroxyacyl nitrates (PANs) — formed from VOCs reacting with NOx in sunlight. They sting the eyes, irritate airways, and reduce plant growth and photosynthesis.
The core reaction, simplified
NO2 + sunlight → NO + O
O + O2 → O3
Ozone forms when sunlight splits nitrogen dioxide, releasing a single oxygen atom that joins an ordinary oxygen molecule. Normally that ozone would be destroyed again almost immediately by reacting with the nitric oxide. VOCs are what breaks the balance: they mop up the nitric oxide, so the ozone survives and accumulates. This is why cutting VOCs matters as much as cutting NOx.
A day in the life of a smoggy city
Photochemical smog follows a daily rhythm, and the shape of that rhythm is strong evidence for the chemistry. Traffic peaks first, then the chemistry catches up.
Notice the evening rush hour produces a second NO peak but almost no ozone. Without strong sunlight the chemistry does not run.
WORKED EXAMPLE
Using the daily cycle above, explain why ozone peaks in mid-afternoon even though traffic is heaviest at about 08:00.
Step 1: what happens at 08:00
Rush-hour engines emit nitric oxide and VOCs directly, so NO peaks first.
Step 2: the first conversion
NO is oxidised in the air to nitrogen dioxide, so the NO2 peak arrives about two hours later.
Step 3: the sunlight-driven step
Sunlight splits NO2 to give oxygen atoms, which combine with O2 to form ozone.
This needs strong insolation, which peaks around midday to mid-afternoon.Step 4: the lag
Each step takes time, and the sunlight is strongest hours after the traffic.
Ozone is a secondary pollutant, so it arrives lateAfter sunset the reaction stops and remaining ozone is destroyed overnight, which is why the curve falls back to a low baseline.
Weather that makes it worse
Strong sunlight
The reactions are photochemical, meaning light supplies the energy. Long sunny days give more hours of reaction and higher peak concentrations, which is why smog is a summer problem in Los Angeles and why warm cities such as New Delhi are badly affected.
Low wind
Stagnant air prevents dispersion. Pollutants stay near ground level, concentrations build, and the reactants spend longer close together, which speeds up the chemistry.
Temperature inversion
Normally air near the ground is warmest, so it rises and carries pollutants upwards. In an inversion this is turned upside down: a layer of warm air sits above cooler air near the surface. Cool air is denser, so it cannot rise through the warm layer, and everything in it stays put.
🧩 How an inversion forms overnight
During the day, the ground absorbs sunlight and warms the air just above it.
That warm air rises, carrying pollutants up and away. Normal dispersal.
At night, the ground loses heat quickly by radiation and cools the air directly above it.
A layer of warmer air remains above the chilled surface layer, acting like a lid.
Pollutants emitted under the lid accumulate until the sun warms the surface again.
Land that makes it worse
Two cities can emit the same amount and have very different air quality. Geography and weather decide what happens after the emission.
Mountains and basins
Cities ringed by high ground, such as Mexico City and Los Angeles, are especially prone to smog. The mountains block horizontal winds, so pollutants that cannot rise through an inversion cannot escape sideways either.
Urban canyons
Streets lined with high-rise buildings form urban canyons that restrict air movement at street level, so exhaust fumes linger exactly where pedestrians are breathing. Hong Kong experiences smog intensified by very dense high-rise development.
A neat way to structure any “explain why this city has severe smog” answer: sources, sunlight, stagnation, shape. Sources gives you the traffic and industry, sunlight the photochemistry, stagnation the inversion and low wind, shape the mountains and buildings. Four headings, full marks.
WORKED EXAMPLE
Two cities have similar populations and vehicle numbers. City P sits on a flat, windy coastal plain; City Q sits in a mountain basin at high altitude with long sunny days. Explain why City Q suffers far worse photochemical smog.
Step 1: emissions are similar
Both emit comparable quantities of NOx and VOCs, so the difference is not the source.
Step 2: sunlight
City Q has stronger and longer insolation, and the reactions are photochemical, so more ozone and PANs are produced per unit of NOx.
Step 3: stagnation
City P’s coastal winds disperse pollutants continually. City Q is more likely to develop temperature inversions, trapping pollutants near the ground.
Step 4: shape
Mountains around City Q block horizontal dispersal, so the pollution has nowhere to go in any direction.
Same emissions, very different dispersalAltitude adds a further problem: thinner air means engines burn fuel less efficiently, producing more carbon monoxide and unburnt hydrocarbons.
💡 Exam tip
Name the ingredients precisely: NOx and VOCs in, ozone and PANs out, with sunlight driving it.
Always call ozone and PANs secondary pollutants.
Explain an inversion using density: cool dense air cannot rise through warmer air above it.
Separate meteorological factors from topographical ones if the question asks about factors.
Use the daily cycle as evidence. The lag between the traffic peak and the ozone peak proves ozone is not emitted.
Keep two named basin cities ready: Los Angeles and Mexico City.
⚠ Common mix-ups
Saying vehicles emit ozone. They emit the ingredients; sunlight makes the ozone.
Confusing photochemical smog with the old smoke-and-fog smog. The historic London type came from coal smoke and sulphur dioxide in cold damp air; photochemical smog needs sunshine.
Describing an inversion as warm air near the ground. It is warm air above cooler air — the reverse of normal.
Ignoring VOCs. Without them the ozone would be destroyed almost as fast as it forms.
Assuming smog is worst in the morning. The ozone peak is mid-afternoon.
Treating topography as a minor detail. It is often the single biggest reason one city is worse than another.
Up next: The Damage Done by Tropospheric Ozone — what this smog actually does to lungs, crops, materials and economies.
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