7.2 Photochemical Smog

• Nitrogen oxides (NOₓ), mostly from vehicle engines and power plants.
• Volatile organic compounds (VOCs), carbon-based chemicals that evaporate easily, from gasoline vapor, paints, solvents and some industries. Trees also give off natural VOCs.
• Sunlight, which supplies the energy for the reactions. That is why it's called photochemical: 'photo' means light.
• Warm temperatures, which speed up the reactions.

Sunlight splits nitrogen dioxide: NO₂ + sunlight → NO + O. The single oxygen atom (O) is very reactive and joins an oxygen molecule to form ozone: O + O₂ → O₃.

Without VOCs, the cycle mostly balances out, because NO reacts with ozone and turns it back into NO₂ and O₂. VOCs break that balance. Sunlight breaks VOCs into reactive fragments that turn NO back into NO₂ without using up ozone, so less NO is left to destroy ozone, and ozone accumulates. VOCs and NOₓ also combine into other smog chemicals such as peroxyacyl nitrates (PANs), which irritate eyes and harm plants.

So smog needs both NOₓ and VOCs plus sunlight. Cutting either NOₓ or VOCs reduces ozone.

If you graph pollutant levels over a sunny weekday in a big city, you'll see a clear sequence. NO rises first, peaking during the morning rush hour as cars start up. NO₂ peaks a little later in the morning as NO reacts in the air. Ozone climbs through the late morning and peaks in the early to mid afternoon, when sunlight is strongest, then falls in the evening as sunlight fades.

Smog is worst in summer, in cities with heavy traffic, lots of sunshine and surrounding mountains that trap air, such as Los Angeles and Mexico City. Thermal inversions (topic 7.3) make it worse.

Ground-level ozone irritates the airways, causes coughing and chest pain, reduces lung function and triggers asthma attacks. It also damages plant leaves, reducing photosynthesis and crop yields. Smog reduces visibility too.

Ways to reduce it include driving less and using public transit, catalytic converters that cut NOₓ and unburned hydrocarbons, vapor recovery nozzles at gas pumps that capture VOCs, lower-VOC paints and solvents, and cleaner power plants.

Don't confuse this ground-level ozone with the stratospheric ozone layer high above, which protects you from UV radiation (topic 9.1). Same molecule, different place: harmful near the ground, helpful high up.