AP® Environmental Science · Unit 7 · Topic 7.2
Photochemical Smog
Photochemical smog is ground-level ozone and haze built when sunlight cooks nitrogen oxides and volatile organic compounds from vehicle exhaust, which is why it takes hours of sun to peak, not the morning rush itself.
Photochemical smog is not something that comes straight out of a tailpipe. Cars and trucks emit nitrogen oxides (NO and NO2) and volatile organic compounds, VOCs, unburned hydrocarbons that evaporate from fuel and exhaust. On their own those are primary pollutants. Ozone, the main ingredient of smog and the thing that burns your throat and hazes the skyline, is a secondary pollutant: sunlight has to build it. UV light splits NO2 into NO plus a free oxygen atom, and that lone oxygen atom attaches to an O2 molecule in the air to form O3, ozone. VOCs feed the cycle by reacting with NO and letting NO2 regenerate faster than it would otherwise, so more ozone keeps forming instead of the reaction stalling out.
That chemistry explains the daily timing. Morning rush hour dumps the raw ingredients, NO2 and VOCs, into the air, but the sunlight-driven reactions need hours of strong UV to build up a real ozone load. So concentrations climb through the morning and peak in mid to late afternoon, well after traffic has thinned, once the sun has been cooking the mix long enough. A cold, overcast, rainy day with the same traffic produces almost no smog, because weak sunlight can't split NO2 efficiently and rain washes the precursors out of the air before they react.
This links smog to the rest of the unit on atmospheric pollution: a valley or an inversion layer traps the precursors close to the ground so the same sunlight has a denser mix to work on, worsening smog, while a scrubber or catalytic converter cuts the NOx and VOCs at the source before sunlight ever gets the chance.
Key terms
- Photochemical smog
- A mix of ozone and haze formed when sunlight drives reactions between nitrogen oxides and volatile organic compounds.
- Secondary pollutant
- A pollutant like ozone that forms in the atmosphere from other emissions rather than being released directly.
- Volatile organic compounds (VOCs)
- Carbon-based chemicals that evaporate easily from fuel and exhaust and help regenerate NO2 in smog formation.
- Nitrogen dioxide (NO2)
- A gas from combustion that sunlight splits apart, releasing an oxygen atom that goes on to form ozone.
- Temperature inversion
- A layer of warm air trapping cooler air (and pollutants) near the ground, concentrating smog precursors.
Watch it explained
- How it worksWalk me through nitrogen dioxide, sunlight, VOCs, and ozone over the course of one day.
- How it worksHow does sunlight split nitrogen dioxide and leave a free oxygen atom to build ozone?
- Common mistakeWhy isn't smog at its worst on a cold cloudy day when everyone is still driving?
- Going furtherWhat happens to the daily smog curve in a city that switches half its cars to electric?
Check yourself
End of topic.