CGround-level ozone in a city on a sunny afternoon
DSoot from a diesel engine
Ground-level ozone is not emitted directly. It forms in the air when sunlight drives reactions involving nitrogen oxides and volatile organic compounds. The other three come straight out of a source, so they are primary pollutants.
Question 2 True / False
In a city, ozone concentration usually peaks in the early morning rush hour, when car traffic is heaviest.
TTrue
FFalse
Answer: False
Morning traffic releases NO and VOCs. Sunlight then drives a chain of reactions over several hours that converts NO to NO₂ and builds up ozone. Ozone therefore usually peaks in the afternoon, often downwind of the city center.
Question 3 Short Answer
Why do smog episodes often occur under a stationary high-pressure system?
Think about your answer, then reveal below.
Model answer: Sinking air in a high-pressure system warms by compression and often creates a temperature inversion aloft. The inversion caps vertical mixing, and light winds do not carry pollutants away. Clear skies also give strong sunlight, which drives the photochemical reactions.
Pollution levels depend on how deeply and quickly emissions are mixed, as well as on how much is emitted. A low mixing height traps the same emissions in a smaller volume of air.
Question 4 Multiple Choice
Why does adding VOCs to air that contains NO and NO₂ allow ozone to build up?
AVOCs absorb ultraviolet light and turn it directly into ozone
BVOC-derived peroxy radicals convert NO to NO₂ without using up ozone, so the reaction NO + O₃ → NO₂ + O₂ no longer removes the ozone that forms
CVOCs react with ozone and make it more stable
DVOCs raise the air temperature, and ozone forms from heat alone
Sunlight splits NO₂ into NO and an oxygen atom, which forms ozone. Without VOCs, NO quickly reacts with that ozone and returns to NO₂, so little ozone builds up. Peroxy radicals from VOC oxidation provide another path from NO to NO₂, so ozone accumulates.