AP Environmental Science (APES) Atmospheric Pollution Practice Test

Which component of urban smog forms primarily through photochemical reactions driven by sunlight?

Nitrogen dioxide Particulate matter Ozone Sulfur dioxide

Correct Answer:

Ozone

Explanation:

Ozone forms primarily through reactions driven by sunlight in photochemical smog. When sunlight hits the atmosphere, it splits nitrogen dioxide into nitrogen oxide and an oxygen atom; the oxygen atom then combines with molecular oxygen to produce ozone. Radicals produced from volatile organic compounds help cycle NO back to NO2, allowing more ozone to accumulate away from the emission sources. This makes ozone a secondary pollutant created by the sun’s energy, rather than a pollutant emitted directly. The other substances—sulfur dioxide and much of the particulate matter—are mainly emitted directly or formed through pathways not dominated by sunlight-driven photochemistry, so they don’t form primarily in this way.

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