Ozone formation in urban areas requires which conditions?

• A. NOx, and sunlight, but not VOCs.
• B. No sunlight is required for ozone formation.
• C. NOx, VOCs, and sunlight.
• D. Only VOCs and no NOx.

Answer: (C)

Ozone in urban air is formed through photochemical reactions that require energy from sunlight and the chemical precursors nitrogen oxides and volatile organic compounds. Sunlight powers the photolysis of NO2 to produce an oxygen atom, which quickly combines with O2 to form O3. But sustaining this production needs a steady supply of reactive radicals generated when VOCs react in the daytime; these radicals help convert NO to NO2 without consuming ozone, keeping the cycle going. Without sunlight, the photochemical steps stop and ozone won’t form. Without either NOx or VOCs, the necessary reactive partners aren’t present to drive the reactions. So, all three—NOx, VOCs, and sunlight—are required for ozone formation in urban environments, though the dominant limiting factor can vary depending on the local mix of pollutants.