Which statement best describes how photochemical smog forms in sunny urban environments, including the roles of NOx, VOCs, and sunlight?

Which statement best describes how photochemical smog forms in sunny urban environments, including the roles of NOx, VOCs, and sunlight?

• A. Ozone forms directly from sunlight acting on oxygen molecules independent of NOx and VOCs.
• B. Sunlight destroys pollutants, preventing smog.
• C. Sunlight drives NO2 photolysis to NO and an O atom; O combines with O2 to form O3; VOCs react with NO to form peroxy radicals that convert NO to NO2 without consuming O3, promoting ozone formation and secondary organic aerosol formation.
• D. Ozone is formed in the absence of light.

Answer: (C)

Photochemical smog in sunny cities comes from sunlight driving radical chemistry that uses NOx and VOCs to build ozone and other secondary pollutants. Sunlight breaks down nitrogen dioxide into nitric oxide and an oxygen atom; that oxygen atom quickly adds to O2 to form ozone. At the same time, volatile organic compounds react with atmospheric radicals to form peroxy radicals, which convert NO to NO2 without destroying ozone. This keeps the NO2 supply replenished so ozone production can continue, especially under sunny, stagnant urban conditions with plenty of NOx and VOCs. Over time, VOC oxidation also leads to secondary organic aerosols. The other ideas miss that ozone formation relies on these radical cycles driven by sunlight and the presence of NOx and VOCs, rather than forming directly from light on oxygen alone, or implying that sunlight would destroy pollutants or that ozone forms without light.

Photochemical smog in sunny cities comes from sunlight driving radical chemistry that uses NOx and VOCs to build ozone and other secondary pollutants. Sunlight breaks down nitrogen dioxide into nitric oxide and an oxygen atom; that oxygen atom quickly adds to O2 to form ozone. At the same time, volatile organic compounds react with atmospheric radicals to form peroxy radicals, which convert NO to NO2 without destroying ozone. This keeps the NO2 supply replenished so ozone production can continue, especially under sunny, stagnant urban conditions with plenty of NOx and VOCs. Over time, VOC oxidation also leads to secondary organic aerosols. The other ideas miss that ozone formation relies on these radical cycles driven by sunlight and the presence of NOx and VOCs, rather than forming directly from light on oxygen alone, or implying that sunlight would destroy pollutants or that ozone forms without light.