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)

Sunlight-driven chemistry involving NOx and VOCs leads to ozone formation in urban photochemical smog. When light shines on nitrogen dioxide, it splits into nitric oxide and a free oxygen atom. The free oxygen quickly joins molecular oxygen to form ozone. But if only this simple path occurred, ozone would be consumed again by NO, so little would accumulate. VOCs change the story by producing peroxy radicals (and other reactive species) when they react with common daytime oxidants like hydroxyl radicals. These peroxy radicals can convert NO to NO2 without using up ozone, allowing ozone to build up rather than be destroyed. This chain of reactions sustains ozone production during sunny parts of the day. VOC oxidation can also lead to secondary organic aerosols, adding to particulate pollution. So the described mechanism—NO2 photolysis creating O and O3, with VOC-derived radicals converting NO to NO2 without consuming O3 and promoting both ozone and secondary organic aerosol formation—best captures how photochemical smog forms in sunny urban environments.

The other statements don’t fit because ozone does not form directly from sunlight acting on oxygen alone, sunlight does not destroy pollutants to prevent smog, and ozone is not formed in the absence of light.