Which description best explains smog formation?

• A. Photochemical oxidants formed from VOCs and NOx under sunlight combine with ozone to create smog.
• B. Smog forms only from smoke and dust in polluted air.
• C. Smog is simply fog created by humidity, with no chemical reactions.
• D. Smog occurs only in rural areas away from traffic.

Answer: (A)

Smog forms from sunlight-driven chemistry in polluted air. Emissions from vehicles and industry release volatile organic compounds (VOCs) and nitrogen oxides (NOx). When sunlight shines on these pollutants, chemical reactions occur that create ground-level ozone and other photochemical oxidants. This buildup of reactive oxidants and ozone produces the hazy, oxidizing mixture we call smog. It’s a chemical process, not just moisture or smoke, and it tends to be a problem in urban areas with lots of traffic and sunlight. The other ideas miss this key point: smog isn’t simply fog from humidity, nor is it caused by dust alone, and it isn’t restricted to rural areas away from traffic.