Which statement correctly describes how smog forms?

• A. Smog forms when primary pollutants are directly emitted as ozone.
• B. Smog formation is caused only by natural factors, not human emissions.
• C. Photochemical oxidants formed from VOCs and NOx, along with ozone, contribute to smog.
• D. Smog only forms in clean, rural environments.

Answer: (C)

Smog forms when sunlight drives chemical reactions in the atmosphere that turn common pollutants into new substances. In places with lots of traffic and industry, volatile organic compounds (VOCs) and nitrogen oxides (NOx) are released into the air. Sunlight then triggers reactions between these precursors to produce photochemical oxidants, including ozone. Ozone is a major part of smog, and these oxidants together create the smoggy mix we associate with sunny, polluted days. This is why the statement describing photochemical oxidants formed from VOCs and NOx, along with ozone, as contributors to smog is the best description. Remember, ozone isn’t usually emitted directly; it forms in the atmosphere. Smog arises from human-emitted pollutants, and it tends to be most problematic in polluted urban areas rather than clean rural environments.