Which statement about photochemical smog is true?

• A. It forms from sunlight-driven reactions of NOx and VOCs producing ozone
• B. It forms from volcanic eruptions
• C. It forms only at night
• D. It forms from carbon dioxide and water

Answer: (A)

Photochemical smog is formed when sunlight drives reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in polluted air, producing ozone at the ground level. Sunlight provides the energy for the chemical reactions that convert these pollutants into ozone and other oxidants, creating the characteristic hazy, smelly air in sunny urban areas. This daytime photochemistry is the key idea: without light, these reactions slow down and ozone levels don’t rise as much.

That’s why the statement describing photochemical smog as arising from sunlight-driven reactions of NOx and VOCs producing ozone is the correct one. The other options don’t fit the process: volcanic eruptions release volcanic gases and particulates that lead to different air quality effects, not the classic NOx–VOC photochemical pathway; photochemical smog is a daytime phenomenon driven by light rather than occurring only at night; and carbon dioxide and water are not the reactive precursors that form photochemical smog.