What causes ground-level ozone to form?

• A. Ozone is emitted directly by vehicles and factories and accumulates near the ground.
• B. Sunlight drives reactions involving nitrogen oxides and volatile organic compounds to form ozone at ground level.
• C. Ground-level ozone forms only in the upper atmosphere.
• D. Ozone at the ground is harmless to humans.

Answer: (B)

Ground-level ozone is a secondary pollutant formed when sunlight drives chemical reactions between nitrogen oxides and volatile organic compounds released from cars, factories, and other sources. Sunlight provides the energy for these reactions, especially the breakdown of nitrogen dioxide to release an oxygen atom that combines with oxygen molecules to make ozone. VOCs help sustain the reaction chain, so warm, sunny conditions in polluted urban or downwind areas lead to higher ozone near the surface. This is different from ozone in the upper atmosphere, which is part of a protective layer. Ground-level ozone is not emitted directly and is harmful to human health and crops.