A secondary pollutant formed when sunlight reacts with NOx and VOCs.

• A. PM2.5
• B. Ground-level ozone
• C. Particulate matter
• D. VOCs

Answer: (B)

Ground-level ozone is the secondary pollutant formed when sunlight drives reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in polluted air. Sunlight splits NO2 to produce an oxygen atom, which quickly joins O2 to make O3. VOCs help sustain this process by producing radicals that convert NO back to NO2 without destroying ozone, allowing more ozone to accumulate near the ground. This is why ozone is considered a secondary pollutant: it isn’t emitted directly, but formed in the atmosphere from other pollutants under sunlight.

PM2.5 (fine particulate matter) and other particles can be present in the air, but they’re not the product of the sunlight-driven reaction between NOx and VOCs. VOCs are precursors, not the product itself, so they don’t fit as the pollutant formed by this photochemical reaction.