Which of the following is a secondary air pollutant?

• A. Carbon Monoxide
• B. Sulfur Dioxide
• C. Nitrogen Dioxide
• D. Ozone

Answer: (D)

A secondary air pollutant is formed in the atmosphere from precursor chemicals, not emitted directly. Ground-level ozone fits this because it isn’t released from a smokestack or tailpipe; it forms when sunlight drives chemical reactions between nitrogen oxides and volatile organic compounds, producing ozone in the air we breathe. This photochemical process makes ozone a daytime pollutant that contributes to smog and health problems.

In contrast, carbon monoxide, sulfur dioxide, and nitrogen dioxide are primarily emitted directly from combustion and industrial processes, so they’re considered primary pollutants. (They can participate in further reactions to create other pollutants, but their defining characteristic here is direct emission.)