Which of the following is a secondary pollutant?

• A. Ozone
• B. Sulfur dioxide
• C. Nitrogen dioxide
• D. Particulate matter

Answer: (A)

Secondary pollutants form in the atmosphere from chemical reactions involving primary pollutants, not from direct emissions. Ground-level ozone is produced when sunlight drives reactions between nitrogen oxides and volatile organic compounds emitted by cars, industry, and solvents, creating ozone in the lower atmosphere. This photochemical process makes ozone a classic secondary pollutant, contributing to smog and respiratory problems. The other options are typically released directly from sources: sulfur dioxide and nitrogen dioxide come from burning fossil fuels and other combustion, and particulate matter is released as tiny particles, though some PM can form secondarily—this question focuses on the well-known secondary formation of ozone.