Which of the following is a secondary pollutant formed in the atmosphere rather than emitted directly?

• A. Carbon monoxide
• B. Ozone
• C. Particulate matter
• D. Nitrogen oxides

Answer: (B)

Ozone is formed in the atmosphere through chemical reactions driven by sunlight, not released directly. When solar radiation hits pollutants like nitrogen oxides and volatile organic compounds from cars, industry, and solvents, it triggers a series of reactions that build up ozone in the lower atmosphere. This makes ozone a secondary pollutant: it isn’t emitted as a pollutant itself, but is created from other emitted substances after they enter the air.

In contrast, carbon monoxide and nitrogen oxides are released directly from combustion processes, and particulate matter is typically released as particles but can also form secondarily. The key idea is that ozone’s harmful presence comes from atmospheric chemistry under sunlight, not from a direct emission.