Which statement correctly identifies a secondary air pollutant?

• A. Directly emitted smoke from a stack.
• B. Particulate matter is a primary pollutant.
• C. Ground-level ozone is formed primarily through biological processes in water.
• D. Ozone formed from NOx and VOCs in the presence of sunlight.

Answer: (D)

Secondary pollutants are not released directly; they form in the atmosphere through chemical reactions, often driven by sunlight. Ground-level ozone is a classic example: sunlight breaks down NOx and VOCs from vehicles and other sources, leading to reactions that produce ozone. So describing ozone formation from NOx and VOCs in the presence of sunlight identifies a pollutant that is formed secondarily, not emitted directly. The other statements reference pollutants that are released as they are (like smoke or particulates) or misstate how ozone forms (in water or by biological processes), which is not how ground-level ozone develops.