Which pollutant is a primary pollutant emitted directly into the atmosphere?

• A. Ozone
• B. Nitrate particles
• C. Carbon monoxide
• D. Sulfate particles

Answer: (C)

Primary pollutants are substances released directly into the air from sources like engines, stoves, and fires, while secondary pollutants form after release through chemical reactions in the atmosphere. Carbon monoxide is produced directly by incomplete combustion of fuels (for example, car exhaust or burning wood), so it is drawn into the air as a product of those processes rather than being formed later in the air. The other options are typically formed in the atmosphere: ozone at ground level is created by reactions involving nitrogen oxides and volatile organic compounds in sunlight; nitrate particles and sulfate particles largely originate from atmospheric chemical reactions of precursor gases. So carbon monoxide best fits the idea of a pollutant emitted directly into the atmosphere.