Which of the following is a primary air pollutant?

• A. CO, CO2, SO2, NO, hydrocarbons, particulates
• B. Ozone
• C. Water vapor
• D. Methane

Answer: (A)

Primary air pollutants are substances released directly into the atmosphere from sources like vehicles, power plants, and industrial processes. They enter the air in a form that can harm health or the environment without needing to undergo chemical reactions first. Ozone, for example, is produced in the air when sunlight drives reactions between nitrogen oxides and volatile organic compounds, so it’s a secondary pollutant rather than a primary one. Water vapor is a natural part of the atmosphere and isn’t treated as a pollutant in this context. Methane is a hydrocarbon and is emitted directly, so it falls under hydrocarbons. The listed emissions—carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen oxides, hydrocarbons, and particulates—are all released directly into the air, which is why they’re considered primary pollutants. Ozone isn’t included because it forms in the atmosphere after emissions, not released directly.