Which one of the following is not a primary pollutant?

The correct answer is D. Nitrogen oxides.

Primary pollutants are pollutants that are emitted directly from a source, such as a smokestack or tailpipe. Secondary pollutants are pollutants that are formed in the air when primary pollutants react with other chemicals or with sunlight.

Particulate matter is a mixture

of solid particles and liquid droplets that are small enough to be suspended in the air. Carbon monoxide is a colorless, odorless gas that is produced when fossil fuels are burned. Sulfur dioxide is a gas that is produced when sulfur-containing fuels are burned. Nitrogen oxides are gases that are produced when fuels are burned at high temperatures.

Nitrogen oxides are not primary pollutants because they are not emitted directly from a source. They are formed in the air when primary pollutants, such as nitrogen monoxide and nitrogen dioxide, react with other chemicals or with sunlight.