Which of the following is commonly considered a primary air pollutant?

Which of the following is commonly considered a primary air pollutant?

• A. Ozone
• B. Carbon Dioxide
• C. Nitrogen Dioxide
• D. Water Vapor

Answer: (C)

Primary pollutants are those released directly into the air from a source. Nitrogen dioxide fits this idea because it is emitted directly from combustion processes like vehicle exhaust and power plants. Ozone, by contrast, forms in the atmosphere from reactions involving NOx and volatile organic compounds under sunlight, so it’s a secondary pollutant. Water vapor is a natural atmospheric constituent and not treated as a pollutant in this framework. Carbon dioxide is also emitted directly, but in many air-quality contexts it’s discussed mainly as a greenhouse gas rather than a traditional ambient pollutant, making nitrogen dioxide the best example among these for a primary pollutant.

Primary pollutants are those released directly into the air from a source. Nitrogen dioxide fits this idea because it is emitted directly from combustion processes like vehicle exhaust and power plants. Ozone, by contrast, forms in the atmosphere from reactions involving NOx and volatile organic compounds under sunlight, so it’s a secondary pollutant. Water vapor is a natural atmospheric constituent and not treated as a pollutant in this framework. Carbon dioxide is also emitted directly, but in many air-quality contexts it’s discussed mainly as a greenhouse gas rather than a traditional ambient pollutant, making nitrogen dioxide the best example among these for a primary pollutant.