Which pollutant is classified as a primary pollutant?

Carbon dioxide is classified as a primary pollutant because it is emitted directly from sources such as vehicle exhaust, industrial processes, and burning fossil fuels. Primary pollutants are those that are released into the atmosphere in their original form, making carbon dioxide a clear example since it does not undergo transformation or chemical reactions in the atmosphere before affecting air quality.

On the other hand, ozone is typically formed through photochemical reactions involving other pollutants, which classifies it as a secondary pollutant rather than a primary one. Smog is a complex mixture that often results from the reaction of primary pollutants, including those that contribute to ozone formation. Particulate matter consists of small particles that can be emitted directly (as primary pollutants) or form through reactions in the atmosphere, but in the context of classifying common pollutants, carbon dioxide stands out more distinctly as a primary pollutant due to its widespread and direct emissions from varied sources.