Which of the following is not a primary air pollutant ?

Air pollutants are categorized based on how they enter the atmosphere. Primary air pollutants are emitted directly from identifiable sources, such as vehicle exhaust or industrial smokestacks. In this context, Carbon Monoxide (CO), Nitric Oxide (NO), and Sulfur Dioxide (SO2) are all primary pollutants because they are released directly into the air. Conversely, Ozone (O3) is a secondary pollutant. It does not come directly from an emission source but forms in the atmosphere through complex photochemical reactions between primary pollutants like nitrogen oxides and volatile organic compounds under sunlight. Therefore, Ozone is the correct answer as it is not a primary air pollutant.