Which of the following is NOT considered a primary pollutant?

Ozone is considered a secondary pollutant because it is not directly emitted into the atmosphere; instead, it forms through chemical reactions between primary pollutants such as volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. In contrast, primary pollutants like carbon monoxide, sulfur dioxide, and particulate matter are directly released into the air from sources such as vehicle emissions, industrial processes, and combustion activities. Understanding this distinction is essential for grasping air pollution dynamics, as it highlights the complex interplay between various pollutants and their origins. The focus on direct emissions is crucial for the development of effective air quality management strategies.