Which of the following is not considered an air pollutant?

Nitrogen, in its molecular form (N2), constitutes about 78% of the Earth's atmosphere and is generally not considered an air pollutant. Unlike other substances that contribute to smog, acid rain, and health problems, nitrogen itself is a non-toxic gas that does not have harmful effects at the concentrations found in the atmosphere.

On the other hand, carbon dioxide, sulfur dioxide, and particulate matter are classified as air pollutants due to their negative impact on air quality and human health. Carbon dioxide is a greenhouse gas that contributes to climate change, sulfur dioxide can lead to the formation of acid rain, and particulate matter is associated with respiratory issues and various health problems. Thus, while increased levels of these substances can be detrimental, nitrogen does not fall into the same category of pollutants under normal atmospheric conditions.