Which of the following substances cannot be a primary air pollutant? Why not?

Substances classified as primary pollutants are those that enter the environment directly from a source and cause pollution without experiencing a secondary reaction.

Smog, on the other hand, is a secondary pollutant that develops through (photo)chemical reaction or physical mixing once primary pollutants are in the environment. It is formed by the reaction of different primary pollutants in the presence of UV light (in the case of photochemical smog) or is a mixture of atmospheric fog and various types of airborne particulates (e.g. smoke).

Inert gases (e.g., helium, neon) cannot be primary air pollutants because they do not react significantly with other substances in the atmosphere to form pollutants. Primary pollutants are directly emitted and undergo chemical transformations to create secondary pollutants.