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

Carbon monoxide
sulfur dioxide
Nitrogen dioxide

1 Answer
Aug 26, 2016



Primary pollutants are substances that are emitted into the environment directly from a source, and then cause pollution without undergoing any further reaction.

Smog, on the other hand, forms via the reaction of various 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). Smog, therefore, forms via (photo)chemical reaction or physical mixing once primary pollutants are in the environment, and is therefore a secondary pollutant.