What are the two classifications of air pollutants?

The classification of air pollutants into primary and secondary categories is widely recognized in environmental science. Primary pollutants are those that are emitted directly into the atmosphere from identifiable sources. These can include substances like carbon monoxide, sulfur dioxide, and particulate matter released from vehicles and industrial processes.

Secondary pollutants, on the other hand, are not emitted directly. Instead, they form in the atmosphere through chemical reactions involving primary pollutants and other atmospheric constituents. Ozone in the lower atmosphere is a common example, as it results from the reaction of sunlight with pollutants like volatile organic compounds (VOCs) and nitrogen oxides.

This classification system helps in understanding the sources and formation processes of pollutants, which is critical for developing effective air quality management strategies and regulations. The other options do not provide a universally accepted or practical classification of air pollutants in the same way that primary and secondary do.