What are the two groups that all air contaminants can be divided into?

All air contaminants can be divided into two main groups: primary pollutants and secondary pollutants. Primary pollutants are directly emitted into the atmosphere from sources such as vehicles, industrial processes, and natural events. In contrast, secondary pollutants are not emitted directly but form in the atmosphere through chemical reactions involving primary pollutants and other atmospheric constituents. Examples include ozone and smog, which develop from the interaction of sunlight with pollutants like nitrogen oxides and volatile organic compounds.