Here's why:

* Primary pollutants are emitted directly into the atmosphere from a source, such as a power plant or car exhaust. Examples include carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2).

* Secondary pollutants are formed in the atmosphere through chemical reactions between primary pollutants and other atmospheric components.

In this case, sulfur trioxide (SO3) is formed from the oxidation of sulfur dioxide (SO2) in the presence of oxygen and water vapor. This reaction occurs in the atmosphere, not directly at the source of emission.

Therefore, SO3 is considered a secondary pollutant.