Ozone is considered a secondary air pollutant because it

Ground-level ozone is a secondary pollutant because it is not emitted directly but forms when sunlight drives reactions among primary pollutants like nitrogen oxides and hydrocarbons.

Being present in the upper atmosphere describes stratospheric ozone's location, not why surface ozone counts as secondary.

Direct production inside industry would make it a primary emission, the opposite of how tropospheric ozone arises.

Having a merely minor role in pollution misreads the term, which refers to origin by reaction rather than degree of harm.

Primary pollutants are released straight from a source, while secondary ones are manufactured in the air afterwards.

Ground-level ozone is a key ingredient of photochemical smog and irritates the lungs.