Which of the following is called the secondary air pollutant?

Primary pollutants are directly emitted from sources (e.g., $\text{CO}$, ${\text{NH}}_3$, ${\text{NO}}_2$). Secondary pollutants are not emitted directly but form in the atmosphere through chemical reactions between primary pollutants and environmental constituents; ground-level ozone (${\text{O}}_3$) is a classic example.