What form of emissions come from chemical reactions after pollutants enter the air?

The correct choice identifies secondary pollutants, which are formed when primary pollutants undergo chemical reactions in the atmosphere. Primary pollutants are directly emitted from sources like vehicles, factories, and natural processes. Once these pollutants enter the air, they can interact with sunlight or other atmospheric compounds, resulting in new substances that are harmful to air quality and human health.

An example of a secondary pollutant is ozone, which is created when sunlight reacts with pollutants like volatile organic compounds and nitrogen oxides. This distinction is crucial because it highlights the transformations that can occur in the atmosphere, emphasizing the complexity of air pollution and the importance of addressing not just direct emissions but also their subsequent reactions and effects. Other options, such as tertiary and compact pollutants, do not represent commonly recognized classifications of air emissions, underscoring the significance of understanding primary and secondary pollutants in air quality discussions.