Which of the following describes a secondary pollutant?

A secondary pollutant is defined as a substance that does not come directly from a source but rather forms in the atmosphere through chemical reactions between primary pollutants and other atmospheric components. This process typically occurs in the presence of sunlight, where primary pollutants, such as nitrogen oxides and volatile organic compounds, react with one another (often aided by sunlight) to create secondary pollutants such as ozone or fine particulate matter.

In contrast, the other options refer to different characteristics of pollutants: primary pollutants are those that are emitted directly into the air from sources like vehicles or industrial processes, natural occurrences refer to pollutants that arise from natural events without human interference, and regulated pollutants are those subject to legal limits set by environmental regulations. Thus, these distinctions highlight the unique process by which secondary pollutants are formed, underscoring their importance in air quality management and public health considerations.