Which of the following is an example of a secondary pollutant?

A secondary pollutant is defined as a substance that is not emitted directly into the air but forms in the atmosphere through chemical reactions. Acid rain is a prime example of this process; it results from the chemical reactions between primary pollutants, such as sulfur dioxide and nitrogen oxides, with water vapor in the atmosphere. This leads to the formation of sulfuric and nitric acids, which then fall to the ground as acid rain.

The other substances listed are examples of primary pollutants, which are directly emitted from sources like vehicles, industrial processes, or natural occurrences. Understanding the distinction between primary and secondary pollutants is crucial for identifying the sources of environmental issues and developing strategies for mitigation and management in public health and environmental contexts.