Which of the following pollutants is specifically categorized as a secondary pollutant?

Ozone is categorized as a secondary pollutant because it is not directly emitted from a source; rather, it forms in the atmosphere through chemical reactions between primary pollutants—such as volatile organic compounds (VOCs) and nitrogen oxides—when exposed to sunlight. This process typically occurs in the lower atmosphere, specifically within the troposphere, contributing to ground-level ozone formation, which is a significant component of smog.

In contrast, the other options represent primary pollutants, which are directly released into the air from various sources. For example, carbon monoxide is produced from incomplete combustion of fossil fuels, sulfur dioxide is primarily emitted from the burning of coal and oil, and lead comes from industrial processes and the combustion of leaded fuels. Understanding the difference between primary and secondary pollutants is crucial for environmental science because it informs pollution control strategies and public health policies.