Which of the following is a secondary air pollutant?
ozone mercury sulfur dioxide carbon monoxide chlorofluorocarbons
Ozone is a secondary air pollutant. It is formed in the atmosphere through chemical reactions between primary pollutants (such as volatile organic compounds and nitrogen oxides) in the presence of sunlight. The other substances listed (mercury, sulfur dioxide, carbon monoxide, and chlorofluorocarbons) are considered primary pollutants, as they are directly emitted into the air from various sources.