Definition

Secondary air pollutants are atmospheric pollutants that are not directly emitted into the air as primary pollutants but are formed as a result of chemical reactions or transformations involving primary pollutants in the atmosphere. These secondary pollutants are often the product of complex chemical reactions that occur in the presence of sunlight, atmospheric compounds, and other catalysts. Common examples of secondary air pollutants include ground-level ozone (formed from reactions involving nitrogen oxides and volatile organic compounds), sulfate aerosols (resulting from the oxidation of sulfur dioxide), and secondary organic aerosols (formed from the atmospheric oxidation of volatile organic compounds).