Written by the Fiveable Content Team • Last updated September 2025
Verified for the 2026 exam
Verified for the 2026 exam•Written by the Fiveable Content Team • Last updated September 2025
Definition
Secondary pollutants are harmful substances that form in the atmosphere through chemical reactions involving primary pollutants and other atmospheric components. They contribute to air pollution indirectly.
Primary pollutants are directly emitted into the environment from human activities or natural sources. Unlike secondary pollutants, they do not require any chemical reactions to form.
Ozone is a secondary pollutant formed by complex chemical reactions involving sunlight, nitrogen oxides (NOx), volatile organic compounds (VOCs), and other pollutants. It is a major component of smog.
Acid rain is a secondary pollutant formed when primary pollutants, such as sulfur dioxide (SO2) and nitrogen oxides (NOx), react with water vapor in the atmosphere. It can have harmful effects on ecosystems, buildings, and human health.