7.1 Introduction to Air Pollution

Primary vs. Secondary Pollutants

Primary pollutants are emitted directly into the atmosphere from a source, like carbon monoxide (CO) from car exhaust or sulfur dioxide (SO2) from burning coal. Secondary pollutants form when primary pollutants react in the atmosphere, often with sunlight — like ozone (O3) and PAN, key parts of photochemical smog. Understanding this distinction helps identify pollution sources and control strategies.

Major Sources of Air Pollution

Stationary sources stay in one place, like power plants and factories, while mobile sources move, like cars and airplanes. Natural sources include volcanic eruptions, wildfires, and dust storms, while anthropogenic sources come from human activity like fossil fuel combustion. Most urban air pollution stems from burning fossil fuels for energy and transportation.

Common Pollutants and Their Origins

Key pollutants include CO (incomplete combustion), SO2 (burning sulfur-containing coal/oil), NOx (high-temperature combustion), particulate matter (PM) (dust, soot, smoke), and VOCs (evaporated fuels, solvents). These combine to form smog and acid precipitation, harming human respiratory health and ecosystems alike.

Key terms

Primary pollutant

A pollutant emitted directly into the atmosphere from a source, such as CO or SO2.

Secondary pollutant

A pollutant formed through chemical reactions of primary pollutants in the atmosphere, such as ozone or PAN.

Stationary source

A fixed pollution source, like a power plant or factory.

Mobile source

A pollution source capable of movement, such as cars, trucks, and airplanes.

Anthropogenic pollution

Pollution caused by human activities, such as burning fossil fuels for energy and transportation.

Try an AP-style question

Which is a secondary air pollutant?

1. Tropospheric ozone
2. Carbon monoxide from cars
3. Soot from diesel engines
4. Sulfur dioxide from coal