Dirty Air: Primary vs Secondary
Identify pollutants and their sources
What this lesson covers
Why it matters
A chimney belches black smoke straight into the sky — that is a primary pollutant. But the choking haze that forms hours later, cooked up by sunlight in the air, is something new: a secondary pollutant.
Predict first
Which kind of pollutant is NOT released directly from a source, but forms later in the atmosphere?
Primary pollutants are emitted directly (SO₂, CO, NO, smoke). Secondary pollutants form in the air by reaction of primaries (SO₃, NO₂, ground-level ozone, acid rain).
- A secondary pollutant — correct
- A primary pollutant
- A natural pollutant
What you do
Sort each pollutant: is it released directly (primary), or formed later in the air (secondary)?
Check yourself
Which is a secondary pollutant?
Ground-level ozone isn't emitted directly — it forms in the air from other pollutants and sunlight.
Carbon monoxide from incomplete combustion is a primary pollutant because it is…
Primary pollutants enter the air straight from the source; CO comes directly out of the exhaust.
- Ground-level ozone formed in photochemical smog — correct
- Carbon monoxide from a car exhaust
- Smoke from burning coal
- emitted directly from the source into the air — correct
- formed by reactions in the atmosphere
- harmless to humans