The atmosphere is a thin layer of gases held to Earth by gravity. Dry air (with the water vapor left out) has almost the same recipe everywhere near the ground.
| Gas | Share of dry air |
|---|---|
| Nitrogen (N₂) | about 78% |
| Oxygen (O₂) | about 21% |
| Argon (Ar) | about 0.9% |
| Carbon dioxide (CO₂) | about 0.04% (about 420 parts per million) |
| Others (neon, helium, methane …) | tiny amounts |
Key idea
Dry air is about 78% nitrogen, 21% oxygen and 1% argon. Carbon dioxide is only about 0.04%, yet it matters a lot for climate and for plants.
Why it works
"Parts per million" (ppm) is a percent with a bigger denominator. 420 ppm means 420 out of every 1,000,000 molecules, which is $420 \div 1{,}000{,}000 = 0.00042$, or 0.042%. Out of every 10,000 molecules of air, only about 4 are carbon dioxide, but those few absorb heat (the greenhouse effect) and feed every plant on Earth.
Worked example: Oxygen in a room
A classroom holds 200,000 liters of air. About how many liters are oxygen?
- Oxygen is about 21% of air: a percent of a number.
- $200{,}000 \times 0.21 = 42{,}000$ L.
Answer: About 42,000 L
Worked example: From ppm to percent
Before the 1800s, air held about 280 ppm of carbon dioxide. What percent is that?
- 280 ppm = $280 \div 1{,}000{,}000 = 0.00028$.
- As a percent: $0.00028 \times 100 = 0.028\%$.
Answer: About 0.028%
Common mistake
Thinking air is mostly oxygen, or that carbon dioxide is a big part of it. Air is mostly nitrogen (78%); oxygen is about a fifth, and carbon dioxide only about 0.04%. Check: the shares must add up to about 100%, and 78 + 21 + 1 already does, leaving very little room for anything else.