Composition of dry air (spec 2.6)
78% N₂, 21% O₂, ~1% Ar, ~0.04% CO₂. Plus variable H₂O and trace gases.
Dry air — the well-mixed gases of the atmosphere with water vapour removed — has a remarkably stable composition near the surface:
| Gas | Formula | % by volume |
|---|---|---|
| Nitrogen | N₂ | ≈ 78% |
| Oxygen | O₂ | ≈ 21% |
| Argon | Ar | ≈ 0.93% (≈ 1%) |
| Carbon dioxide | CO₂ | ≈ 0.04% (~420 ppm and rising) |
| Other (Ne, He, Kr, Xe, CH₄, H₂) | — | trace amounts |
Why N₂ is essentially constant. Nitrogen is by far the largest reservoir (78%). The N≡N triple bond is very strong (~945 kJ/mol), so N₂ is generally unreactive. The natural rates of nitrogen fixation (by lightning, bacteria, and the Haber process) and denitrification are tiny compared with the atmospheric reservoir, so the % stays essentially constant.
Why CO₂ varies. Carbon dioxide is only 0.04% — a small reservoir. It is involved in many fast processes:
- Removed by: photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂).
- Released by: respiration (reverse of photosynthesis), combustion of fossil fuels and biomass, volcanic activity.
- Exchanged with: the oceans (CO₂ dissolves more in cold water).
Because the inflows and outflows are large relative to the small reservoir, CO₂ levels vary seasonally (lower in summer when photosynthesis dominates), geographically (higher near cities and fires), and historically (rising from ~280 ppm in 1800 to >420 ppm today due to human activity).
Water vapour. Real atmospheric air contains 0-4% water vapour, depending on temperature and humidity. The 'dry' composition above is air with this variable amount of H₂O removed.
Where does the air come from? Today's atmosphere evolved over billions of years. The original atmosphere (4 billion years ago) was mostly CO₂ + N₂ + water vapour from volcanic outgassing. The current ~21% O₂ was produced over ~2 billion years by photosynthetic cyanobacteria and later plants. The current N₂ is the long-standing 'leftover' that doesn't react easily.
- N₂ 78%, O₂ 21%, Ar ~1%, CO₂ ~0.04% — memorise these.
- N₂ constant (large unreactive reservoir, strong triple bond).
- CO₂ variable (small reservoir; photosynthesis/respiration/combustion cycle).
- Air also contains variable water vapour (0-4%) and trace gases.