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Composition of Air: Major Gases and Their Roles

Dry air at sea level is a mixture of gases with remarkably constant proportions:

GasVolume %Primary Roles
Nitrogen (N₂)78.08%Inert diluent; nitrogen fixation → biological N cycle; industrial feedstock.
Oxygen (O₂)20.95%Essential for aerobic respiration, combustion, ozone formation.
Argon (Ar)0.93%Inert; used in lighting, welding, insulating windows.
Carbon Dioxide (CO₂)0.04% (420 ppm)Greenhouse gas; photosynthesis substrate; pH buffer in blood/oceans.
Trace Gases<0.01%Neon, helium, methane, krypton, hydrogen, xenon, ozone (variable).

Variable Components

  • Water Vapor (H₂O): 0–4% (humidity); major greenhouse gas, weather driver.
  • Aerosols/Particulates: Dust, pollen, sea salt, pollution.

Physiological Note

Inspired air: 21% O₂, 0.04% CO₂. Expired air: ~16% O₂, ~4% CO₂, saturated with H₂O at 37°C.

Air composition is stable globally but local pollution alters trace gases and particulates.

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