Table of Elements and Compounds in Air

Below is the composition of air in percent by volume, at sea level at 15 C and 101325 Pa.

• Nitrogen -- N₂ -- 78.084%
• Oxygen -- O₂ -- 20.9476%
• Argon -- Ar -- 0.934%
• Carbon Dioxide -- CO₂ -- 0.04%
• Neon -- Ne -- 0.001818%
• Methane -- CH₄ -- 0.0002%
• Helium -- He -- 0.000524%
• Krypton -- Kr -- 0.000114%
• Hydrogen -- H₂ -- 0.00005%
• Xenon -- Xe -- 0.0000087%
• Ozone -- O₃ -- 0.000007%
• Nitrogen Dioxide -- NO₂ -- 0.000002%
• Iodine -- I₂ -- 0.000001%
• Carbon Monoxide -- CO -- trace
• Ammonia -- NH₃ -- trace

Nitrogen, oxygen and argon are the three main components of the atmosphere. Water concentration varies, but averages around 0.25% of the atmosphere by mass. Carbon dioxide and all of the other elements and compounds are trace gases. Trace gases include the greenhouse gases carbon dioxide, methane, nitrous oxide, and ozone. Except for argon, other noble gases are trace elements. These include neon, helium, krypton, and xenon. Industrial pollutants include chlorine and its compounds, fluorine and its compounds, elemental mercury vapor, sulfur dioxide, and hydrogen sulfide. Other components of the atmosphere include spores, pollen, volcanic ash, and salt from sea spray.

Water Vapor in the Atmosphere

Although this CRC table does not list water vapor (H₂O), air can contain as much as 5% water vapor, more commonly ranging from 1-3%. The 1-5% range places water vapor as the third most common gas (which alters the other percentages accordingly). Water content varies according to air temperature. Dry air is denser than humid air. However, sometimes humid air contains actual water droplets, which can make it more dense than humid air that only contains water vapor.