The Chemical Composition of Air

Nearly all of the Earth's atmosphere is made up of only five gases: nitrogen, oxygen, water vapor, argon, and carbon dioxide. Several other compounds are also present.

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.

Air Pollution

Air pollution varies according to geographical location and also where it occurs in the air column. Pollutants include chemicals, particulates like dust and ash, and biological matter, like pollen and bacteria.

The Ozone Layer

Ozone (O₃) is unevenly distributed throughout the Earth's atmosphere. The ozone layer is a portion of the stratosphere from 15 to 35 kilometers (9.3 to 21.7 miles). However, its thickness varies geographically and seasonally. The ozone layer contains about 90% of the atmospheric ozone, with a concentration of 2 to 8 parts per million. Even though this is a much higher concentration of ozone than occurs in the troposphere, ozone is still just a trace gas in the ozone layer.

Homosphere and Heterosphere

The homosphere is the portion of the atmosphere with a fairly uniform composition due to atmospheric turbulence. In contrast, the heterosphere is the part of the atmosphere where chemical composition varies mainly according to altitude.

The homosphere includes the lower layers of the atmosphere: the troposphere, stratosphere, mesosphere, and lower thermosphere. The turbopause, at around 100 kilometers or 62 miles, is the edge of space and roughly the limit of the homosphere.

Above this layer, the heterosphere includes the exosphere and thermosphere. The lower portion of the heterosphere contains oxygen and nitrogen, but these heavier elements do no occur higher up. The upper heterosphere consists almost entirely of hydrogen.

Sources

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• Lide, David R. (1997). CRC Handbook of Chemistry and Physics. Boca Raton, FL: CRC. 14-17.
• Lutgens, Frederick K.; Tarbuck, Edward J. (1995). The Atmosphere (6th ed.). Prentice Hall. ISBN 0-13-350612-6.
• Martin, Daniel; McKenna, Helen; Livina, Valerie (2016). "The human physiological impact of global deoxygenation". The Journal of Physiological Sciences. 67 (1): 97–106. doi:10.1007/s12576-016-0501-0
  
• Wallace, John M.; Hobbs, Peter V. (2006). Atmospheric Science: An Introductory Survey (2nd ed.). Elsevier. ISBN 978-0-12-732951-2.