Monday 26 November 2012

Air Composition

ir is a mixture of gases - 78% nitrogen and 21% oxygen - with traces of water vapor, carbon dioxide, argon, and various other components. Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.
Gas Ratio compared to Dry Air (%) Molecular Mass
- M -
(kg/kmol)
Chemical Symbol Boiling Point
By volume By weight (K) (oC)
Oxygen 20.95 23.20 32.00 O2 90.2 -182.95
Nitrogen 78.09 75.47 28.02 N2 77.4 -195.79
Carbon Dioxide 0.03 0.046 44.01 CO2 194.7 -78.5
Hydrogen 0.00005 ~ 0 2.02 H2 20.3 -252.87
Argon 0.933 1.28 39.94 Ar 84.2 -186
Neon 0.0018 0.0012 20.18 Ne 27.2 -246
Helium 0.0005 0.00007 4.00 He 4.2 -269
Krypton 0.0001 0.0003 83.8 Kr 119.8 -153.4
Xenon 9 10-6 0.00004 131.29 Xe 165.1 -108.1
  • The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature
  • The composition of air is unchanged until elevation of approximately 10.000 m
  • The average air temperature diminishes at the rate of 0.6oC for each 100 m vertical height
  • "One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0oC sea level and at standard gravity (32.174 ft/sec2)
Other components in air
  • Sulfur dioxide - SO2 - 1.0 parts/million (ppm)
  • Methane - CH4 - 2.0 parts/million (ppm)
  • Nitrous oxide - N2O - 0.5 parts/million (ppm)
  • Ozone - O3 - 0 to 0.07 parts/million (ppm)
  • Nitrogen dioxide - NO2 - 0.02 parts/million (ppm)
  • Iodine - I2 - 0.01 parts/million (ppm)
  • Carbon monoxide - CO - 0 to trace (ppm)
  • Ammonia - NH3 - 0 to trace (ppm)

Common Pressure Units frequently used as alternative to "one Atmosphere"

  • 76 Centimeters (760 mm) of Mercury
  • 29.921 Inches of Mercury
  • 10.332 Meters of Water
  • 406.78 Inches of Water
  • 33.899 Feet of Water
  • 14.696 Pound-Force per Square Inch
  • 2116.2 Pounds-Force per Square Foot
  • 1.033 Kilograms-Force per Square Centimeter
  • 101.33 Kilopascal

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