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Chapter: 3 Chemistry
    Section: 3.1 The elements
        SubSection: 3.1.4 Composition of the Earth’s atmosphere



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Updated: 16 October 2012
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3.1.4 Composition of the Earth′s atmosphere

Values for the amount fraction of the major components of the atmosphere are required for various reasons including the calculation of the molecular weight and the density of air.

The values given here are expressed as amount fractions in the unit mol/mol. Formulae for the conversion of these quantities to quantities expressed volumetrically, for example in units of m3/m3 are available in ISO Standard 14912.

In the case of the conversion of an amount fraction (in mol/mol) to a volume fraction (in m3/m3), the numerical values are the same for an ideal gas.

The three most abundant components

The three most abundant compounds in the atmosphere are nitrogen, oxygen and argon, which each have amount fractions that are stable.

In order to achieve closure (ie. a sum for all components of unity when expressed in fractional units) with a relative uncertainty of 10-6, it is necessary to consider the presence of carbon dioxide, which does not have a constant amount fraction. The values listed here are based on measured values of oxygen (Machta 1970) and argon (Park 2004) and nitrogen which is calculated by completion. For reference, we also include the CIPM 81/91 values (Giacomo 1982).

Component    Values from CIPM 81/91    Recommended values and uncertainties
mol/mol mol/mol
N2 0.78101   0.78082  ± 0.00012
O2 0.20939   0.20945  ± 0.00012
Ar 0.00917    0.009332 ± 0.000006

The molecular weight of air can be calculated using the formula

Formula for calculating the molecular weight of air

where the index A covers the components with amount fraction xA and molecular weight MA. Calculation of the uncertainty in the molecular weight of air should take account of the fact that the uncertainty in the value for nitrogen is negatively correlated with the uncertainties in the values of the other components. It also requires a value for the amount fraction of water vapour, which should be measured locally.

The greenhouse gases

The amount fractions of the major greenhouse gases in the atmosphere are not constant. Values have been given by the IPCC (2007) for the volume mixing ratios in 2005 and the change since 1998.

Component    2005 value    Change since 1998
  Carbon dioxide [10 -6 m3/m3]   379 ± 0.65 + 13
  Methane [10 -9 m3/m3]   1774 ± 1.8    + 11
  Nitrous oxide [10 -9 m3/m3]   319 ± 0.12 + 5

Water vapour, ozone and halogenated compounds

Ozone and water vapour are present in both the troposphere and the stratosphere. Their amount fractions vary significantly with altitude, latitude, longitude, time of year and time of day.

Halogenated compounds, of which the most abundant is CFC-12, exist in the atmosphere at volume fractions below 1 ppb. (IPCC 2007).

The Noble gases (except argon) plus hydrogen and carbon monoxide

The best available values for the levels of the noble gases in the atmosphere are from Glueckauf (1951). These values have been reproduced in the US Standard Atmosphere (1976) and "rounded" before incorporation in the CIPM 81/91 formula (Giacomo 1982).

Component    Values from US Standard Atmosphere     Values from CIPM 81/91
10-6 m3/m3 µmol/mol
   Neon 18.18  18.2  
   Helium 5.24 5.2
   Krypton 1.14 1.1
   Xenon   0.087 0.1
   Hydrogen 0.5 0.5
   Carbon monoxide    - 0.2


  • Machta L and Hughes E (1970)
    Atmospheric Oxygen in 1967 to 1970
    Science 168 1582-1584
  • Giacomo P (1982)
    Equation for the determination of the density of moist air
    Metrologia 18 33-40
  • Park S Y, Kim J S, Esler M B, Davis R S and Wielgosz R I (2004)
    A redetermination of the argon content of air for buoyancy corrections in mass standard comparisons
    Metrologia 41 387-95
  • IPCC (2007), Climate Change: The Physical Science Basis.
    Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Table 2.1)
  • Glueckauf E (1951)
    The composition of atmospheric air
    Compendium of Meteorology pp 3-10
    American Meteorological Society, Boston 
  • US Standard Atmosphere (1976), published by the U.S. Government Printing Office, Washington, D.C.

Martin Milton


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