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3.1.4 Composition of the Earth′s atmosphereValues 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 m^{3}/m^{3} are available in ISO Standard 14912. In the case of the conversion of an amount fraction (in mol/mol) to a volume fraction (in m^{3}/m^{3}), 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).
The molecular weight of air can be calculated using the formula where the index A covers the components with amount fraction x_{A} and molecular weight M_{A}. 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.
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 CFC12, 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).
References
Martin Milton 
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