Terrestrial Air

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The percent by volume of those gases found in relatively constant amount in dry air near sea level in very nearly as follows:

ELEMENT	%
nitrogen (N2)	78.084
oxygen (O2)	20.9476
argon (A)	0.934
carbon dioxide (CO2)	0.0314 (variable)
neon (Ne)	0.001818
helium (He)	0.000524
methane (CH4)	0.0002 (variable)
krypton (Kr)	0.000114
hydrogen (H2)	0.00005
nitruous oxide (N2O)	0.00005
xenon (Xe)	0.0000087

In addition to the above constituents there are many variable constituents. Chief of these is water vapor, which may vary from zero to volume percentages close to 4 percent. Ozone, sulfur dioxide, ammonia, carbon monoxide, iodine, and other trace gases occur in small and varying amounts.
The above composition of dry air is true to about 90 kilometers. See upper atmosphere.

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use