Figure 1

Composition of the Earth's atmosphere below 100 km
ConstituentMolecular WeightContent (fraction of total molecules
Nitrogen(N2)28.0160.7808(75.51% by mass)
Oxygen(O2)32.000.2095(23.14% by mass)
Argon(Ar)39.940.0093(1.28% by mass)
Water Vapor(H2O)18.020-0.04
Carbon Dioxide(CO2)44.01364 parts per million
Neon(Ne)20.1818 parts per million
Helium(He)4.005 parts per million
Krypton(Kr)83.71 parts per million
Hydrogen(H)2.020.5 parts per million
Ozone(03)48.000-12 parts per million

Adapted from J. M. Wallace and P. V. Hobbs, 1977:
Atmospheric Science - An Introductory Survey. Academic Press, New York. 467 pp.