The earth's atmosphere near the surface is composed primarily of Nitrogen and Oxygen. Together, the two comprise about 99% of the gas in the atmosphere. Here's a listing of the key components of the lower atmosphere...
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Nitrogen - 78.084%
Oxygen - 20.95%
Argon - 0.934%
Carbon Dioxide - 0.036%
Neon - 0.0018%
Helium - 0.0005%
Methane - 0.00017%
Hydrogen - 0.00005%
Nitrous Oxide - 0.00003%
Ozone - 0.000004%
In addition, water vapor is variable but typically makes up about 1-4% of the atmosphere.
