PRESENT COMPOSITION OF THE ATMOSPHERE:

The atmosphere is comprised of gases which are considered to be permanent (gases which remain essentially constant by percent) and gases considered to be variable (gases which have changing concentrations over a finite period of time).

 

  

PERMANENT gases in the atmosphere by percent are:

        Nitrogen    78.1%
        Oxygen      20.9%

(Note that these two permanent gases together comprise 99% of the atmosphere)

Other permanent gases:

        Argon                0.9%
        Neon                 0.002%
        Helium               0.0005%
        Krypton              0.0001%    
        Hydrogen             0.00005%

VARIABLE gases in the atmosphere and typical percentage values are:

        Water vapor     0 to 4%
        Carbon Dioxide  0.035%
        Methane         0.0002%
        Ozone           0.000004%

Source: http://www.met.fsu.edu/explores/atmcomp.html  [Jan99]