Composition of Earth's atmosphere as at Dec. 1987.
The lower pie represents the trace gases which together compose 0.038% of the atmosphere.
Values normalized for illustration.

Composition of dry atmosphere, by volume[2] ppmv: parts per million by volume
(note: volume fraction is equal to mole fraction for ideal gas only, see Gas volume#Partial volume)

Gas Nitrogen (N2) Oxygen (O2) Argon (Ar) Carbon dioxide (CO2) Neon (Ne) Helium (He) Methane (CH4) Krypton (Kr) Hydrogen (H2) Nitrous oxide (N2O) Xenon (Xe) Ozone (O3) Nitrogen dioxide (NO2) Iodine (I) Carbon monoxide (CO) Ammonia (NH3) Not included in above dry atmosphere: Water vapor (H2O)

Volume 780,840 ppmv (78.084%) 209,460 ppmv (20.946%) 9,340 ppmv (0.9340%) 387 ppmv (0.0387%) 18.18 ppmv (0.001818%) 5.24 ppmv (0.000524%) 1.79 ppmv (0.000179%) 1.14 ppmv (0.000114%) 0.55 ppmv (0.000055%) 0.3 ppmv (0.00003%) 0.09 ppmv (9x10−6%) 0.0 to 0.07 ppmv (0% to 7x10−6%) 0.02 ppmv (2x10−6%) 0.01 ppmv (1x10−6%) 0.1 ppmv trace ~0.40% over full atmosphere, typically 1%-4% at surface

http://en.wikipedia.org/wiki/Earth%27s_atmosphere