clipped from: www.physicalgeography.net   

0.000004%

O3

*Ozone

*Nitrous Oxide

N2O
0.00003%

0.00005%

H2

Hydrogen

*Methane

0.00017%

Argon

*Water

H2O

0 to 4%

0.93%

0.0360%

20.95%

78.08%

Percent Volume

Chemical Formula

N2

O2

Nitrogen

Gas Name

Oxygen

*Water

Ar

0.0018%

CO2

n

Ne

0.0018%

Helium

He

0.0005%

*Methane

CH4

0.00

Neon

Helium

*Methane

He

0.0005%

Ne
CH4

0.00017%

0.000004%

H2

*Nitrous Oxide

Average composition of the atmosphere up to an altitude of 25 km.
Ozone is also highly concentrated at the Earth's surface in and around cities. Most of this ozone is created as a by product of human created photochemical smog. This buildup of ozone is toxic to organisms living at the Earth's surface.