Atmosphere Contents

1. Structure And Composition

The predominant gases in the atmosphere are nitrogen, oxygen, argon and carbon dioxide

Nitrogen and oxygen are the predominant gases in the atmosphere, accounting for just over 99% by volume. Another very important constituent is water vapour, the concentration of which is extremely variable both in space and time. Water vapour is found predominantly in the lower atmosphere below 15km. Water is unique in that at typical Earth surface temperatures and pressures it is the only substance that can exist in all of its three states; solid, liquid and gas. Both water and carbon dioxide are termed 'greenhouse gases', as they insulate the Earth against heat loss.

The composition of the dry atmosphere is fairly constant up to 80 km: the concentrations of most gases do not vary with height within the atmosphere. A major exception to this is ozone, which is predominantly found at altitudes between 15 km and 40 km.

Gas Symbol Percent by
(volume dry air) 		Gas (and particles) Symbol Percent
(by volume) 		Parts per million (ppm)
Permanent Gases Variable Gases
Nitrogen N2 78.08 Water Vapour H2O 0 to 4
Oxygen O2 20.95 Carbon dioxide CO2 0.035 365
Argon Ar 0.93 Methane CH4 0.00017 1.7
Neon Ne 0.0018 Nitrous oxide N2O 0.00003 0.3
Helium He 0.0005 Ozone O3 0.000004 0.04
Hydrogen H2 0.00005 Particles (dust,soot,etc)   0.000001 0.01
Xenon Xe 0.000009 Chloroflurocarbons
(CFCs) 		  0.00000001 0.0001

What are the predominant gases in the atmosphere?