The atmosphere

The atmosphere of Earth is a layer of gasessurrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperatureextremes between day and night (the diurnal temperature variation).

The common name given to the atmospheric gases used in breathing and photosynthesis isair. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen,^[1] 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. Although air content andatmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants andterrestrial animals currently is only known to be found in Earth's troposphere and artificial atmospheres.

The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of the Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reenty of spacecraft at an altitude of around 120 km (75 mi). Several  layerscan be distinguished in the atmosphere, based on characteristics such as temperature and composition 

 Composition

Air is mainly composed of nitrogen, oxygen, andargon, which together constitute the major gases of the atmosphere. Water vapor accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppmv in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically provided for dry air without any water vapor.^[3] The remaining gases are often referred to as trace gases,^[4] among which are thegreenhouse gases such as carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, andvolcanic ash. Various industrial pollutants also may be present as gases or aerosols, such aschlorine (elemental or in compounds), fluorinecompounds and elemental mercury vapor. Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO₂) may be derived from natural sources or from industrial air pollution.

Composition of dry atmosphere, by volume^[5]

ppmv: parts per million by volume (note: volume fractionis equal to mole fraction for ideal gas only, see volume (thermodynamics))
Gas	Volume
Nitrogen(N2)	780,840 ppmv (78.084%)
Oxygen (O2)	209,460 ppmv (20.946%)
Argon (Ar)	9,340 ppmv (0.9340%)
Carbon dioxide(CO2)	397 ppmv (0.0397%)
Neon (Ne)	18.18 ppmv (0.001818%)
Helium (He)	5.24 ppmv (0.000524%)
Methane(CH4)	1.79 ppmv (0.000179%)
Krypton (Kr)	1.14 ppmv (0.000114%)
Hydrogen(H2)	0.55 ppmv (0.000055%)
Nitrous oxide (N2O)	0.325 ppmv (0.0000325%)
Carbon monoxide(CO)	0.1 ppmv (0.00001%)
Xenon (Xe)	0.09 ppmv (9×10−6%) (0.000009%)
Ozone (O3)	0.0 to 0.07 ppmv (0 to 7×10−6%)
Nitrogen dioxide(NO2)	0.02 ppmv (2×10−6%) (0.000002%)
Iodine (I2)	0.01 ppmv (1×10−6%) (0.000001%)
Ammonia(NH3)	trace
Not included in above dry atmosphere:
Water vapor(H2O)	~0.25% by mass over full atmosphere, locally 0.001%–5%[3]