Atmospheric Chemistry
Composition of the Atmosphere
The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. 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 ppm by volume 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 quoted in terms of dry air (without water vapor). The
remaining gases are often referred to as trace gases, among which are the greenhouse gases,
principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, already
mentioned, other noble gases, neon, helium, krypton, and xenon are also present. 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,
and volcanic ash. Various industrial pollutants also may be present as gases or aerosols, such
as chlorine (elemental or in compounds), fluorine compounds and elemental mercury vapor.
Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO 2) may be derived from
natural sources or from industrial air pollution.
Major constituents of dry air, by volume
Gas Volume(A)
Name Formula in ppmv(B) in %
Nitrogen N2 780,840 78.084
Oxygen O2 209,460 20.946
, Argon Ar 9,340 0.9340
Carbon dioxide
CO2
(December, 2020) (C) 415.00 0.041500
Neon Ne 18.18 0.001818
Helium He 5.24 0.000524
Methane CH4 1.87 0.000187
Krypton Kr 1.14 0.000114
Not included in above dry atmosphere:
Water vapor(D) H2O 0–30,000(D) 0–3%(E)
notes:
(A)
volume fraction is equal to mole fraction for ideal gas only,
also see volume (thermodynamics)
(B)
ppmv: parts per million by volume
(C)
The concentration of CO
2 has been increasing in recent decades
(D)
Water vapor is about 0.25% by mass over full atmosphere
(E)
Water vapor varies significantly locally
The average molecular weight of dry air, which can be used to calculate densities or to convert
between mole fraction and mass fraction, is about 28.946 or 28.96 g/mol. This is decreased when
the air is humid. The relative concentration of gases remains constant until about 10,000 m
(33,000 ft).
,Structure or Stratification of the Atmosphere
,
Composition of the Atmosphere
The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. 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 ppm by volume 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 quoted in terms of dry air (without water vapor). The
remaining gases are often referred to as trace gases, among which are the greenhouse gases,
principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, already
mentioned, other noble gases, neon, helium, krypton, and xenon are also present. 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,
and volcanic ash. Various industrial pollutants also may be present as gases or aerosols, such
as chlorine (elemental or in compounds), fluorine compounds and elemental mercury vapor.
Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO 2) may be derived from
natural sources or from industrial air pollution.
Major constituents of dry air, by volume
Gas Volume(A)
Name Formula in ppmv(B) in %
Nitrogen N2 780,840 78.084
Oxygen O2 209,460 20.946
, Argon Ar 9,340 0.9340
Carbon dioxide
CO2
(December, 2020) (C) 415.00 0.041500
Neon Ne 18.18 0.001818
Helium He 5.24 0.000524
Methane CH4 1.87 0.000187
Krypton Kr 1.14 0.000114
Not included in above dry atmosphere:
Water vapor(D) H2O 0–30,000(D) 0–3%(E)
notes:
(A)
volume fraction is equal to mole fraction for ideal gas only,
also see volume (thermodynamics)
(B)
ppmv: parts per million by volume
(C)
The concentration of CO
2 has been increasing in recent decades
(D)
Water vapor is about 0.25% by mass over full atmosphere
(E)
Water vapor varies significantly locally
The average molecular weight of dry air, which can be used to calculate densities or to convert
between mole fraction and mass fraction, is about 28.946 or 28.96 g/mol. This is decreased when
the air is humid. The relative concentration of gases remains constant until about 10,000 m
(33,000 ft).
,Structure or Stratification of the Atmosphere
,
