The equations of pressure by altitude in
the above references can be used directly to estimate
atmospheric thickness. However, the following published
data are given for reference:
- 50% of the atmosphere by mass is
below an altitude of 5.6 km.
- 90% of the atmosphere by mass is
below an altitude of 16 km. The common cruising
altitude of commercial airliners is about 10 km.
- 99.99997% of the atmosphere by
mass is below 100 km (almost all of it). The highest
X-15 plane flight in 1963 reached an altitude of
354,300 ft or 108 km.
Therefore, most of the atmosphere
(99.9997%) is below 100 km, although in the rarefied
region above this there are auroras and other
atmospheric effects.
Composition
Composition
of Earth's atmosphere as at 1987.Dec. The
lower pie represents the least common gases
that compose 0.038% of the atmosphere.
Values normalized for illustration.
Composition of
dry atmosphere, by volume
| ppmv:
parts per million by volume |
| 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) |
383 ppmv (0.0383%) |
| Neon (Ne) |
18.18 ppmv |
| Helium (He) |
5.24 ppmv |
| Methane (CH4) |
1.745 ppmv |
| Krypton (Kr) |
1.14 ppmv |
| Hydrogen (H2) |
0.55 ppmv |
| Not included
in above dry atmosphere: |
| Water vapor (H2O) |
~0.25% over full atmosphere,
typically 1% to 4% near surface |
Minor components of air not listed
above include
| Gas |
Volume |
| nitrous oxide |
0.5 ppmv |
| xenon |
0.09 ppmv |
| ozone |
0.0 to 0.07 ppmv |
| nitrogen dioxide |
0.02 ppmv |
| iodine |
0.01 ppmv |
| carbon monoxide |
trace |
| ammonia |
trace |
The mean molar mass of air is 28.97
g/mol. Note that the composition figures above are by
volume-fraction (V%), which for ideal gases is equal to
mole-fraction (that is, fraction of total molecules). By
contrast, mass-fraction abundances of gases,
particularly for gases with significantly different
molecular (molar) mass from that of air will differ from
those by volume. For example, in air, helium is 5.2 ppm
by volume-fraction and mole-fraction, but
only about (4/29) × 5.2 ppm = 0.72 ppm by
mass-fraction.
Heterosphere
Below the turbopause at an altitude of
about 100 km (not far from the mesopause), the Earth's
atmosphere has a more-or-less uniform composition (apart
from water vapor) as described above; this constitutes
the homosphere. However, above about 100 km, the
Earth's atmosphere begins to have a composition which
varies with altitude. This is essentially because, in
the absence of mixing, the density of a gas falls off
exponentially with increasing altitude, but at a rate
which depends on the molar mass. Thus higher mass
constituents, such as oxygen and nitrogen, fall off more
quickly than lighter constituents such as helium,
molecular hydrogen, and atomic hydrogen. Thus there is a
layer, called the heterosphere, in which the
earth's atmosphere has varying composition. As the
altitude increases, the atmosphere is dominated
successively by helium, molecular hydrogen, and atomic
hydrogen. The precise altitude of the heterosphere and
the layers it contains varies significantly with
temperature.
Density
and mass
-
Earth's atmosphere from space
The density of air at sea level is
about 1.2 kg/m3(1.2 g/L). Natural variations
of the barometric pressure occur at any one altitude as
a consequence of weather. This variation is relatively
small for inhabited altitudes but much more pronounced
in the outer atmosphere and space due to variable solar
radiation.
Temperature and mass density
against altitude from the NRLMSISE-00
standard atmosphere model
The atmospheric density decreases as
the altitude increases. This variation can be
approximately modeled using the barometric formula. More
sophisticated models are used by meteorologists and
space agencies to predict weather and orbital decay of
satellites.
The average mass of the atmosphere is
about 5,000 trillion metric tons or 1/1,200,000 the mass
of Earth. According to the National Center for
Atmospheric Research, "The total mean mass of the
atmosphere is 5.1480×1018 kg with an annual
range due to water vapor of 1.2 or 1.5×1015
kg depending on whether surface pressure or water vapor
data are used; somewhat smaller than the previous
estimate. The mean mass of water vapor is estimated as
1.27×1016 kg and the dry air mass as 5.1352
±0.0003×1018 kg."
Evolution on Earth
Atmospheric gases scatter
blue light more than other wavelengths,
giving the Earth a blue halo when seen from
space.
The history of the Earth's atmosphere
prior to one billion years ago is poorly understood and
an active area of scientific research. The following
discussion presents a plausible scenario.
The modern atmosphere is sometimes
referred to as Earth's "third atmosphere", in order to
distinguish the current chemical composition from two
notably different previous compositions. The original
atmosphere was primarily helium and hydrogen. Heat from
the still-molten crust, and the sun, plus a probably
enhanced solar wind, dissipated this atmosphere.
About 4.4 billion years ago, the
surface had cooled enough to form a crust, still heavily
populated with volcanoes which released steam, carbon
dioxide, and ammonia. This led to the early "second
atmosphere", which was primarily carbon dioxide and
water vapor, with some nitrogen but virtually no oxygen.
This second atmosphere had approximately 100 times as
much gas as the current atmosphere, but as it cooled
much of the carbon dioxide was dissolved in the seas and
precipitated out as carbonates. The later "second
atmosphere" contained largely nitrogen and carbon
dioxide. However, simulations run at the University of
Waterloo and University of Colorado in 2005 suggest that
it may have had up to 40% hydrogen. It is generally
believed that the
greenhouse effect, caused by high levels of carbon
dioxide and methane, kept the Earth from freezing.
One of the earliest types of bacteria
were the
cyanobacteria. Fossil evidence indicates that
bacteria shaped like these existed approximately 3.3
billion years ago and were the first oxygen-producing
evolving phototropic organisms. They were responsible
for the initial conversion of the earth's atmosphere
from an anoxic state to an oxic state (that is, from a
state without oxygen to a state with oxygen) during the
period 2.7 to 2.2 billion years ago. Being the first to
carry out oxygenic photosynthesis, they were able to
produce oxygen while sequestering carbon dioxide in
organic molecules, playing a major role in oxygenating
the atmosphere.
Photosynthesising plants would later
evolve and continue releasing oxygen and sequestering
carbon dioxide. Over time, excess carbon became locked
in fossil fuels, sedimentary rocks (notably limestone),
and animal shells. As oxygen was released, it reacted
with ammonia to release nitrogen; in addition, bacteria
would also convert ammonia into nitrogen. But most of
the nitrogen currently present in the atmosphere results
from sunlight-powered photolysis of ammonia released
steadily over the aeons from volcanoes.
As more plants appeared, the levels of
oxygen increased significantly, while carbon dioxide
levels dropped. At first the oxygen combined with
various elements (such as iron), but eventually oxygen
accumulated in the atmosphere, resulting in mass
extinctions and further evolution. With the appearance
of an ozone layer (ozone is an
allotrope of oxygen) lifeforms were better protected
from ultraviolet radiation. This oxygen-nitrogen
atmosphere is the "third atmosphere". 200 – 250 million
years ago, up to 35 percent of the atmosphere was oxygen
(bubbles of ancient atmosphere were found in an amber).
This modern atmosphere has a
composition which is enforced by oceanic blue-green
algae as well as geological processes. O2
does not remain naturally free in an atmosphere, but
tends to be consumed (by inorganic chemical reactions,
as well as by animals, bacteria, and even land plants at
night), while CO2 tends to be produced by
respiration and decomposition and oxidation of organic
matter. Oxygen would vanish within a few million years
due to chemical reactions and CO2 dissolves
easily in water and would be gone in millennia if not
replaced. Both are maintained by biological productivity
and geological forces seemingly working hand-in-hand to
maintain reasonably steady levels over millions of years
(see
Gaia theory).
Air
pollution
Before
desulfurization filters were installed,
the emissions from this power plant in New
Mexico contained excessive amounts of sulfur
dioxide.
Air pollution
is a chemical, physical (e.g. particulate matter) or
biological agent that modifies the natural
characteristics of the atmosphere in an unwanted way.
Stratospheric ozone depletion due to air pollution
(chiefly from chlorofluorocarbons) has long been
recognized as a threat to human health as well as to the
earth's ecosystems.
Worldwide air pollution is responsible
for large numbers of deaths and cases of respiratory
disease. Enforced air quality standards, like the
Clean Air Act in the United States, have reduced the
presence of some pollutants. While major stationary
sources are often identified with air pollution, the
greatest source of emissions are actually mobile
sources, principally the automobile. Gases such as
carbon dioxide, methane, and fluorocarbons contribute to
global warming, and these gases, or excess amounts
of some of emitted from fossil fuel burning, have
recently been identified by the United States and many
other countries (see
Kyoto accord), as pollutants.
Another depiction of the
layers of the atmosphere.
![Layers of Atmosphere—not to scale (NOAA) [1]](http://en.wikipedia.org/wiki/Image:Atmosphere_layers.svg)
