The infrared atmospheric window refers to a region of the
Infrared spectrum where there is relatively little absorption of terrestrial thermal radiation by atmospheric gases.
The window plays an important role in the atmospheric greenhouse effect by maintaining the balance between incoming solar radiation and outgoing IR to space. In the
Earth's atmosphere this window is roughly the region between 8 and 14 μm although it can be narrowed or closed at times and places of high humidity because of the strong absorption in the water vapor continuum or because of blocking by clouds.
It covers a substantial part of the spectrum from surface thermal emission which starts at roughly 5
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
. Principally it is a large gap in the
absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which s ...
spectrum of water vapor.
Carbon dioxide plays an important role in setting the boundary at the long wavelength end.
Ozone partly blocks transmission in the middle of the window.
The importance of the infrared atmospheric window in the atmospheric
energy balance was discovered by
George Simpson in 1928, based on G. Hettner's 1918
laboratory studies of the gap in the absorption spectrum of water vapor. In those days, computers were not available, and Simpson notes that he used approximations; he writes about the need for this in order to calculate outgoing IR radiation: "There is no hope of getting an exact solution; but by making suitable simplifying assumptions . . . ."
[ ] Nowadays, accurate line-by-line computations are possible, and careful studies of the
spectroscopy of infrared atmospheric gases have been published.
Mechanisms in the infrared atmospheric window
The principal natural
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
es in order of their importance are water vapor , carbon dioxide , ozone , methane and nitrous oxide . The concentration of the least common of these, , is about 400 ppbV.
Other gases which contribute to the greenhouse effect are present at pptV levels. These include the chlorofluorocarbons (CFCs) and hydrofluororcarbons (HFC and HCFCs). As discussed below, a major reason that they are so effective as greenhouse gases is that they have strong vibrational bands that fall in the infrared atmospheric window. IR absorption by at 14.7
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
sets the long wavelength limit of the infrared atmospheric window together with absorption by rotational transitions of at slightly longer wavelengths. The short wavelength boundary of the atmospheric IR window is set by absorption in the lowest frequency vibrational bands of water vapor. There is a strong band of ozone at 9.6
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
in the middle of the window which is why it acts as such a strong greenhouse gas. Water vapor has a continuum absorption due to collisional broadening of absorption lines which extends through the window.
Local very high humidity can completely block the infrared vibrational window.
Over the
Atlas Mountains
The Atlas Mountains are a mountain range in the Maghreb in North Africa. It separates the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean; the name "Atlantic" is derived from the mountain range. It stretches around through Moroc ...
, interferometrically recorded spectra of outgoing longwave radiation
show emission that has arisen from the land surface at a temperature of about 320 K and passed through the atmospheric window, and non-window emission that has arisen mainly from the troposphere at temperatures about 260 K.
Over
Côte d'Ivoire
Ivory Coast, also known as Côte d'Ivoire, officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital is Yamoussoukro, in the centre of the country, while its largest city and economic centre is ...
, interferometrically recorded spectra of outgoing longwave radiation
show emission that has arisen from the cloud tops at a temperature of about 265 K and passed through the atmospheric window, and non-window emission that has arisen mainly from the troposphere at temperatures about 240 K. This means that, at the scarcely absorbed continuum of wavelengths (8 to 14 μm), the radiation emitted, by the Earth's surface into a dry atmosphere, and by the cloud tops, mostly passes unabsorbed through the atmosphere, and is emitted directly to space; there is also partial window transmission in far infrared spectral lines between about 16 and 28 μm. Clouds are excellent emitters of infrared radiation. Window radiation from cloud tops arises at altitudes where the air temperature is low, but as seen from those altitudes, the water vapor content of the air above is much lower than that of the air at the land-sea surface. Moreover,
the water vapour continuum absorptivity, molecule for molecule, decreases with pressure decrease. Thus water vapour above the clouds, besides being less concentrated, is also less absorptive than water vapour at lower altitudes. Consequently, the effective window as seen from the cloud-top altitudes is more open, with the result that the cloud tops are effectively strong sources of window radiation; that is to say, in effect the clouds obstruct the window only to a small degree (see another opinion about this, proposed by Ahrens (2009) on page 43
).
Importance for life
Without the infrared atmospheric window, the Earth would become much too warm to support life, and possibly so warm that it would lose its water, as
Venus did early in
Solar System history. Thus, the existence of an atmospheric window is critical to Earth remaining a
habitable planet
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from a ...
.
As a proposed management strategy for global warming,
passive daytime radiative cooling
Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zero ...
(PDRC) surfaces use the infrared window to send heat back into outer space with the aim of reversing rising temperature increases caused by
climate change.
Threats
In recent decades, the existence of the infrared atmospheric window has become threatened by the development of highly unreactive gases containing bonds between
fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
and
carbon,
sulfur or
nitrogen. The impact of these compounds was first discovered by Indian–American atmospheric scientist
Veerabhadran Ramanathan
Veerabhadran "Ram" Ramanathan (born 24 November 1944) is Edward A. Frieman Endowed Presidential Chair in Climate Sustainability Scripps Institution of Oceanography, University of California, San Diego. He has contributed to many areas of the a ...
in 1975, one year after
Roland and
Molina's much-more-celebrated paper on the ability of chlorofluorocarbons to destroy
stratospheric ozone
The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relat ...
.
The "stretching frequencies" of bonds between fluorine and other light
nonmetal
In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differentl ...
s are such that strong absorption in the atmospheric window will always be characteristic of compounds containing such bonds,
[Bera, Partha P.; Francisco, Joseph S. and Lee, Timothy J.; 'Identifying the Molecular Origin of Global Warming'; ''Journal of Physical Chemistry''; 113 (2009), pp. 12694-12699] although fluorides of nonmetals other than carbon, nitrogen or sulfur are short-lived due to
hydrolysis. This absorption is strengthened because these bonds are highly polar due to the extreme
electronegativity of the fluorine atom. Bonds to other
halogens
The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
also absorb in the atmospheric window, though much less strongly.
Moreover, the unreactive nature of such compounds that makes them so valuable for many industrial purposes means that they are not removable in the natural circulation of the Earth's lower atmosphere. Extremely small natural sources created by means of
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
oxidation of
fluorite and subsequent reaction with sulfate or carbonate minerals produce via
degassing
Degassing, also known as degasification, is the removal of dissolved gases from liquids, especially water or aqueous solutions. There are numerous methods for removing gases from liquids.
Gases are removed for various reasons. Chemists remove gas ...
atmospheric concentrations of about 40
ppt for all perfluorocarbons and 0.01 ppt for sulfur hexafluoride, but the only natural ceiling is via photolysis in the
mesosphere
The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
and upper stratosphere. It is estimated that
perfluorocarbons (, , ), originating from commercial production of anesthetics, refrigerants, and polymers
[ ] can stay in the atmosphere for between two thousand six hundred and fifty thousand years.
[Midgeley, P.M. and McCulloch, A.; ''Properties and Applications of Industrial Halocarbons'', in Fabian, Peter and Singh, Onkar N. (editors); ''Reactive Halogen Compounds in the Atmosphere, Volume 4'', p. 134 ]
This means that such compounds have an enormous
global warming potential
Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (). GWP is 1 for . For other gases it depends on the gas and the time ...
. One kilogram of
sulfur hexafluoride
Sulfur hexafluoride or sulphur hexafluoride ( British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non-flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached ...
will, for example, cause as much warming as 23 tonnes of carbon dioxide over 100 years. Perfluorocarbons are similar in this respect, and even
carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemic ...
() has a global warming potential of 1800 compared to carbon dioxide. These compounds still remain highly problematic with an ongoing effort to find substitutes for them.
See also
*
Greenhouse effect
*
Greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
*
Infrared astronomy
*
Optical window
*
Ozone depletion
*
Radio window
References
Books
*
External links
IR Atmospheric Window
{{DEFAULTSORT:Infrared Window
Atmospheric radiation
Satellite meteorology
Electromagnetic spectrum
Climatology
Climate change
Atmosphere