The infrared atmospheric window refers to a region of the

Infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

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 The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

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. Principally it is a large gap in the absorption spectrum of water vapor.

Carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

plays an important role in setting the boundary at the long wavelength end.

Ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

partly blocks transmission in the middle of the window. The importance of the infrared atmospheric window in the atmospheric

energy balance Energy balance may refer to: * Earth's energy balance, the relationship between incoming solar radiation, outgoing radiation of all types, and global temperature change. * Energy accounting, a system used within industry, where measuring and anal ...

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 gases 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 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 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, 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, 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 Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

did early in

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

history. Thus, the existence of an atmospheric window is critical to Earth remaining a habitable planet. 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 zer ...

(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 In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

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 and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

, sulfur or

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

. 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 nonmetals 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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

. This absorption is strengthened because these bonds are highly polar due to the extreme

electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...

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 i ...

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 consi ...

oxidation of

fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs sca ...

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 ga ...

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 and upper stratosphere. It is estimated that

perfluorocarbon Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...

s (, , ), 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 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 () 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.

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