Infrared window

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As the main part of the 'window' spectrum, a clear electromagnetic spectral transmission 'window' can be seen between 8 and 14 mm. A fragmented part of the 'window' spectrum (one might say a louvred part of the 'window') can also be seen in the visible to mid-wavelength infrared between 0.2 and 5.5 mm. Atmosfaerisk spredning.png
As the main part of the 'window' spectrum, a clear electromagnetic spectral transmission 'window' can be seen between 8 and 14 μm. A fragmented part of the 'window' spectrum (one might say a louvred part of the 'window') can also be seen in the visible to mid-wavelength infrared between 0.2 and 5.5 μm.

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. [1] 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. [2] [3] [4] [5] [6] 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 plays an important role in setting the boundary at the long wavelength end. Ozone partly blocks transmission in the middle of the window.

Contents

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 [7] 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 ... ." [8] Nowadays, accurate line-by-line computations are possible, and careful studies of the spectroscopy of infrared atmospheric gases have been published.

Mechanisms

The principal natural greenhouse gases in order of their importance are water vapor H
2O , carbon dioxide CO
2 , ozone O
3 , methane CH
4 and nitrous oxide N
2O . The concentration of the least common of these, N
2O , is about 400 ppb (by volume).[ clarification needed ] [9] Other gases which contribute to the greenhouse effect are present at ppt levels. These include the chlorofluorocarbons (CFCs), halons and hydrofluorocarbons (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 CO
2 at 14.7 μm sets the long wavelength limit of the infrared atmospheric window together with absorption by rotational transitions of H
2O 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. [2] [3] [4] [5] [6] [7] [8] [10] Local very high humidity can completely block the infrared vibrational window.

Over the Atlas Mountains, interferometrically recorded spectra of outgoing longwave radiation [11] 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 [11] 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, [10] 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 [12] ).

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.

As a proposed management strategy for global warming, passive daytime radiative cooling (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. [13] [14]

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, sulfur or nitrogen. The impact of these compounds was first discovered by Indian–American atmospheric scientist Veerabhadran Ramanathan in 1975, [15] one year after Roland and Molina's much-more-celebrated paper on the ability of chlorofluorocarbons to destroy stratospheric ozone.

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, [16] 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 chlorine [16] and bromine [17] 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 oxidation of fluorite and subsequent reaction with sulfate or carbonate minerals produce via degassing atmospheric concentrations of about 40 ppt for all perfluorocarbons and 0.01 ppt for sulfur hexafluoride, [18] but the only natural ceiling is via photolysis in the mesosphere and upper stratosphere. [19] It is estimated that perfluorocarbons ( CF
4 , C
2F
6 , C
3F
8 ), originating from commercial production of anesthetics, refrigerants, and polymers [20] can stay in the atmosphere for between two thousand six hundred and fifty thousand years. [21]

This means that such compounds possess enormous global warming potential. One kilogram of sulfur hexafluoride will, for example, cause as much warming as 26.7 tonnes of carbon dioxide over 100 years, and as much as 37.6 tonnes over 500 years. [22] Perfluorocarbons are similar in this respect, and even carbon tetrachloride (CCl
4) has a global warming potential of 2310 compared to carbon dioxide. [22] Quite short-lived halogenated compounds can still have fairly high global warming potentials: for instance chloroform, with a lifetime of 0.5 years, still has a global warming potential of 22; halothane, with a lifetime of only one year, has a GWP of 47 over 100 years, [22] and Halon 1202, with a lifetime of 2.9 years, has a 100 year global warming potential 231 times that of carbon dioxide. [23] These compounds still remain highly problematic with an ongoing effort to find substitutes for them.

See also

Related Research Articles

<span class="mw-page-title-main">Greenhouse effect</span> Atmospheric phenomenon causing planetary warming

The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source as in the case of Jupiter, or from its host star as in the case of the Earth. In the case of Earth, the Sun emits shortwave radiation (sunlight) that passes through greenhouse gases to heat the Earth's surface. In response, the Earth's surface emits longwave radiation (heat) that is mostly absorbed by greenhouse gases. That heat absorption reduces the rate at which the Earth can cool off in response to being warmed by the Sun. Adding to greenhouse gases further reduces the rate a planet emits radiation to space, raising its average surface temperature.

<span class="mw-page-title-main">Global warming potential</span> Potential heat absorbed by a greenhouse gas

Global Warming Potential (GWP) is an index to measure of how much infrared thermal radiation a greenhouse gas would absorb over a given time frame after it has been added to the atmosphere. The GWP makes different greenhouse gases comparable with regards to their "effectiveness in causing radiative forcing". It is expressed as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide, which is taken as a reference gas. Therefore, the GWP is one for CO2. For other gases it depends on how strongly the gas absorbs infrared thermal radiation, how quickly the gas leaves the atmosphere, and the time frame being considered.

<span class="mw-page-title-main">Satellite temperature measurement</span> Measurements of atmospheric, land surface or sea temperature by satellites.

Satellite temperature measurements are inferences of the temperature of the atmosphere at various altitudes as well as sea and land surface temperatures obtained from radiometric measurements by satellites. These measurements can be used to locate weather fronts, monitor the El Niño-Southern Oscillation, determine the strength of tropical cyclones, study urban heat islands and monitor the global climate. Wildfires, volcanos, and industrial hot spots can also be found via thermal imaging from weather satellites.

<span class="mw-page-title-main">Cloud feedback</span> Type of climate change feedback mechanism

Cloud feedback is a type of climate change feedback that has been difficult to quantify in climate models. Clouds can either amplify or dampen the effects of climate change by influencing Earth's energy balance. This is because clouds can affect the magnitude of climate change resulting from external radiative forcings. On the other hand, clouds can affect the magnitude of internally generated climate variability. Climate models represent clouds in different ways, and small changes in cloud cover in the models have a large impact on the predicted climate. Changes in cloud cover are closely coupled with other feedbacks, including the water vapor feedback and ice–albedo feedback.

<span class="mw-page-title-main">Atmosphere of Earth</span> Gas layer surrounding Earth

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 creates pressure, absorbs most meteoroids and ultraviolet solar radiation, warms the surface through heat retention, and reduces temperature extremes between day and night, maintaining conditions allowing life and liquid water to exist on the Earth's surface.

<span class="mw-page-title-main">Radiative forcing</span> Difference between solar irradiance absorbed by the Earth and energy radiated back to space

Radiative forcing is a concept used in climate science to quantify the change in energy balance in the Earth's atmosphere caused by various factors, such as concentrations of greenhouse gases, aerosols, and changes in solar radiation. In more technical terms, it is "the change in the net, downward minus upward, radiative flux due to a change in an external driver of climate change." These external drivers are distinguished from feedbacks and variability that are internal to the climate system, and that further influence the direction and magnitude of imbalance.

<span class="mw-page-title-main">Hydroxyl radical</span> Neutral form of the hydroxide ion (OH−)

The hydroxyl radical, •HO, is the neutral form of the hydroxide ion (HO–). Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H2O2 (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

<span class="mw-page-title-main">Emissivity</span> Capacity of an object to radiate electromagnetic energy

The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is not visible to human eyes. A portion of the thermal radiation from very hot objects is easily visible to the eye.

<span class="mw-page-title-main">Earth's energy budget</span> Accounting of the energy flows which determine Earths surface temperature and drive its climate

Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but make a tiny contribution compared to solar energy. The energy budget also accounts for how energy moves through the climate system. The Sun heats the equatorial tropics more than the polar regions. Therefore, the amount of solar irradiance received by a certain region is unevenly distributed. As the energy seeks equilibrium across the planet, it drives interactions in Earth's climate system, i.e., Earth's water, ice, atmosphere, rocky crust, and all living things. The result is Earth's climate.

Trace gases are gases that are present in small amounts within an environment such as a planet's atmosphere. Trace gases in Earth's atmosphere are gases other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of its atmosphere.

A runaway greenhouse effect will occur when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A runaway version of the greenhouse effect can be defined by a limit on a planet's outgoing longwave radiation which is asymptotically reached due to higher surface temperatures evaporating water into the atmosphere, increasing its optical depth. This positive feedback means the planet cannot cool down through longwave radiation and continues to heat up until it can radiate outside of the absorption bands of the water vapour.

<span class="mw-page-title-main">Outgoing longwave radiation</span> Energy transfer mechanism which enables planetary cooling

In climate science, longwave radiation (LWR) is electromagnetic thermal radiation emitted by Earth's surface, atmosphere, and clouds. It may also be referred to as terrestrial radiation. This radiation is in the infrared portion of the spectrum, but is distinct from the shortwave (SW) near-infrared radiation found in sunlight.

<span class="mw-page-title-main">Atmospheric infrared sounder</span> Science instrument on NASAs Aqua satellite

The atmospheric infrared sounder (AIRS) is one of six instruments flying on board NASA's Aqua satellite, launched on May 4, 2002. The instrument is designed to support climate research and improve weather forecasting.

<span class="mw-page-title-main">Electromagnetic absorption by water</span>

The absorption of electromagnetic radiation by water depends on the state of the water.

<span class="mw-page-title-main">Atmospheric window</span>

An atmospheric window is a region of the electromagnetic spectrum that can pass through the atmosphere of Earth. The optical, infrared and radio windows comprise the three main atmospheric windows. The windows provide direct channels for Earth's surface to receive electromagnetic energy from the Sun, and for thermal radiation from the surface to leave to space. Atmospheric windows are useful for astronomy, remote sensing, telecommunications and other science and technology applications.

<span class="mw-page-title-main">Idealized greenhouse model</span> Mathematical estimate of planetary temperatures

The temperatures of a planet's surface and atmosphere are governed by a delicate balancing of their energy flows. The idealized greenhouse model is based on the fact that certain gases in the Earth's atmosphere, including carbon dioxide and water vapour, are transparent to the high-frequency solar radiation, but are much more opaque to the lower frequency infrared radiation leaving Earth's surface. Thus heat is easily let in, but is partially trapped by these gases as it tries to leave. Rather than get hotter and hotter, Kirchhoff's law of thermal radiation says that the gases of the atmosphere also have to re-emit the infrared energy that they absorb, and they do so, also at long infrared wavelengths, both upwards into space as well as downwards back towards the Earth's surface. In the long-term, the planet's thermal inertia is surmounted and a new thermal equilibrium is reached when all energy arriving on the planet is leaving again at the same rate. In this steady-state model, the greenhouse gases cause the surface of the planet to be warmer than it would be without them, in order for a balanced amount of heat energy to finally be radiated out into space from the top of the atmosphere.

<span class="mw-page-title-main">Greenhouse gas</span> Gas in an atmosphere that absorbs and emits radiation at thermal infrared wavelengths

Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).

<span class="mw-page-title-main">Atmospheric methane</span> Methane in Earths atmosphere

Atmospheric methane is the methane present in Earth's atmosphere. The concentration of atmospheric methane is increasing due to methane emissions, and is causing climate change. Methane is one of the most potent greenhouse gases. Methane's radiative forcing (RF) of climate is direct, and it is the second largest contributor to human-caused climate forcing in the historical period. Methane is a major source of water vapour in the stratosphere through oxidation; and water vapour adds about 15% to methane's radiative forcing effect. The global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe.

<span class="mw-page-title-main">Water vapor windows</span>

Water vapor windows are wavelengths of infrared light that have little absorption by water vapor in Earth's atmosphere. Because of this weak absorption, these wavelengths are allowed to reach the Earth's surface barring effects from other atmospheric components. This process is highly impacted by greenhouse gases because of the effective emission temperature. The water vapor continuum and greenhouse gases are significantly linked due to water vapor's benefits on climate change.

In the study of heat transfer, Schwarzschild's equation is used to calculate radiative transfer through a medium in local thermodynamic equilibrium that both absorbs and emits radiation.

References

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  8. 1 2 "Archived copy". Archived from the original on 2008-04-22. Retrieved 2009-06-26.{{cite web}}: CS1 maint: archived copy as title (link)Simpson, G.C. (1928). "Further Studies in Terrestrial Radiation". Memoirs of the Royal Meteorological Society. 3 (21): 1–26.
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  13. ↑ Chen, Meijie; Pang, Dan; Chen, Xingyu; Yan, Hongjie; Yang, Yuan (2022). "Passive daytime radiative cooling: Fundamentals, material designs, and applications". EcoMat. 4. doi: 10.1002/eom2.12153 . S2CID   240331557.
  14. ↑ Munday, Jeremy (2019). "Tackling Climate Change through Radiative Cooling". Joule. 3 (9): 2057–2060. doi: 10.1016/j.joule.2019.07.010 . S2CID   201590290.
  15. ↑ Ramanathan, Veerabhadran; 'Greenhouse Effect Due to Chlorofluorocarbons: Climatic Implications'; Science , vol. 190, no. 4209 (October 3, 1975), pp. 50–52
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  17. ↑ Gann, Richard G.; Barnes, John D.; Davis, Sanford; Harris, Jonice S.; Harris Jr., Richard H.; Herron, John T.; Levin, Barbara C.; Mospik, Frederick I.; Notarianni, Kathy A.; Nyden, Marc R.; Paabo, Maya; Ricker, Richard E. (1990). Preliminary screening procedures and criteria for replacements for Halons 1211 and 1301 (Technical report). p. 135.
  18. ↑ Harnisch, Jochen; Eisenhauer, Anton (1998). "Natural CF4 and SF6 on Earth". Geophysical Research Letters. 25 (13): 2401–2404. Bibcode:1998GeoRL..25.2401H. doi: 10.1029/98GL01779 . S2CID   129805049.
  19. ↑ Kovács, Tamás; Feng, Wuhu; Totterdill, Anna; Plane, John M.C.; Dhomse, Sandip; Gómez-Martín, Juan Carlos; Stiller, Gabriele P.; Haenel, Florian J.; Smith, Christopher; Forster, Piers M.; García, Rolando R.; Marsh, Daniel R.; Chipperfield, Martyn P. (January 20, 2017). "Determination of the atmospheric lifetime and global warming potential of sulfur hexafluoride using a three-dimensional model". Atmospheric Chemistry and Physics. 17 (2): 883–898. Bibcode:2017ACP....17..883K. doi: 10.5194/acp-17-883-2017 . S2CID   54586431.
  20. ↑ Lemal DM (January 2004). "Perspective on fluorocarbon chemistry". J. Org. Chem. 69 (1): 1–11. doi:10.1021/jo0302556. PMID   14703372.
  21. ↑ Midgeley, P.M.; McCulloch, A. (9 April 1999). "Properties and Applications of Industrial Halocarbons". In Fabian, Peter; Singh, Onkar N. (eds.). Reactive Halogen Compounds in the Atmosphere, Volume 4. Springer. p. 134. ISBN   3540640908.
  22. 1 2 3 Hodnebrog, Øivind; Åmås, Borgar; Fuglestvedt, Jan; Marston, George; Myhre, Gunnar; Nielsen, Claus Jørgen; Sandstad, Marit; Shine, Keith P.; Wallington, Tim J. (July 9, 2020). "Updated Global Warming Potentials and Radiative Efficiencies of Halocarbons and Other Weak Atmospheric Absorbers". Reviews of Geophysics. 58 (3): e2019RG000691. Bibcode:2020RvGeo..5800691H. doi:10.1029/2019RG000691. PMC   7518032 . PMID   33015672.
  23. ↑ Hodnebrog, Øivind; Etminan, Maryam; Fuglestvedt, Jan; Marston, George; Myhre, Gunnar; Nielsen, Claus Jørgen; Shine, Keith P.; Wallington, Tim J. (April 24, 2013). "Global warming potentials and radiative efficiencies of halocarbons and related compounds: A comprehensive review" (PDF). Reviews of Geophysics (2. 51 (2): 300–378. Bibcode:2013RvGeo..51..300H. doi:10.1002/rog.20013.

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