Water absorption

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Solar spectrum above the atmosphere (blue curve) and transmitted to the Earth's surface (orange curve) showing major absorption bands.
Solar spectrum above the atmosphere (blue curve) and transmitted to the Earth's surface (orange curve) showing major absorption bands.

Water absorption is a phenomenon in the transmission of electromagnetic radiation through a medium containing water molecules. Water molecules are excited by radiation at certain wavelengths and tend to selectively absorb portions of the spectrum while allowing the balance of the spectrum to be transmitted with minimal effect.

Strong water vapor absorption bands occur at wavelengths around 2500, 1950 and 1450 nanometers (nm),[1][2] with weaker absorption around 1200 and 970 nm,[3] and three additional sets of water-vapor absorption lines near 930, 820, and 730 nm,[4] all in the infrared spectrum. Water has a complex absorption spectrum — the 2007 HITRAN spectroscopy database update lists more than 64,000 spectral lines corresponding to significant transitions of water vapor ranging from the microwave region to the visible spectrum.[5]

The liquid water absorption features are offset to longer wavelengths from the water vapor absorption features by about 60 nm.[6] In hexagonal ice, the features are shifted even further. In liquid water and ice the infrared and Raman spectra are far more complex than in the vapor.[7]

Contents

[edit] Atmospheric effects

Water vapor is a greenhouse gas in the Earth's atmosphere, responsible for 70% of the known absorption of incoming sunlight, particularly in the infrared region, and about 60% of the atmospheric absorption of thermal radiation by the Earth known as the greenhouse effect.[8] It is also an important factor in multispectral imaging and hyperspectral imaging used in remote sensing[5] because water vapor absorbs radiation differently in different spectral bands. Its effects are also an important consideration in infrared astronomy and radio astronomy in the microwave or millimeter wave bands. The South Pole Telescope was constructed in Antarctica in part because the elevation and low temperatures there mean there is very little water vapor in the atmosphere.[9]

Similarly, carbon dioxide absorption bands occur around 1400, 1600 and 2000 nm,[10] but its presence in the Earth's atmosphere accounts for just 26% of the greenhouse effect.[8] Carbon dioxide gas absorbs energy in some small segments of the thermal infrared spectrum that water vapor misses. This extra absorption within the atmosphere causes the air to warm just a bit more and the warmer the atmosphere the greater its capacity to hold more water vapor. This extra water vapor absorption then further enhances the Earth’s greenhouse effect.[11]

Conversely, there is an atmospheric window between approximately 800 and 1400 nm, in the near-infrared spectrum where carbon dioxide and water absorption is weak.[12] This window allows most of the thermal radiation in this band to be radiated out to space, keeping the Earth's atmosphere from going into thermal runaway. This band is also used for remote sensing of the Earth from space, for example with VNIR imaging.

[edit] Technical explanation

The water vapor absorption bands are related to molecular vibrations involving different combinations of the water molecule's three fundamental vibrational transitions:

  • V1: symmetric stretch mode
  • V2: bending mode
  • V3: asymmetric stretch mode

The absorption feature centered near 970 nm is attributed to a 2V1 + V3 combination, the one near 1200 nm to a V1 + V2 + V3 combination, the one near 1450 nm to a V1 + V3 combination, and the one near 1950 nm to a V2 + V3 combination.[3]

In liquid water, rotations tend to be restricted by hydrogen bonds, leading to librations, or rocking motions. Also stretching is shifted to a lower frequency while the bending frequency increased by hydrogen bonding.[7]

Three fundamental vibrations of the water molecule
Symmetrical
stretching
(V1)		 Bending or
Scissoring
(V2)		 Antisymmetrical
stretching
(V3)

[edit] See also

[edit] References

  1. ^ Carter, G.A.; McCain, D.C. (1993). "Relationship of leaf spectral reflectance to chloroplast water content determined using NMR microscopy". Remote Sensing of Environment 46 (3): 305–310. doi:10.1016/0034-4257(93)90050-8. “Reflectance responses to leaf water content were greatest in the water absorption bands near 1450 nm, 1950 nm, and 2500 nm wavelengths” 
  2. ^ Rossel, R.A.V.; McBratney, A.B. (1998). "Laboratory evaluation of a proximal sensing technique for simultaneous measurement of soil clay and water content". Geoderma 85 (1): 19–39. doi:10.1016/S0016-7061(98)00023-8. “the strong absorption bands of OH groups in soil water at around 1450, 1950 and 2500 nm.” 
  3. ^ a b Jacquemoud, S.; Ustin, S.L. (2003). "Application of radiative transfer models to moisture content estimation and burned land mapping" ([dead link] – Scholar search). Joint European Association of Remote Sensing Laboratories (EARSeL) and GOFC/GOLD-Fire Program, 4th Workshop on Forest Fires, University Ghent, Belgium 5--7 June 2003. “...in the action spectrum of water the three main peaks near 1400, 1950, and 2500 nm, and two minor ones at 970 and 1200 nm,” 
  4. ^ Duarte, Edited (1995). Tunable Laser Applications. New York: M. Dekker. ISBN 0824789288. “There are three sets of water-vapor absorption lines in the near-IR spectral region. Those near 730 and 820 nm are useful for lower tropo- spheric measurements, whereas those near 930 nm are useful for upper- tropospheric measurements...” 
  5. ^ a b Gordon, Iouli E.; Laurence S. Rothman, Robert R. Gamache, David Jacquemart, Chris Boone, Peter F. Bernathd, Mark W. Shephard, Jennifer S. Delamere, Shepard A. Clough (2007-06-24). Current updates of the water-vapor line list in HITRAN: A new ‘‘Diet’’ for air-broadened half-widths (pdf). Journal of Quantitative Spectroscopy & Radiative Transfer. Retrieved on 2007-11-03. “Water vapor is the principal absorber of longwave radiation in the terrestrial atmosphere and it has a profound effect on the atmospheric energy budget in many spectral regions. The HITRAN database lists more than 64,000 significant transitions of water vapor ranging from the microwave region to the visible, with intensities that cover many orders of magnitude. These transitions are used, or have to be accounted for, in various remote-sensing applications.”
  6. ^ Toselli, F. (1992). Imaging Spectroscopy. Boston: Kluwer Academic Publishers. ISBN 0792315359. “The liquid water absorption features are offset to longer wavelengths from the water vapor absorption features by about 60 nm.” 
  7. ^ a b Chaplin, Martin (2007-10-28). Water Absorption Spectrum. Retrieved on 2007-11-04. “In the liquid, rotations tend to be restricted by hydrogen bonds, giving the librations. Also, spectral lines are broader causing overlap of many of the absorption peaks. The main stretching band in liquid water is shifted to a lower frequency and the bending frequency increased by hydrogen bonding.”
  8. ^ a b Maurellis, Ahilleas (2003-05-01). The climatic effects of water vapour - physicsworld.com. Physics World. Institute of Physics. Retrieved on 2007-11-03.
  9. ^ South Pole Telescope: South Pole : Why is the telescope at the South Pole?. University of Chicago. Retrieved on 2007-11-03. “Quick Answer: Because the South Pole is probably the best place on Earth for this telescope. It is extremely dry, making the atmosphere exceptionally transparent for SPT.”
  10. ^ Prieto-Blanco, Ana; Peter R. J. North , Nigel Fox , Michael J. Barnsley. Satellite estimation of surface/atmosphere parameters: a sensitivity study (pdf). Retrieved on 2007-10-31. “...water absorption bands (around 940nm, 1100nm, 1450nm, 1950nm and 2500nm) and carbon dioxide absorption bands (1400nm, 1600nm and 2000nm)...”
  11. ^ EO Study: Does the Earth have an Iris Analog. NASA. Retrieved on 2007-11-04.
  12. ^ Cotton, William (2006). Human Impacts on Weather and Climate. Cambridge: Cambridge University Press. ISBN 0521840864. “Little absorption is evident in the region called the atmospheric window between 8 and 14 μm”