Atmospheric window

The atmospheric window refers to those parts of the electromagnetic spectrum that are, with the earth's atmosphere in its natural state, not absorbed at all. The atmospheric window lies approximately at wavelengths of infrared radiation between 8 and 14 micrometres.

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Causes

The absorptions of the principal natural greenhouse gases are concentrated in two belts. Gases such as CO₂ and CH₄ (along with less abundant hydrocarbons) absorb at wavelengths longer than this window due to the presence of relatively long C-H and carbonyl bonds. The bonds of H₂O and NH₃ absorb at wavelengths shorter than 8 micrometres. Except for the bonds in O₃, no bonds between carbon, hydrogen, oxygen and nitrogen atoms absorb between these two ranges. This means that most of the radiation emitted by the earth's surface at wavelengths within in the atmospheric window pass through the Earth's atmosphere without heating it; instead, this radiation is emitted into space. Without this 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 a window in the electromagnetic spectrum is critical to Earth remaining a habitable planet.

Threats

In recent decades, the existence of the atmospheric window has become threatened by the development of highly unreactive gases containing bonds between fluorine and either carbon or sulfur. 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. This absorption is strengthened because these bonds are highly polar because of the extreme electronegativity of the fluorine atom. Bonds to other halogens 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 atmosphere. It is estimated, for instance, that perfluorocarbons (CF₄, C₂F₆, C₃F₈) can stay in the atmosphere for over fifty thousand years, a figure which may be an underestimate given the absence of natural sources of these gases.

This means that such compounds have an enormous global warming potential. 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 (CCl₄) has a global warming potential of 1800 compared to carbon dioxide.

Efforts to find substitutes for these compounds are still going on and remain highly problematic.

See also

• Astronomical window
• Greenhouse effect
• Greenhouse gas
• Optical window
• Ozone depletion
• Radio window