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 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 due to 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.
Read more about this topic: Infrared Window
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