The new study looked at a broader class of chemicals to identify molecular-level features that make them more or less efficient at trapping radiation in the atmospheric window. The study employed results from atomic-scale quantum chemistry calculations done on computers from NASA and Information Technology at Purdue (ITaP), Purdue's central information technology organization.
"We specifically looked at molecules that we felt would have potential for industrial use as replacements for chlorofluorocarbons," says Francisco, whose research focuses on the chemistry of molecules in the atmosphere.
Among other things, the study looked at how the number and placement of electronegative atoms in a molecule's structure affects its radiative efficiency. The number and placement of fluorine atoms proved to be a key factor because they're very electronegative and form highly polar bonds with carbon and sulfur.
Fluorine atoms thus tend to change the bond-polarity of the molecules -- modifying the bonds holding the atoms in the structure. This, in turn, affects how a molecule will absorb infrared radiation that normally passes through Earth's atmosphere and into space.
"The polarity change is what makes for an efficient absorber of infrared radiation," says Lee, chief of the Space Science and Astrobiology Division at NASA Ames Research Center.
One message from the study: Avoid allowing fluorines to bunch up in a molecular structure. "In other words, don't put them all on one atom," Francisco says. "Spread them out."
The fluorinated compounds also persist longer in the atmosphere than carbon dioxide and other major global warming agents, Lee and Francisco note. Even if emitted in lower quantities, fluorine-containing chemicals might have a powerful cumulative effect. Some don't break down for thousands of ye
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