Ruth Marlaire
NASA Ames Research Center, Moffett Field, Calif.

May 3, 2010
NASA Designs Molecular Strategies to Minimize Global Warming
MOFFETT FIELD, Calif. – NASA scientists have developed strategies to minimize the environmental impact of greenhouse gas on global warming.

After analyzing more than a dozen molecules involved in global warming, researchers discovered that several fluorinated compounds tend to be stronger greenhouse gases than compounds containing chlorine and-or hydrogen. Researchers then ranked the greenhouse gases as global warming agents and developed a screening method for industry to produce chemical compounds that are less efficient at absorbing radiated heat in the atmosphere.

“Once we discovered the molecular properties that cause molecules to absorb radiated heat more efficiently, we were able to design strategies that will minimize the global warming contribution of materials,” said Timothy Lee, chief of the Space Science and Astrobiology Division, at NASA’s Ames Research Center, Moffett Field, Calif.

“The paper does not address toxicity, atmospheric lifetime, nor the atmospheric fate of molecules. Rather, we argue that radiative efficiency is another fundamental property that should be considered in addition to these other properties,” said Partha Bera, a post-doctoral fellow at NASA’s Ames Research Center, Moffett Field, Calif..

Lee and Bera are co-authors of “Design Strategies to Minimize the Radiative Efficiency of Global Warming Molecules,” published today by the Proceedings of the National Academy of Sciences.

Some of the most potent and common anthropogenic greenhouse gases are hydrofluorocarbons (HFC), perfluorocarbons (PFC) and chlorofluorocarbons (CFC). When scientists discovered that CFCs contributed to the destruction of the ozone layer, their production and industrial use was discontinued. Today, HFCs and PFCs are still used in various industries. They are used heavily in electronics, air conditioning, appliances, and carpet manufacturing industries.

Because fluorinated compounds are highly efficient as global warming agents, scientists looked at five classes of them: perofluorcarbons, perfluorethers, perfluorinated sulfur/carbon compounds, perfluoroolefins, and perfluorinated nitrogen/carbon compounds. They then categorized the compounds by their ability to absorb radiated heat and trap it in the Earth’s atmosphere (called a molecule’s “radiative efficiency”), and decided which were more harmful. For partially fluorinated compounds used in industry, they examined how placement of the fluorine atoms in the molecule affects their ability to absorb radiated heat.

“Our paper makes specific suggestions for the use and number of fluorine atoms that should be used in molecules. In addition, a conceptual framework is provided to compare a molecule’s radiative efficiency to carbon dioxide,” said Joseph Francisco, a professor of Earth and atmospheric sciences and chemistry at Purdue University, West Lafayette, Indiana, and the third author of the paper.

If chemists need to use a set number of fluorine atoms, more than one carbon species can be used. “It isn’t just about limiting the number of fluorine atoms in a compound. It’s about where they are placed as well. We have let industry chemists know that they should consider which isomers they use in their applications,” explained Lee.

Researchers also looked at the length of carbon chains in each class, wanting to know if there is an optimum chain length. Or does it matter? “In some applications, it may be better to have a longer carbon chain,” said Lee.

“We showed that the longer the carbon chain, the greater the absorption in the infrared window,” said Lee. “As you increase the length of the carbon chain, the molecule absorbs radiated heat more efficiently, but the absorption per bond decreases.”

Screening materials for their radiative efficiency is one additional constraint an industrialist chemist may use in designing molecules. The first constraint, of course, is that the molecule work for whatever purpose they have. A proposal cannot be made for a molecule for a particular application if it doesn’t work. So efficacy is one constraint, another is atmospheric lifetime, according to these researchers.

“We have laid out the strategy for industrial chemists to use a molecule’s radiative efficiency as an additional constraint in their optimization process,” Lee concluded.

Researchers found that molecules have unique vibrational patterns, which are determined by the bonds between atoms. Bonds can stretch or contract, and can be thought of as bouncing springs.
Animation Credit: NASA / Partha Bera

For additional information, please read:
Bera, Partha P., Joseph S. Francisco and Timothy J. Lee. “Identifying the Molecular Origin of Global Warming,” J. Phys. Chem. A, 2009, 113, 12694 – 12699.

Bera, Partha P., Joseph S. Francisco and Timothy J. Lee. “Design Strategies to Minimize the Radiative Efficiency of Global Warming Molecules,” Proceedings of the Naional Academy of Sciences, 2010.

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