NASA Traces Molecular Characteristics That Heat Earth
Every molecule has a unique vibrational pattern, which is determined by the bond between atoms. These vibrating bonds are where heat is absorbed and retained. Animation Credit: NASA / Partha Bera
Not all greenhouse gases are created equal. Some trace gases are much more efficient global warming agents than the abundant carbon dioxide, which is why scientists are studying the physical characteristics that cause heat absorption to vary among different types of molecules.
Carbon dioxide receives a lot of attention because it is abundant and its concentration is increasing in the atmosphere. Methane is a trace gas that absorbs heat about 200 times more efficiently than carbon dioxide and has a relatively short lifetime of 12 years. Other trace gases contain fluorine atoms, such as hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), and perfluorocarbons. These compounds are many times more efficient as heat absorbers than methane, and can last in the atmosphere thousands of years, which makes monitoring their accumulation in the atmosphere critical to understanding Earth’s climate.
"To protect Earth’s climate, we need to understand the molecular properties that determine heat absorption, and design molecules that are more environmentally friendly," said Timothy Lee, an astrochemist at NASA’s Ames Research Center, Moffett Field, Calif. "Our paper is the first to focus on the inherent capabilities of molecules to absorb Earth’s radiated heat."
"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. and first author of both research papers.
Lee and Bera are co-authors of "Identifying the Molecular Origin of Global Warming," published last year in the Journal of Physical Chemistry A 2009, vol. 113, and is the first of two research papers about industry’s use of global warming agents. This week, the second paper, titled “Design Strategies to Minimize the Radiative Efficiency of Global Warming Molecules,” was published in the Proceedings of the National Academy of Sciences.
The first paper identifies the physical properties that cause molecules to be efficient heat absorbers. The second paper suggests design strategies to create benign alternatives and test new chemicals before they are produced and used by industry.
"The paper is important because it suggests that a researcher can get a first estimate of the radiative property of a molecule just by knowing the combination of atoms in the molecule," said Joseph Francisco, a professor of Earth and atmospheric sciences and chemistry at Purdue University, West Lafayette, Indiana, and co-author of both papers.
In the past, studies have shown that using CFCs and HCFCs lead to an ozone depletion problem in the stratosphere. To remedy this problem, industry has tried to replace chlorine with fluorine, and started manufacturing fluorinated compounds. These compounds are heavily used in the electronics, air conditioning, appliances, and carpet manufacturing industries. Industry uses them because they are very stable compounds and less harmful to humans. It also means they can last a very long time in the atmosphere.
“When hydrogen atoms are replaced with fluorine atoms in molecules, they have a greater heating capacity. They become more efficient at trapping heat. Scientists already knew that compounds with fluorine atoms are more efficient global warming agents. But no one looked at their molecular structure,” said Lee.
Scientists say that as Earth absorbs the sun’s radiation, it heats and starts to radiate its own heat. This rising thermal energy is absorbed by particles in the atmosphere, exciting them at the atomic level. As trace gases in the atmosphere absorb this thermal energy, they become excited, causing them to vibrate in definite patterns.
"Every molecule has a unique vibrational pattern, which is determined by the bond between atoms. These vibrating bonds are where heat is absorbed and retained," explained Bera.
Greenhouse gases, and fluorine-containing molecules in particular, become sinks, trapping and carrying extraordinary amounts of heat. When this happens, highly intense vibrating fluorocarbons migrate toward outlets in the atmosphere, called “infrared windows.” These infrared windows are where heat is released into space, helping our planet maintain a balanced temperature, explained Bera.
"The infrared windows can be compared to glass in a car window," said Bera. "Sunlight on the window can transfer heat into an enclosed car, but no gases are exchanged. Similarly, the infrared windows can release heat into space, but the gases are retained in the atmosphere."
Most of Earth’s atmosphere looks opaque, except for its infrared window, which look transparent. After these large, high-intensity fluorinated molecules move to the infrared window, they still carry large amounts of heat, which eventually effects the transparency of the window, turning it opaque and eventually closing the window, according to Lee.
The study recommends that fluorinated compounds be monitored because, as they continue to accumulate in these atmospheric windows, the windows’ opacity starts to block the transmission of heat into space. Over time, the windows close and create a greenhouse effect on Earth.
"Unlike high concentrations of carbon dioxide in the atmosphere that can squeeze the infrared window, these fluorinated molecules have the potential to close the infrared window," concluded Lee.