New Device Sniffs Out Origins of Harmful Methane
In the fight against climate change, most experts focus on controlling emissions of carbon dioxide, but methane is actually the more potent greenhouse gas, even more effective at trapping heat in Earth's atmosphere. Now, thanks to a new device, scientists are sniffing out the origins of harmful methane, helping them to better understand its role in warming the planet.
Now, researchers from MIT, along with teams from the Woods Hole Oceanographic Institution and the University of Toronto, have developed an instrument that can rapidly and precisely analyze samples of environmental methane to determine how the gas was formed. The new approach, called tunable infrared laser direct absorption spectroscopy, relies on the ratio of methane isotopes. This "fingerprint" allows scientists to differentiate between two common sources of methane: microbial and thermogenic.
Microbial sources refer to microorganisms that typically live in wetlands or the guts of animals, like cows, which produce methane as a metabolic byproduct. Thermogenic origins, on the other hand, are when organic matter buried deep within the Earth decays to methane at extremely high temperatures.
The team collected samples of methane from settings such as lakes, swamps, natural gas reservoirs, the digestive tracts of cows, and deep ancient groundwater, as well as methane made by microbes in the lab.
"We are interested in the question, 'Where does methane come from?'" Shuhei Ono, an assistant professor of geochemistry in MIT's Department of Earth, Atmospheric and Planetary Sciences, said in a statement. "If we can partition how much is from cows, natural gas, and other sources, we can more reliably strategize what to do about global warming."
The results were published in the journal Science.
Methane is a molecule composed of one carbon atom linked to four hydrogen atoms. Carbon can come as one of two isotopes (carbon-12 or carbon-13), as can hydrogen, including a form called deuterium.
Specifically, the team focused on 13CH3D - a molecule with both an atom of carbon-13 and a deuterium atom. This isotope is crucial for finding the origin of methane, thermogenic or microbial, as it may be a signal of the temperature at which methane formed.
Using the new infrared spectroscopy technique, Ono and his colleagues built an instrument that can detect 13CH3D by identifying different frequencies that correspond to different isotopes.
What the team ended up finding was surprising. Based on the isotope ratios they detected in cow rumen, they calculated that this methane formed at 400 degrees Celsius (752 degrees Fahrenheit). That's impossible, considering cow stomachs are typically at about only 40 C (104 F). The same anomaly occurred when they tested methane samples from lakes and swamps.
So what does this mean? It turns out that this inconsistency can be explained by a previously unknown relationship between a feature of the bonds linking carbon and hydrogen in methane molecules, and the rate at which methane was produced. They refer to it as the "clumpiness" factor, meaning the clumpier the bond, the slower the rate of methanogenesis - the formation of methane by microbes.
"Cow guts produce methane at very high rates - up to 500 liters a day per cow. They're giant methane fermenters, and they prefer to make less-clumped methane, compared to geologic processes, which happen very slowly," researcher David Wang explained. "We're measuring a degree of clumpiness of the carbon and hydrogen isotopes that helps us get an idea of how fast the methane formed."
With greenhouse gas levels hitting a record high in 2013, and this past summer clocking in as the hottest the Earth has experienced in more than 130 years, controlling emissions of greenhouse gases like methane and determining where they come from is critical in combating climate change.
The researchers hope that with their unique approach, they can better understand how methane forms in environments "on the Earth and beyond."
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