Atmospheric science goes to ground: Researchers present new findings on the natural hydrogen cycle
New evidence suggest earths soil may be as important as its atmosphere
New evidence is emerging on the probable effects of an anticipated reliance on hydrogen as a fuel: surprisingly, we may need to look down in the ground rather up in the air, for answers.
In the August 21 issue of the journal Nature, a group of researchers from the California Institute of Technology and other institutions reports results of a study of the atmospheric chemical reactions that produce and destroy hydrogen in the stratosphere. Funded in part by the National Science Foundation (NSF), the study concludes that most of the hydrogen eliminated from the atmosphere goes into the ground, and therefore that scientists will need to turn their focus toward developing an understanding of soil destruction of hydrogen to accurately predict whether hydrogen emissions will eventually accumulate in the air.
The researchers reached this conclusion after carefully measuring the abundance of a rare isotope of hydrogen known as deuterium. It has long been known that atmospheric hydrogen is rich in deuterium, but it was unclear why. The only reasonable explanation, scientists believed, is that atmospheric hydrogen is mostly destroyed by chemical reactions in the air, and that those reactions are relatively slow for deuterium-rich hydrogen, so it accumulates like salt in an evaporating pan of water.
If correct, this would mean that oxidizing atmospheric trace gases control the natural hydrogen cycle and that soils are relatively unimportant. But new research results suggest that one of the main natural sources of atmospheric hydrogen--the breakdown of methane--is actually responsible for the atmospheres enrichment in deuterium. This result implies that reactions with atmospheric oxidants may be less important to the hydrogen cycle, and that uptake by soils, where microbial processes involve methane, is the driving force.
Hydrogen is a highly reactive element, but answers to the questions of when and where it reacts, and under what circumstances, are difficult to unravel. These questions are simplified in the stratosphere, where its easier to single out and understand specific reactions. According to John Eiler, a geochemist at the California Institute of Technology and an author of the Nature paper, the new data were gathered from air samples taken from the stratosphere with one of the high-flying ER-2 planes operated by the NASA Dryden Flight Research Center in the Mojave Desert.
"We wanted to look at hydrogen in the stratosphere because its easy to study the production of hydrogen from methane separate from other influences," Eiler explains. "It may seem odd to go to the stratosphere to understand whats happening in the ground, but this was the best way to get a global perspective on the importance of soils to the hydrogen cycle."
With precise information on the deuterium content of hydrogen formed from methane, the researchers were able to calculate that the soil uptake of hydrogen is as high as 80 percent. It is suspected that this hydrogen is used by soil-living microbes to carry on their biological functions, although the details of this process are poorly understood and have been the subject of only a few previous studies.
It seems likely, according to the scientists, that the hydrogen taken up by soils is relatively free of environmental consequences, but the question still remains of how much more hydrogen the soil can "consume." If future use of hydrogen in transportation results in a significant amount of leakage, then soil uptake must increase dramatically or it will be inadequate to cleanse the released hydrogen from the atmosphere, Eiler says.
"An analogy would be the discovery that trees and other plants get rid of some of the carbon dioxide that cars emit, but by no means all of it," he says. "So the question as we look toward a future hydrogen economy is whether [soil] microbes will be able to eat the hydrogen fast enough."
Bruce Doddridge, program director in NSFs division of atmospheric sciences, which co-funded the research, said, "This carefully conducted research investigating the natural chemistry of sources and sinks affecting the abundance of hydrogen in the atmosphere results in the most accurate information to date, and appears to account for the atmospheric deuterium excess previously observed.
"A more accurate molecular hydrogen budget may have important implications as global fuel technology shifts its focus from fossil fuels to other sources," Doddridge added.
The lead author of the paper is Thom Rahn, a former post-doctoral fellow of Eilers who is now affiliated with Los Alamos National Laboratory. The other authors are Paul Wennberg, an atmospheric chemist at Caltech; Kristie A. Boering and Michael McCarthy, both of UC Berkeley; Stanley Tyler of UC Irvine; and Sue Schauffler of the National Center for Atmospheric Research in Boulder, Colorado.
In addition to NSF, an independent federal agency that supports fundamental research in science and engineering, other supporters of the research were the Davidow Fund and General Motors Corp., the David and Lucile Packard Foundation, the NASA Upper Atmosphere Research Program, and the National Center for Atmospheric Research.
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Cheryl Dybas | NSF