|( NASA photo)|
Balancing Earth's hydrogen budget
BERKELEY – Discovery of the last piece of a long-standing puzzle - what happens to hydrogen gas in the atmosphere - will help scientists assess the impact of additional hydrogen escaping into the atmosphere if America moves to hydrogen-fueled vehicles.
In a study published in the Aug. 21 issue of Nature, a team of scientists from the University of California, Berkeley, the California Institute of Technology, the National Center for Atmospheric Research in Boulder, Colo., and UC Irvine finally balances the Earth's hydrogen budget, accounting for where the hydrogen comes from and where it goes.
"A balanced budget now means that we may be better able to predict what will happen if and when humans introduce and leak into the atmosphere vast quantities of hydrogen for fuel cells," said Kristie Boering, professor of chemistry and of earth and planetary science at UC Berkeley.
Until now, scientists thought they understood most sources and sinks of hydrogen - where it is produced and how it gets taken up in chemical reactions in the soil, the oceans and the atmosphere. But two distinct methods used to track the hydrogen provided significantly different results.
Scientists measuring hydrogen concentrations found that the major sink for hydrogen was in the soil - microbes metabolize the hydrogen gas (H2) for energy, thus removing hydrogen from the air, where it is one of the most abundant trace gases after methane.
Those tracing the course of hydrogen by looking at the relative amounts of two of its isotopes - standard hydrogen, whose nucleus consists of only one proton, and deuterium, which harbors an extra neutron in its nucleus - got a different answer. The data seemed to point to the major sink being reaction of hydrogen gas in the atmosphere with OH radicals, which "cleanse" the atmosphere of many reactive gases.
What led many scientists to this last conclusion was the significant enrichment of deuterium in H2, or molecular hydrogen, in the atmosphere near the Earth's surface. Deuterium, which on Earth is only one ten-thousandth as common as hydrogen, is 12 percent more enriched in atmospheric H2 than is water (H2O) in the world's oceans. For many years, no other source or sink for hydrogen besides reaction with OH radicals in the atmosphere was thought capable of producing the large deuterium enrichment observed.
UC Berkeley's Boering, who specializes in isotopic analysis of greenhouse gases, decided to look for an answer to this enigma in the upper atmosphere, or stratosphere. The stratosphere stretches from 10 to 50 kilometers (6 to 30 miles) above the surface, and contains the ozone layer protecting the Earth from ultraviolet rays. Stratospheric H2 is extremely enriched in deuterium, with an enrichment up to 32 percent larger than in the H2 near the Earth's surface.
"Hydrogen in the stratosphere is the most isotopically enriched material found on Earth apart from compounds in unusual meteorites," Boering said. "These first-ever measurements in the stratosphere - far from the soil microbe sink and surface sources such as fossil fuel combustion - allowed us to examine how deuterium is enriched or depleted by processes solely in the atmosphere."
Samples of stratospheric air were captured by a modified U-2 spy plane, an ER-2, operated by NASA. Nearly 500 samples of air were obtained during flights dating back to 1996.
Her isotopic analysis showed that the extreme deuterium enrichment observed in stratospheric H2 must result not only from deuterium enrichment by the destruction of H2 when it reacts with OH, but also from deuterium enrichment in the series of chemical reactions occurring as methane (CH4) is oxidized to produce H2. In both instances, reactions involving deuterium proceed at a different rate than those involving hydrogen, leading to products with a deuterium/hydrogen ratio different from the ratio in the reacting chemicals.
"The global atmospheric hydrogen budget was an enigma for some time because people didn't realize that deuterium enrichment in atmospheric H2 could be due to the methane source," Boering said. "Our measurements resolve this, showing that the H2 produced from methane oxidation produces quite enriched H2 and helps to 'balance' the H2 budget."
When these new data are combined with the known deuterium-hydrogen ratios of other reactions involving hydrogen - uptake of H2 by soil microbes and the ocean, plus production by incomplete burning of fossil fuels, biomass burning and production by microbes - the sources and sinks finally balance, Boering said.
"Our measurements and analysis are a step forward in understanding the H2 budget because they help resolve the discrepancy between the budget derived from H2 concentration measurements versus that derived from H2 isotope measurements," she said.
Boering's analysis was possible thanks to the development of a new technique by Caltech geochemist John Eiler to measure isotope ratios using as little as 50-100 milliliters of air - less than half a cup - instead of the 4,000 liters needed previously - the approximate volume of a large home propane tank. Obtaining stratospheric air in such large quantities would be difficult, Boering said. Eiler's technique involves mass spectrometry to separate different isotopes based on their slight weight differences.
Coauthors on the paper include Caltech's Eiler and post-doctoral researcher Thom Rahn, now at Los Alamos National Laboratory; UC Berkeley graduate student Michael McCarthy; and colleagues from the National Center for Atmospheric Research and UC Irvine.
Boering's research is supported by the National Science Foundation, the David and Lucile Packard Foundation and the NASA Upper Atmosphere Research Program.