Hydrogen would inevitably leak if we shifted to a hydrogen economy and the question arises: would large quantities of leaked hydrogen cause environmental problems?

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 atmosphere's 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.

Air samples from the stratosphere indicate that most atmospheric hydrogen is taken up by the soil.

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 it's 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.

The big question is whether the inevitable hydrogen leakage in an economy converted to hydrogen would accumulate in the atmosphere and cause harmful effects.

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.

It is assumed by most advocates of hydrogen as an energy storage form that it will not have any harmful pollutant effects. Well, this assumption might turn out to be correct. But for atmospheric scientists this is still an open question.