Hydrogen could harm stratosphere

According to conventional wisdom, hydrogen-fueled cars are environmentally friendly because they emit only water vapor—a naturally abundant atmospheric gas. But a new Caltech study shows that leakage of the hydrogen-gas fuel could cause problems for the upper atmosphere.

In an article in last week’s issue of Science, Tracey Tromp, John Eiler, Yuk Yung, and Run-Lie Shia of Caltech and Mark Allen of JPL report that leaked hydrogen gas inevitably resulting from a hydrogen economy could accumulate and indirectly cause up to a 10 percent decrease in atmospheric ozone.
If hydrogen were to replace fossil fuel entirely, the researchers estimate that, assuming a 10 to 20 percent leakage rate, 60 to 120 trillion grams of hydrogen would be released yearly—about four to eight times the amount now humanly generated. It would also double or triple the total hydrogen input (both natural and human sources) to the atmosphere.

Because molecular hydrogen freely moves up and mixes with stratospheric air, it would create additional water at high altitudes and, consequently, increased dampening of the stratosphere. This in turn would result in a cooling of the lower stratosphere and a disturbance of ozone chemistry. In this respect, hydrogen would be similar to chlorofluorocarbons, once widely used in air conditioning and refrigeration, which leak into the atmosphere and attack the ozone layer.

Whether or not hydrogen is causing damage hinges on the planet’s ability to consume excess hydrogen, explains Eiler, an assistant professor of geochemistry. The human-made hydrogen might either be absorbed by soil or react with other atmospheric compounds. “The balance of these two processes will be key to the outcome,” says Eiler. “If soils dominate, a hydrogen economy might have little effect on the environment. But if the atmosphere is the big player, the stratospheric cooling and destruction of ozone modeled in this Science paper are more likely to occur.” Determining which process is dominant, Eiler believes, “should be a solvable problem.”

Ideally, vehicles using hydrogen fuel cells have no environmental impact, producing energy by combining hydrogen with oxygen from the atmosphere, and emitting water as a byproduct. By comparison, internal combustion engines use fossil fuels and produce many pollutants, including the “greenhouse gas” carbon dioxide. While an economy based on hydrogen fuel cells would almost certainly improve air quality, it has the potential for unexpected consequences due to hydrogen leakage from cars and production facilities and during transport.

The Caltech team says that unlike CFCs and other chemicals released into the environment before their consequences were known, the current situation gives society an opportunity to understand potential effects well in advance.

“We have an unprecedented opportunity this time to understand what we’re getting into before we even switch to the new technology,” says lead author Tromp, a physics research scientist. “It won’t be like the case with the internal-combustion engine, when we started learning the effects of carbon dioxide decades later.”

Even if hydrogen is shown to be a potential environmental hazard, Eiler and Tromp don’t believe the move toward hydrogen-fueled cars should necessarily be abandoned. “If it’s the best way to provide a new energy source for our needs, then we can, and probably should, do it,” Tromp says, noting that any knowledge of hydrogen’s negative effects should then direct technology development and investment toward minimizing leakage.
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