GLOBAL WARMING
Finding Clues in the Stratosphere
Kristie Boering discusses atmospheric gas samples with graduate students Kate Hoag and Aaron Johnson.
The next time you fly in a commercial passenger jet, look up. Above you is the stratosphere, and compared to the troposphere below you, with its turbulence and weather, the stratosphere appears calm and unexciting. Atmospheric chemist Kristie Boering would disagree. The stratosphere is her turf, about 6 to 30 miles above the ground, where for her the chemistry really starts to get interesting. "The reactions that led to the growth of the ozone hole are the best known examples of chemistry in the stratosphere, but there is a lot more going on up there that we need to understand," says Boering.
One major challenge to studying chemistry in the stratosphere is collecting samples from altitudes well above those at which airplanes can fly. For Boering, there are two ways to collect air samples. One method is high-altitude balloons, which can reach 120,000 feet. Another is the NASA ER-2, the research version of the famous U-2 spy plane that caused an international incident when pilot Gary Powers was shot down over the former Soviet Union in 1960.
The ER-2 can reach altitudes of 70,000 feet or higher even when fully loaded with instruments. The aircraft's instruments can analyze air while flying, or collect samples for further analysis on the ground. Boering has spent many hours chasing ER-2s to remote locations such as Fiji and New Zealand.
Boering is particularly interested in measuring the different isotopic compositions of greenhouse gases that make their way into the stratosphere. Oxygen has three stable isotopes, 16O, 17O and 18O, each with eight protons but with eight, nine and ten neutrons, respectively. The hydrogen isotope deuterium contains one neutron in its nucleus, while the nucleus of the nitrogen isotope 15N contains seven protons and eight neutrons.
These heavy isotopes are components of water vapor, carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). These greenhouse gases undergo isotopic enrichment in the stratosphere. The lighter versions of these molecules interact more readily in the stratosphere's intense ultraviolet light with highly reactive free radicals, leaving behind molecules unusually rich in the isotopes of nitrogen, carbon and hydrogen. Through isotopic enrichment, the stratosphere leaves its mark on these greenhouse gases, and the chemical changes they undergo allow them to be tracked as they circulate through the stratosphere or re-enter the troposphere.
"These isotope signatures allow us to use these gases as tracers of chemical processes and circulation patterns both in the stratosphere itself and on the earth's surface below," says Boering, "but we need to understand more about why and at what rate this isotopic enrichment happens. And the sooner the better," Boering adds, "because measuring the concentration of these gases provides a benchmark that can help us monitor climate change."
Boering has already made a major contribution with her isotopic enrichment observations. In a paper that appeared in Nature magazine in 2003, Boering and colleagues reported on research that helped balance the molecular hydrogen (H2) budget for the planet. Just as a household budget tracks income and expenses, a budget for an element like hydrogen attempts to account for all the sources and sinks.
The NASA ER-2 research aircraft can carry a full range of instruments to altitudes of 70,000 feet.
The pattern of hydrogen concentration in the atmosphere and its isotopic composition had been at odds with what scientists expected, throwing the hydrogen budget out of balance. It appeared that both atmospheric reactions and the soil were acting as large sinks. Researchers knew that soil microbes metabolized hydrogen, removing it from the atmosphere. Other researchers noted the high concentration of deuterium in the atmosphere near earth's surface and assumed this was due to isotopic enrichment of hydrogen as it reacts with other atmospheric gases. Boering's observations explained the discrepancies. Using the stratosphere as her laboratory, she and her co-authors demonstrated that the high concentration of deuterium in the stratosphere, and by analogy in the lower atmosphere, was also due to the oxidation of methane, which is a major source of hydrogen.
With the hydrogen sources and sinks more precisely calculated, the hydrogen budget balanced. "It's a good thing to understand the hydrogen budget today," says Boering, "because if we switch to a hydrogen–based energy system, due to leaks there may be a large new source of hydrogen in the atmosphere, and we need to understand where the leaking hydrogen will go, what will happen to it chemically, and what the implications might be for climate change."
For Boering, many questions are left to be answered. As the troposphere warms, it appears the stratosphere is cooling, while the amount of water vapor in the stratosphere is rising. Add to this mix plans for a new generation of high-altitude commercial jetliners — next generation Concordes — that could inject combustion by-products directly into the stratosphere.
"We don't yet know what the outcome of these combined changes will be," concludes Boering, "but there is a possibility that we'll see increased chlorine activation and the loss of ozone. And there may be additional implications for climate change that we don't understand yet. As an atmospheric chemist, I think it's imperative to find new observational and modeling tools to help us understand and predict these changes."