Chasing air masses, measuring greenhouse gases

As policymakers scratch their heads over what to do about increasing levels of greenhouse gases in the atmosphere, Harvard atmospheric chemistry researchers are pioneering new ways to measure these levels. “We’re chasing air masses,” says Christoph Gerbig, a research associate working with Steven Wofsy, Abbot Lawrence Rotch Professor of Atmospheric and Environmental Science. Wofsy, Gerbig, and graduate student John Lin are working on a pilot experiment to determine carbon dioxide amounts on regional and continental scales by chasing blocks of air with an airplane.

Essential to plants for photosynthesis and a major regulator of climate for the planet, carbon dioxide (CO2) is an especially important trace gas in the atmosphere. It is a major contributor to the “greenhouse effect,” in which certain gases absorb heat from the Earth’s surface, warm the atmosphere, and radiate that heat back to the surface. Natural levels of CO2 in the atmosphere help the surface keep from getting too cold for life; but when a certain level of greenhouse gases is reached in the atmosphere, the surface will warm too much. Throughout the past 400,000 years, the concentration of carbon dioxide in the atmosphere has stayed within 20 percent of an average of 240 parts per million (ppm). A rapid increase to greater than 370 ppm – more than 30 percent higher than the maximum over the last 400,000 years – has come from the burning of fossil fuels and land-use changes begun during the Industrial Revolution.

Currently, levels of carbon dioxide in the atmosphere continue to rise. Yet, a wide body of research from the past decade has shown that only about half the carbon that humans put in the atmosphere stays there. The other half is taken up by the oceans and by terrestrial vegetation.

Policymakers throughout the world have taken notice of the increasing levels of carbon in the atmosphere and have begun negotiations on how to mitigate the problem through accords such as the Kyoto Protocol. One major sticking point of the Kyoto Protocol, which the United States rejected, was how to monitor compliance. There is now no good way to tell how much carbon is being emitted from – or taken up by – land on a country-sized scale. This makes accords such as the Kyoto Protocol, should they be agreed upon, nearly impossible to enforce. Currently, researchers can either accurately measure carbon dioxide exchange for a local patch of forest, or use large computer simulations to get a good measurement for a hemisphere. There is a gap in scale between the small local measurements and the big global simulations. “We want to bridge that gap,” says Gerbig, who figures he and his colleagues can do it by chasing air masses in the COBRA (CO2 Budget and Rectification Airborne) Study.

No one has yet followed the changes in carbon dioxide in a single block of air as it moves through the atmosphere, which is governed by turbulent motions. In a COBRA campaign over North America last summer, the researchers were among the first to detect to what extent forests and crops directly interact with the atmosphere by taking up carbon dioxide during photosynthesis. Such measurements are critical to understanding carbon amounts at regional and continental scales.

“It’s possible to see changes that reflect the underlying vegetation when you chase an air mass. For instance, you see higher levels of CO2 over dry areas that don’t have as much active vegetation than over areas with plants that are doing photosynthesis more actively,” says Gerbig. Lin puts it more simply, “It’s like detecting the breathing of the biosphere.”

An unexpected but pleasant result from the COBRA Study has been seeing the signature of forest fires within the data. Along with CO2, the researchers also measured carbon monoxide, CO, which is emitted in relatively large quantities from burning forests. As they followed blocks of air, the researchers could see the emissions as they traveled great distances from the source fires. “We never expected to see so much CO so far from fires,” says Lin. The fires also proved to be a stroke of luck for validating the method. By pointing to the carbon monoxide from the fires, Wofsy, Lin, and Gerbig can prove to other skeptical scientists that they truly were chasing a single air mass as it traveled across a state.

A typical field campaign lasts about a month. Long before the fieldwork begins, the researchers fix the bugs in the instruments that measure atmospheric gases, and make arrangements to use a Cessna Citation II private jet specially outfitted for atmospheric research. During the appointed month, each day begins with a few hours of flight planning to see if weather conditions over the following few days will be right for chasing air masses. If the conditions look good, the scientists confer with the pilot and pick a number of places to go flying.

It’s not like airline flying. Because a crucial component of the data is taking measurements of the vertical gradients of gases within the target air mass, there are plenty of climbs, descents, and spirals. Once the data is in, Wofsy, Gerbig, and Lin return to the lab for many months of detailed analysis. Wofsy is personally excited by the experiment, saying, “We’ve gotten the first airborne measurements of the direct interaction of an entire temperate landscape with the atmosphere.” He is eagerly awaiting the opportunity to do it again.

Next up for the team of atmospheric chemists is a mission to measure the carbon amount of the Amazon Basin in Brazil during the summer of 2002. Given its vastness and the number of trees present, the Amazon Basin is a very important component of the global carbon cycle. It probably exerts a large influence on the amount of CO2 in the atmosphere. Moreover, observations from the area are relatively sparse, so there is a real chance for this research campaign to generate some new insights. Ultimately, Wofsy hopes COBRA can serve as the basis for more advanced aircraft studies, as well as satellite measurements.

NASA, the Department of Energy, the National Science Foundation, and the National Oceanographic and Atmospheric Administration fund the COBRA project.

Daniel Matross is working toward his Ph.D. in Earth and Planetary Sciences.