It is well established that the concentration of CO2 in the atmosphere is rising, but the potential and actual role of terrestrial and marine ecosystems in offsetting this increase is open to much debate. The IU tower, which is one of AmeriFlux's twenty-nine data collection sites in North America, is equipped to measure not only the concentration of CO2 in the atmosphere but also the "flux," the actual exchange processes between the Earth's surface and the atmosphere. Data collection at the site began in January 1998.
Sue Grimmond, Associate Professor of Geography, and Brian Offerle, Tower Operations Director, both at Indiana University Bloomington, near a below-canopy station. In the foreground is a radiometer measuring four components of the electromagnetic spectrum. Just out of view is a 3D sonic anemoter thermometer. In the upper left corner of the photo the lower legs of the tower are visible. --credit
Funding for AmeriFlux comes from a variety of federal sources including the Departments of Energy, Commerce, and Agriculture, the National Aeronautic and Space Administration, along with the National Science Foundation. Financial support for IU's tower and instrumentation comes primarily from the Department of Energy's National Institute for Global Environmental Change.
Measurements of temperature, water vapor, CO2, and wind are each recorded ten times per second at all three levels. All three dimensions of wind speed (horizontal, lateral, and vertical) are observed using sonic anemometers. These measurements are made as air passes by the anemometer's six prongs, which resemble the structure of the legs of a spider. Sound waves are transmitted between the short distance separating three downward-facing prongs from three facing upward. By measuring the time it takes a sound wave transmitted from a downward prong to its upward mate, the instrument can calculate the wind speed in all three directions, while the triangulation of measurements made between the three pairs of prongs allows the calculation of wind direction. This state-of-the-art instrumentation produces a much more accurate reading than a conventional anemometer, which uses a wheel that simply spins to measure speed and a tail-wing that points the instrument in the wind's general direction.
Carbon dioxide measurements are taken at a dozen points along the tower by a network of Teflon tubes. Air samples are sucked in at these points, both within and above the forest canopy, and measured for CO2 concentration by an infrared gas analyzer housed in a building near the base of the tower. Like the use of sound waves for wind measurements, CO2 concentration is determined by measuring the length of time it takes for electromagnetic waves to pass through the air sample. Carbon dioxide absorbs radiation, so researchers know the CO2 levels based on the amount of radiation that is detected. The water vapor and ozone in the air samples are measured using the same method but different parts of the electromagnetic spectrum.
Taken collectively, all these measurements allow the researchers to isolate how much CO2 is being exchanged between the atmosphere and the Earth's surface, while controlling for the various conditions that can affect this exchange, like sunlight, the seasons, synoptic condition (weather), temperature, humidity, and state of the forest.
Hans Peter Schmid, an assistant professor of geography at IUB, explains how the actual flux measurement is calculated using what is known as the eddy correlation approach: "By measuring the velocities and the concentrations (of CO2, for example), we can detect the movement of CO2 between the forest and the atmosphere and vice versa. We simply separate what we measure into a mean component over a period of half an hour, and then we take the deviations from that mean. It's the correlation of those deviations between vertical velocity and CO2 concentrations that give us the flux."
Schmid, who was also involved in constructing the tower and placing the instruments, will direct the work that helps scale-up these small area measurements into a model representative of the region. The research team also includes Sara Pryor, an assistant professor of geography at IUB, who is investigating the links between surface flux observations and synoptic conditions. Pryor and Rebecca Barthelmie, an associate scientist with the department, are making additional measurements to identify the effect of nitrogen fertilization on carbon exchanges. Brian Offerle, the department's instrument scientist, is the tower operations director. The climatologists are working closely with others on campus, notably the forest ecology group working under J. C. Randolph, professor of public and environmental affairs at the IUB School of Public and Environmental Affairs.
These regional models, according to the AmeriFlux research plan, will contribute to a better understanding of CO2 levels and provide the scientific foundation for a range of CO2 policy and mitigation actions. By making this data available through the Internet to researchers, policymakers, and the public, these actions can be implemented on a local, regional, or global scale. More information about the IU research site can be found on the World Wide Web (www.indiana.edu/~climate/co2). --Aaron Conley
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