WASHINGTON, Jan. 22 (Xinhua) -- Scientists at UC Irvine have mapped fossil fuel air pollution in the United States by analyzing corn collected from nearly 70 locations nationwide.

    Atmospheric scientists typically measure carbon dioxide by collecting air samples, but this is the first time fossil-fuel-emitted carbon dioxide has been mapped using plants. The study appears Jan. 23 in Geophysical Research Letters.

    In summer 2004, UCI scientists collected corn from farms and gardens in 31 states. They chose corn because it is widely grown and, as an annual plant, all of its carbon is derived from a single growing season. Back in the laboratory, the scientists dried samples of corn leaves and husks, then converted them to graphite using a series of chemical reactions.

    The graphite then was analyzed in the W.M. Keck Carbon Cycle Accelerator Mass Spectrometer, which measures a rare isotope of carbon, called radiocarbon. Carbon dioxide derived from fossil fuels contains no radiocarbon so it is easily distinguishable from other sources.

    With measurements from this machine, scientists calculated overall levels of carbon dioxide produced by fossil fuels at the location where the corn samples were collected.

    California and the Ohio Valley had the most fossil-fuel-emitted carbon dioxide, while the Colorado region had the least. The scientists expected pollution from California and other western coastal states to drift east, but they found that the Rocky Mountains appeared to provide a barrier for the movement of carbon dioxide from fossil fuels.

    This new method may complement existing air sampling techniques because plants provide a cost-effective way to record average daytime conditions over several months. Plants take in carbon dioxide gas, from both background and fossil fuel sources, during photosynthesis, and it becomes part of the plant tissue.

    This novel way to measure carbon dioxide produced by burning coal, oil and natural gas will help atmospheric scientists better understand where pollution is located and how it mixes and moves in the air.

    Tracking fossil-fuel-emitted carbon dioxide will be important as countries throughout the world adhere to the Kyoto Protocol, an agreement among nations to reduce greenhouse gas emissions. The United States signed the protocol, but the treaty has not been ratified by the U.S. Senate.

    "Many nations are facing increasing pressure to monitor and regulate the release of carbon dioxide from fossil fuel sources to limit greenhouse gas warming," said James Randerson, associate professor of Earth system science at UCI and co-author of the study. "This method can help determine how much fossil fuel carbon dioxide is coming from different regions."