Environmental Science & Engineering - www.esemag.com - March 2003
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Global pollutants offset each other

Researchers at Columbia University's Lamont Doherty Earth Observatory and the University of Colorado at Boulder have found, ironically, that two pollutants - carbon dioxide and hydrocarbons emitted from agricultural forest trees - offset each other somewhat in mitigating air quality problems. The study, Increased CO₂ Uncouples Growth from Isoprene Emission in an Agriforest Ecosystem, conducted at Columbia's Biosphere 2 Laboratory, located outside of Tucson, AZ., appears in the January 16, 2003, issue of NATURE Magazine. The study was published online January 5.

In an attempt to isolate leaf cells and chloroplasts - small bodies located inside plant cells that contain chlorophyll - CU-Boulder scientists discovered that increases in CO₂ in the laboratory caused the isoprene emissions from the leaf cells to decrease. The finding was duplicated by Columbia scientists in Biosphere 2's agriforestry area. In the study, scientists demonstrated that growth of an intact Eastern Cottonwood plantation under increased CO₂ (800 and 1200 parts per million) reduced ecosystem isoprene production by 21% and 41% respectively.

Biosphere 2's agriforest is a halfacre space that has been divided into three separate chambers for a multiyear study of Eastern Cottonwoods (Populus deltoides) response to three levels of CO₂ (400, 800 and 1200 ppm).

Planted in 1998, the stands have grown under elevated CO₂ conditions since 1999.

Believed by scientists to be a major factor in global warming, increase levels of carbon dioxide, one of several greenhouse gases, has been shown to reduce "agriforest" emissions of hydrocarbons, such as isoprene that contribute to ground-based ozone pollution, said Todd Rosenstiel, CU-Boulder, co-chief author of the study. Commercial agriforests made up of trees including poplars, Eucalyptus and Acacia emit high levels of isoprene, a highly reactive chemical species believed to contribute heavily to ground-based ozone, Rosenstiel said.

Ozone maintains its chemical structure whether it occurs miles above the earth or at ground level, and can be "good" or "bad," depending on its location in the atmosphere. "Good" ozone occurs naturally in the stratosphere, forming a layer that protects life on earth from the sun's harmful rays. In the lower atmosphere, ground-level ozone is considered "bad." When inhaling ozone, the gas travels throughout the respiratory tract, damaging the bronchioles and alveoli in the lungs, air sacs that are important for gas exchange. Repeated ozone exposure can inflame lung tissues and cause respiratory infections, according to the US Environmental Protection Agency Website.

While some may view this finding as environmentally positive, Rosenstiel is more cautious. "The effects of CO₂ are unpredictable. The bigger picture is the rapidly growing amount of these agriforests worldwide emitting hydrocarbons like isoprene in much larger volumes, since we still do not know enough about the basic chemistry and biochemistry of isoprene to predict what may happen in the future," Rosenstiel said. "One thing we have shown is that 'tweaking' environmental conditions where such trees grow through changes in water consumption, temperature and soil conditions may have significant effects on isoprene emissions."

Writing in NATURE Magazine, the researchers note, "As almost all commercial agriforest species emit high levels of isoprene, proliferation of agriforest plantations has significant potential to increase regional ozone pollution and enhance the lifetime of methane, an important determinant of global climate."

"As agriforests replace natural forests, the potential for producing hydrocarbons like isoprene will increase. The news here is that we have discovered a situation in which elevated CO₂ concentrations work in a positive way to reduce pollution that is caused by isoprene. In the presence of sunlight and nitrogen oxide pollution from automobiles and industrial processes, isoprene reacts in a way that enhances the production of ozone pollution, which can cause serious respiratory problems in people."

An estimated 500 million tons of isoprene are emitted into Earth's atmosphere each year. The Southeast U.S. has large amounts of forest trees contributing to the isoprene emissions.

The CU-Boulder team's work, combined with Columbia researchers' activity inside the Biosphere 2 Lab, suggests it may be possible to genetically engineer environmentally friendly poplar trees by lessening their isoprene output.

Biosphere 2 is a 3.1 acre, glassenclosed, research laboratory and the world's largest controlled environment facility for terrestrial and marine plant growth, and experimental climate change science. Columbia University has managed the campus and research laboratory since January 1996.

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