Carbon dioxide, believed by scientists to be a major factor in greenhouse warming, has been shown to reduce "agriforest" emissions of hydrocarbons that contribute to ground-based ozone pollution, according to CU-Boulder doctoral candidate Todd Rosenstiel of the environmental, population and organismic, or EPO, biology department.
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, said Rosenstiel, co-chief author of the study.
While this may seem like a good thing environmentally to some people, 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.
"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."
As people replace natural forests with agriforests, the species do produce significant amounts of hydrocarbons like isoprene," said Russell Monson, chair of CU-Boulder's EPO biology department. "The news here is that we have found a situation where elevated CO2 concentrations work in a positive way to reduce pollution from isoprene, that combines with sunlight and vehicle and industrial pollution to form smog and related lung problems in people."
A paper on the subject was published electronically today by Nature magazine. The primary authors are Rosenstiel and Mark Potosnak of Columbia University, now with the National Center for Atmospheric Research in Boulder. Other authors include Kevin Griffin of Columbia University, Ray Fall of CU-Boulder and Monson.
Fall, a professor of the chemistry and biochemistry department at CU-Boulder, said about 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, said Fall, who also is a member with Monson at the CU-headquartered Cooperative Institute for Research in Environmental Sciences, or CIRES.
CIRES is a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration in Boulder.
The CU-Boulder team's work, combined with research in the Biosphere II near Tucson, Ariz., primarily by Columbia University researchers, indicates it may be possible to genetically engineer environmentally friendly poplar trees by lessening their isoprene output, said Fall.
"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," the researchers wrote in Nature.
The Fall and Monson groups have been growing poplar trees in the chemistry and biochemistry department greenhouse in an attempt to isolate leaf cells and chloroplasts -- small bodies located inside plant cells that contain chlorophyll. They discovered that increases in CO2 in the laboratory caused the isoprene emissions from the leaf cells to decrease, a finding duplicated at the Biosphere II facility.
They currently are working on a number of further research projects related to the isoprene activity, including inhibiting an enzyme inside the plant cells that appears to control the amount of isoprene emitted by trees.