Trees and VOCs
Trees and VOCs

INSTAAR scientists are busy climbing trees.  They are in the middle of an experiment to study volatile organic compounds (VOCs) that different tree species emit into the air.

A flux experiment on loblolly pine trees in the Duke Forest, North Carolina, August 2005. The Teflon bag around the branch holds in VOCs emitted by the pine needles so that they can be measured. Photo by John Ortega, INSTAAR.

VOCs are organic chemicals that turn into vapor easily.  The sharp scent of pine needles, for instance, is caused by monoterpenes, which are among the group of VOCs.  Trees use VOCs to attract pollinators and repel harmful insects and animals.  Trees also produce VOCs in response to stress.

In the atmosphere, VOCs interact with nitrogen oxides and sunlight to produce ozone and other chemicals.  While much is known about how trees remove pollutants from the air by absorbing carbon dioxide, less is known about the more subtle effects of VOC acting upon the oxidizing chemistry of the atmosphere.

The team is focusing on sesquiterpene, a particular kind of VOC that may be especially important in atmospheric chemistry.  Sesquiterpenes are suspected to be main ingredients in the blue haze that gives its name to the Smoky Mountains and Blue Ridge Mountains of Appalachia, U.S.A., and the Blue Mountains of New South Wales, Australia.

The team wants to collect data on sesquiterpene emission rates from different species of trees, and find out how those rates depend on local environmental conditions, like the time of day, temperature, and cloudiness.

The scientists will use that data to contribute to models of how the world’s atmosphere works, fine-tuning information about how trees in urban areas and forests affect the chemical balance of the air.  They also hope to provide suggestions to urban planners and foresters about what species of trees best offset air pollution.

To measure the amount of sesquiterpene emitted by a tree, a researcher ties a thin Teflon bag around a tree branch, making sure that the seal is air tight.  An instrument pumps scrubbed air into the bag and routes outgoing air through another, highly sensitive instrument that measures emissions from the leaves.  Instruments also keep track of the temperature and sunlight.  Once enough data has been collected, the team cuts off the branch and weighs the leaves.

At the end of the experiments, the team will know how much sesquiterpene a particular tree branch emitted.  They can extrapolate from that branch how much the whole tree or an entire forest of those trees would emit.  They will also know more about how local conditions, like day or night, or cloudy vs. sunny days, affect emissions from the different species.

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