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October 2001

More than a change of color: Autumn foliage may affect air quality, climate

Fading leaves emit volatile organic compounds that may significantly affect air quality. (Photo by Carlye Calvin.)

Those autumn leaves that light up hillsides in bold strokes of gold and other colors also appear to play a role in regional air quality and climate.

Alex Guenther (ACD) and a team of colleagues are seeking to measure levels of chemicals that are emitted by various types of vegetation, including leaves that change color and fall. The chemicals, known as volatile organic compounds (VOCs), can combine with certain types of industrial emissions to affect the formation of pollutants such as smog and particulate matter.

"Some of the same or similar compounds are also produced by cars and industry, and they, together with other compounds in the atmosphere, form smog, have an influence on the climate, and so are very important for regional and global air quality issues," Alex explains.

Plant emissions of VOCs may also play an important role in determining the global distribution of trace gases and particles. Some of the emissions could have an impact on climate by affecting the rate at which greenhouse gases such as methane are oxidized in the atmosphere.

Scientists have known for years that trees and other vegetation produce certain hydrocarbon compounds, such as monoterpenes and isoprene. These VOCs (the source of the appealing scents associated with pine needles and cut grass) are used by plants to attract pollinating insects or to repel leaf-eating ones. Other chemicals are produced in response to such stressors as weather changes or wounds.

But only recently, with the development of new equipment, have scientists at NCAR and other institutions been able to probe the effects that emissions from changing foliage, crop harvesting, lawn mowing, and other events may have on the atmosphere.

Contributing to pollutants

Trees, industry, and the atmosphere interact in complex ways. Emissions from industrial sources and vegetation can lead to the formation of ground-level ozone, which harms both people and plants. (Photo by Carlye Calvin.)

Research shows that VOCs play a significant role in the formation of one of the most damaging pollutants: ground-level ozone, which is the major component of what is more commonly known as smog. The ozone forms in the presence of sunlight when volatile organic compounds react with nitrogen oxides emitted by cars and industrial plants.

The highly reactive plant emissions can also elevate levels of fine particulate matter, a type of pollution that is damaging to the human respiratory system.

Most VOCs in the atmosphere come from vegetation. Plants, however, are not the only culprit. Humans also produce significant amounts of the compounds through such activities as driving motor vehicles, operating factories, and using chemical solvents.

Alex emphasizes that plant emissions are harmless in the absence of human-generated nitrogen oxides.

In fact, trees and other types of vegetation can even benefit air quality by absorbing ozone and some other types of pollutants, as well as giving off scents that people find pleasing. But, he explains: "In an atmosphere modified by people, plant emissions do contribute to pollution."

For example, a team of U.S. and European scientists recently found that mowed grass emits hydrocarbons at a level of 20 to 60 parts per billion—which is comparable to the level released by the gasoline-powered mowers cutting the grass. In an article published this year in Environmental Science & Technology, the team reported: "The results of these experiments suggest that common lawn mowing releases substantial amounts of reactive VOCs and should be considered in urban air-quality control strategies."

At the same time, Alex and his colleagues measured the effects that hay harvesting in Austria and alfalfa harvesting in Colorado had on atmospheric chemistry. They detected significent emissions from both growing and cut vegetation. In fact, the hay-cutting study—a collaboration between ACD and Austria's University of Innsbruck—found VOC levels that were comparable to Los Angeles, which has suffered from some of the worst air pollution in the United States.

Thomas Karl (ACD) will measure levels of chemicals that autumn leaves release into the atmosphere. (Photo by Carlye Calvin.)

One of the researchers who worked on the lawn-mowing study, Thomas Karl, is a former student at Innsbruck who is now an ACD postdoc on Alex's team. He's spending much of September and October at an NCAR flux tower in northern Michigan to try to quantify leaf emissions ( see sidebar).

"The assumption is the leaves could have some significant impacts," Thomas says.

The researchers believe that foliage in colder climates has an especially dramatic atmospheric effect because a heavy, abrupt frost may result suddenly in high emissions. In contrast, when a leaf slowly loses its summer green and withers, the result is likely to be lower emissions that linger over a longer period of time.

In addition to Thomas and Alex, researchers taking part in the study include the University of Colorado's Ray Fall, Central Michigan University's Barkley Sive, and the Environmental Protection Agency's Chris Geron. In a related project, ACD scientists Peter Harley and Jim Greenberg are helping to investigate emissions from a wide variety of grasses and crops that are grown in greenhouses at NCAR and CU.

Cleaning up the air

The long-term goal of Thomas's study, which is funded in part by the EPA, is to help generate strategies to combat ground-level ozone.

In the past, some of the government's anti-smog efforts have focused on reducing VOCs produced by motor vehicles and other human sources. But that may be of little value since the bulk of those compounds actually come from vegetation. Instead, policy makers may consider fighting smog by curtailing human-generated nitrogen oxide emissions.

"We hope to develop scenarios in which we can have forests and people and cars and power plants and factories, all existing together, without creating toxic levels of pollutants," Alex says.

Ozone has attracted the attention of regulators because it aggravates asthma and other respiratory ailments and weakens resistance to infections. It also damages plants, causing an estimated several billion dollars a year in crop damage. The pollutant is so pervasive that the EPA in 1998 reported that 131 million people lived in counties with unhealthy levels of smog.

In addition to helping with urban air quality, ACD's research may benefit forest conservation efforts. Ozone is highly damaging to trees, and it can form in otherwise pristine environments. Environmentalists warn that ozone levels in some national parks, such as Acadia in Maine and Great Smoky Mountains in North Carolina and Tennessee, are comparable to the levels that have been recorded in large cities such as Boston.

Although researchers are uncertain about the exact atmospheric effects of particular species, they know that different types of trees absorb ozone and emit VOCS at different rates. For example, oak, eucalyptus, and sweet gum emit considerable amounts of isoprene and monoterpenes.

The issue of biogenic emissions has stirred some unusual debate in California. In July, the California Air Resources Board urged residents to consider planting low-emitting trees to protect the environment. The board recommended such species as Chinese hackberry, avocado, ash, and eastern redbud over such high- emitting trees as California sycamore, Chinese sweet gum, and certain types of oaks.

Regardless of the species, Alex believes that a mix of trees generally is better for the environment than single-species tree plantations, which he hopes to study in more detail in the near future. Fast-growing trees such as poplars that are used for pulp and other commercial purposes emit especially high levels of VOCs.

"What tends to happen with the monoculture plantations," says Alex, "is that you get extremes, and you have an atmosphere that's not the natural atmosphere."

• David Hosansky

On the Web:
More on VOC emissions from plants

The Technology

In order to measure foliage emissions in Michigan, Thomas Karl (ACD) is using a system that combines proton-transfer-reaction mass spectrometry with a disjunct eddy covariance sampler that was developed by Alex Guenther and others at NCAR. The system, which is taking continuous measurements 10 meters (33 feet) above the forest canopy, works in part by monitoring ions that collide with volatile organic compounds and transfer protons to the VOCs. It also records the concentrations associated with both downdrafts and updrafts above the canopy.

The goal of such a measuring approach is to capture fluxes in VOCs that occur when winds loft the compounds above the canopy. The situation is somewhat analogous to a person blowing air into a tube, thereby creating a spike in certain gases.

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Edited by David Hosansky, hosansky@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Thu Oct 25 11:18:36 MDT 2001