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The Falling Leaves Drift Into Ozone

Leaf-peeping scientists find autumn foliage and human pollution interact to create smog

SUNDAY, Nov. 4, 2001 (HealthDayNews) -- The red, gold and orange leaves that make autumn a colorful spectacle in many parts of the United States also play a role in the formation of ozone, a big component of smog.

As leaves change color, they emit even greater amounts of powerful chemicals known as volatile organic compounds, or VOCs, according to scientists from the National Center for Atmospheric Research.

"Natural VOCs aren't toxic," says Alex Guenther, one of the center's scientists wrapping up a recent study of autumn leaf emissions near Pellston, Mich. "But when they interact with human-generated nitrogen oxides in the presence of sunlight, these chemicals form ground-level ozone, the major component of smog."

Nitrogen oxides are chemicals produced by humans through the burning of fossil fuels, particularly coal and oil. Volatile organic compounds are chemicals as well, produced both naturally and from such human activities as the burning of fossil fuels and the manufacture and use of products like solvents and aerosol sprays.

To better understand the impact of seasonal changes in leaves on VOC production, Guenther and his colleagues set up a sophisticated system to monitor the chemicals produced by fall foliage above the forest canopy in Pellston. Their instruments measured updrafts and downdrafts of air over a several-week period when fall colors were at their peak.

Guenther and his associates chose the site specifically because northern Michigan's abrupt, heavy frosts create some of the most spectacular leaf changes in the nation. Guenther theorized that the freezing temperatures that prompt leaves' dramatic color transformations also burst their cells and boost the amount of VOCs they release into the air.

He was right.

"We found significant increases in the level of VOCs as the trees changed colors," Guenther says. "Some chemical emissions -- particularly methanol and acetaldehyde -- more than doubled during the last week of September."

Guenther's findings provide another piece of the intriguing puzzle of how naturally occurring and man-made chemicals interact to create ozone and degrade air quality to the detriment of human health.

Although ozone can't be seen or tasted, it can irritate lungs and make breathing difficult, according to the Environmental Protection Agency (EPA). And there's a link between ozone levels in the outdoor air and increased hospital admissions for such respiratory conditions as asthma. High levels of ozone are believed responsible for up to 20 percent of summertime respiratory hospital visits and admissions, with children at greatest risk because they tend to spend more time outdoors.

Ozone reduction efforts in the past focused primarily on reducing man-made VOCs -- rather than nitrogen oxides -- produced by motor vehicles and a variety of industrial sources. But this approach didn't work because not enough was known about the VOC emissions coming from plants and other types of vegetation, Guenther says.

As Chris Geron, an environmental scientist with the EPA, points out, some U.S. cities, like Atlanta, had initiatives that significantly reduced human-produced VOCs but failed to improve the high levels of ozone or smog.

"On a global basis, plants produce 10 times the amount of VOCs that humans produce," Geron says. "That's why even large cuts in human production [of VOCs] will never resolve our ozone problems."

Experts now agree that reduction of nitrogen oxides -- often referred to as NOx in air quality circles -- is critical to long-range ozone and smog reduction efforts.

"Plant emissions of VOCs -- even the heightened emissions that we've now documented occurring during fall color changes -- are harmless in the absence of human-generated NOx," Guenther says. "In fact, trees and other vegetation can even benefit air quality by absorbing ozone and other types of pollutants, as well as giving off scents that people find pleasing. But in atmospheres into which NOx has been introduced by human activities, plant emissions do contribute to pollution."

Many cities and states now have new initiatives focused on the reduction or elimination of nitrogen oxides, rather than VOCs.

Heather Evans, an environmental scientist at the Texas Natural Resource Conservation Commission, says her state is working to reduce levels of nitrogen oxides as a way of meeting stricter federal air quality standards.

"We know we can't just focus on decreasing VOCs created by human beings, because it's increasingly clear plants make the greatest contribution to global levels of these chemicals," Evans says. "That's why our present approach to ozone reduction is primarily directed to reducing [nitrogen oxides]. It just happens that, in many cases, man-made VOCs are reduced at the same time because both types of chemicals are created in fuel-burning processes."

What to Do: Curious about volatile organic compounds created by the plants of planet Earth? A simple explanation and a useful diagram are available at this University of Colorado Web site. Learn more about how ozone affects human health and how to reduce its harmful influences at this EPA site.

SOURCES: Interviews with Alex Guenther, Ph.D., National Center for Atmospheric Research, Boulder, Colo.; Chris Geron, environmental scientist, Atmospheric Protection Branch, Air Pollution Prevention and Control Division, National Risk Management Research Laboratory, Office of Research and Development, Environmental Protection Agency, Triangle Park, N.C.; Heather Evans, acting team leader, Texas State Implementation Plan for Attainment of Federal Ozone Requirements, Texas Natural Resource Conservation Commission, Austin
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