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.
This month, scientists from the National Center for Atmospheric Research (NCAR) are measuring levels of chemicals that are emitted as leaves change color and fall to the ground at a research site in northern Michigan.
The chemicals, known as volatile organic compounds (VOCs), have far-ranging effects: they combine with certain types of industrial emissions to create smog, and in some cases they play a role in global warming.
"VOCs are very important for both regional and global air quality issues," said NCAR's Alex Guenther. "VOCs, which are the source of the appealing scents associated with pine needles and cut grass, pose no harm in a natural setting. But they can react with human-generated nitrogen oxides in the presence of sunlight to form ground-level ozone, the major component of smog. The emissions also have an impact on climate by slowing the rate at which greenhouse gases are oxidized in the atmosphere."
To measure foliage emissions, scientists from NCAR and other institutions are taking continuous readings above the forest canopy near Pellston, Michigan. Their instruments track both updrafts and downdrafts, monitoring ions that collide and transfer protons to the VOCs.
"It's a very sophisticated observing system," Guenther explained.
The scientists picked Michigan in part because the area's abrupt, heavy frosts are likely to result in high emissions. In contrast, when leaves lose their summer green and wither more gradually, as in the southeast, the result is likely to be lower emissions that linger over a longer period of time.
Preliminary findings show that concentrations of at least two VOCs, methanol and acetaldehyde, more than doubled after a week of colder weather (September 21-28), which signaled the beginning of autumn colors and falling leaves in that region.
Funded by the National Science Foundation and the Environmental Protection Agency, the study is intended to help guide anti-pollution efforts by quantifying the chemicals that are given off by plants, as well as by human activities.
"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," Guenther said.
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 commonly known as smog. Ozone forms in the presence of sunlight when volatile organic compounds react with nitrogen oxides emitted by cars and industrial plants.
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, said Guenther, since the bulk of those compounds actually comes from vegetation. A more effective way to fight smog would be to curtail human-generated nitrogen oxide emissions.
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 in 1998 the EPA reported that 131 million people lived in counties with unhealthy levels of smog.
Guenther emphasized 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, Guenther explained: "In an atmosphere modified by people, plant emissions do contribute to pollution."
Scientists from Central Michigan University and the University of Colorado also participated in the study. NCAR's primary sponsor is the National Science Foundation.
For more on plant emissions, see this website.
For a two-minute QuickTime video of sound bites by Alex Guenther, go to this URL.