Since the 1990s, bark beetles have bored through more than 6 billion trees in the western United States and British Columbia, killing many of them. Now, a new study shows the tiny bugs are also contributing to air pollution in forested areas.

The study, published yesterday in the journal Environmental Science & Technology, found the beetles can cause trees to release up to 20 times more volatile organic compounds, which combine with nitrogen oxides to create smog and can contribute to the formation of particulate matter.

Western North America has seen a population explosion of mountain pine beetles in recent years, due in part to prolonged drought, which weakens trees and renders them less resistant to beetle attacks. The beetles, which attack pine, spruce and other tree species, bore into the bark and lay their eggs beneath it.

To determine how beetle attacks might affect the atmosphere, the researchers, who include Kara Huff Hartz of Southern Illinois University Carbondale and Anna Gannet Hallar of the Desert Research Institute's Storm Peak Laboratory in Steamboat Springs, Colo., measured VOC levels in the air near both healthy and infected lodgepole pine trees on Mount Werner in Colorado's Routt National Forest.

They found that beetle-infested trees released between five and 20 times more VOCs than healthy trees.

"The total VOCs emitted from the trunks of bark beetle infested lodgepole pine trees were higher than the total VOCs emitted from the trunks of healthy, noninfested lodgepole pine trees," the study concluded.

As the bugs tunnel into the bark, they create pathways for more VOCs to be released from the tree, explained Huff Hartz, the lead author of the study. At the same time, the tree emits more of its stores of VOCs as a defense against the attack.

"You'd think if a tree dies, it's no longer contributing those VOCs [after decomposing]," she said. "But what could be happening is the beetles are opening up the trees where normally the tree would be closing down."

Consequently, the bark beetle epidemic in the western United States is likely contributing to higher concentrations of VOCs in the air, resulting in more haze in forested areas, according to the study.

Haze is already a problem in many areas of the West, and sometimes obscures the view at the Grand Canyon and other national parks and monuments.

It also can cause respiratory problems and affect climate. But unlike some airborne pollutants, VOCs, in contributing to the formation of particulate matter, can have a cooling effect. They also can increase or decrease precipitation, depending on location and other factors.

"These results highlight one of the many potential feedbacks due to aerosols, which continue to be the greatest challenge to improving predictive models for air quality, visibility and climate," said Alex Pszenny, program director in the National Science Foundation's Division of Atmospheric and Geospace Sciences, which funded the research.

Click here to read the study.

Reese writes from Santa Fe, N.M.