Trees Exacerbate Forest Smog

Smog generators. Gases released by trees combine with industrial pollution to produce ozone, a new study finds.

KINGS BEACH, CALIFORNIA--Gases released by industry and trees combine in a noxious cocktail that produces damaging ozone in California forests, according to a study presented here on 9 October at the Sierra Science Symposium. The research illustrates the unpredictable consequences of atmospheric pollution.

Ozone is a major component of smog, which damages the lungs. It's even worse for the health of trees such as ponderosa and Jeffrey pines, the dominant species in the Sierra Nevada mountains of California and staples of the timber industry. Some experts believe that ozone might be affecting the majority of the Sierra Nevada. And damage is likely to worsen as the climate becomes warmer, hiking ozone levels in the lower atmosphere. Vehicles, industry, and agriculture are widely blamed for releasing nitrogen oxides and hydrocarbons that waft into the mountains and, in a series of oxidation reactions, produce ozone.

The new research adds a surprising twist: A natural hydrocarbon called isoprene from oak trees in the foothills reacts with the rising, wind-borne nitrogen oxides, producing about half of the ozone in the Sierras. That reaction produces various byproducts. By measuring the concentration of these byproducts, a team led by atmospheric chemist Allen Goldstein and undergraduate Gabrielle Dreyfus of the University of California, Berkeley, determined how much ozone the trees produced. They found that isoprene accounts for 40% to 70% of ozone production in the forest. Such reactions are likely wherever forests are downwind of industrial pollution, says Goldstein, who reported the findings. The study is also published in the October 2002 issue of the Journal of Geophysical Research—Atmospheres.

The research is "of considerable importance," says atmospheric chemist Chris Geron of the Environmental Protection Agency in Research Triangle Park, North Carolina. Natural isoprene bumps forest ozone up to concentrations thought to reduce pine tree growth and endanger the health of people with respiratory problems, he says. But don't go reaching for your chainsaw just yet. "The solution is not to cut down all the trees," Goldstein says, "but to cut back on nitrogen oxide emissions."

Related sites
Goldstein's lab
EPA's air quality and ozone site
JGR home page (subscription required for content)

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