The study examined a class of chemicals that are emitted as unburned fuel from automobile tailpipes and as vapors from industrial chemicals, but also come naturally from tree leaves. These chemicals, known collectively as VOCs, react with other pollutants to form ozone, a bluish, irritating and pungent gas that is a major form of smog in the lower atmosphere.

While clean-air laws have reduced the level of man-made VOCs (volatile organic compounds), the tree-produced varieties have increased dramatically in some parts of the country, the study found. The increase stems from intensified tree farming and other land use changes that have altered the mix of trees in the landscape, said Drew Purves, the lead author of the study that included scientists from four universities.

"There are seemingly natural but ultimately anthropogenic (human-caused) processes in the landscape that have had larger effects on VOC emissions than the deliberate legislated decreases," said Purves.

Although scientists knew that trees contribute substantial amounts of VOCs to the atmosphere, the rate of increase in recent decades was previously unrecognized. "If we don't understand what's going on with biogenic (plant-produced) VOCs, we are not going to be able to weigh different air-quality strategies properly," said Purves. "It's a big enough part of the puzzle that it really needs to go in there with the rest."

The study may help explain why ozone levels have not improved in some parts of the country as much as was anticipated with the enactment of clean-air laws, Purves said. Environmental technologies such as catalytic converters and hoses that collect fumes at gas pumps have substantially reduced human-produced VOCs. However, in some parts of the country -- particularly the area extending f
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Contact: Steven Schultz
sschultz@princeton.edu
609-258-5729
Princeton University
28-Sep-2004