30 September 2004
Tree smog hike hurts ozone protection
Through legislation, man-made ozone pollution is on the decline. That’s the good news. However, the bad news is nature seems to be picking up where people have left off, because changes in U.S. forests caused by land use practices may have inadvertently worsened ozone pollution.
A study, led by Princeton University scientists, examined a class of chemicals emitted as unburned fuel from automobile tailpipes and as vapors from industrial chemicals, but also come naturally from tree leaves. These chemicals, known as volatile organic compounds (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.
Although clean-air laws have reduced the level of man-made VOCs, 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,” Purves said.
Although scientists know 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,” Purves said. “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, such as the area extending from Alabama up through the Tennessee Valley and Virginia, increased VOC emissions from forests caused by tree growth in abandoned farmland and increases in plantation forestry seem to outweigh the legislated improvements.
While it did not decrease ozone levels in all areas, Purves said cutting the human-caused sources may have been worthwhile. “Even keeping the air quality the same might have been an achievement, because if we hadn’t done anything it might have worsened,” said Purves.
The study did not measure actual ozone levels. Instead, it focused on VOCs, a crucial part of the chemical reaction that produces ozone. The other critical ingredient is a class of gasses known as NOx (various combinations of nitrogen and oxygen), which are almost entirely man-made. The ozone-producing reaction happens most readily in hot weather, which is also when trees produce the most VOCs.
The team conducted the study by analyzing data collected by the U.S. Forest Service, which measured and cataloged 2.7 million trees on 250,000 plots of land across the country. Researchers calculated the VOC emissions for each tree and each plot and used their findings to map VOC levels nationally. The scientists compared survey data taken in the 1980s with those taken in the 1990s to determine how levels were changing over time.
They found that abandoned farmland areas have early generations of trees that produce higher levels of VOCs than older growth forests. In the South, pine plantations used for their fast-growing supplies of timber have proven to be havens for sweet gum trees, which are major producers of VOCs. Indeed, virtually every tree that grows fast, a key element for forestry production, is a heavy emitter of VOCs.
For related information, go to www.isa.org/environment.