Where there's smoke, there's not always ozone

Vegetation-generated hydrocarbons share the blame

Each day for nearly a month during the summer of 1999, a team of scientists boarded a federal turboprop, buckled themselves behind instrument consoles, and endured what team member Tom Ryerson calls "six to eight hours of quality turbulence" in a quest to crack the problem of ozone pollution in the Southeastern US.

Ozone's recipe is time-tested: Mix nitrogen oxides (NOx) from burning fossil fuel with a dash of hydrocarbons, and bake at 80 or 90 degrees Fahrenheit.

Efforts to control the formation of this noxious brew, which initially focused on auto emissions, have met with mixed results, however. Even with tight auto-emission standards, summertime ozone levels have exceeded allowable limits over large areas of the South for years, according to Dr. Ryerson, an atmospheric scientist at the National Oceanic and Atmospheric Administration's Aeronomy Laboratory in Boulder, Colo. Indeed, researchers say that on some days, if no one drove a vehicle, some cities would either exceed standards or come close to exceeding them.

"We've been trying to sort out the scientific reasons for that," he says.

The region's coal-fired power plants, which produce copious amounts of NOx, have emerged as key pieces in the Southern ozone puzzle.

After spending more than a year poring over data, the scientists were surprised to find that the more NOx a coal-fired power plant produces, the less efficiently the NOx is converted to ozone. Moreover, the amount of ozone produced downwind of the plant depends not just on human sources of hydrocarbons, such as automobiles, but on biological sources, such as trees and shrubs.

The results, reported in the current issue of the journal Science, could help shape the way the region designs its NOx-control strategies.

The 1999 effort marked the third summer the team focused on the region around Nashville, Tenn., which shares characteristics with many other Southern cities having trouble meeting ozone standards. It is growing; it is surrounded by woods and farmland, sources of natural hydrocarbons; and it counts coal-fired power plants among its regional neighbors. The work was conducted under the aegis of the Southern Oxidant Study, a broader 13-year effort to study Southern air-pollution.

The study had two immediate goals, says Ryerson, the lead author of the Science report: to measure the NOx production rate from two power plants and the production rates of the region's biologically produced hydrocarbons, particularly isoprene. It noted that a plant at Cumberland, Tenn., pumped eight times as much NOx into the atmosphere as did a plant near Johnsonville, Tenn. Yet the Johnsonville plant's NOx produced ozone much more rapidly, even though the isoprene-producing vegetation downwind of both plants was similar. "Keep track of the isoprene, and you have 90 percent of the ozone problem understood," Ryerson says.

US Environmental Protection Agency efforts to clamp down on NOx emissions from 329 large sources, mostly power plants, are based on an emissions-trading scheme. The researchers suggest that for ozone controls, factors such as size and location should be taken into account when placing a value on NOx emitted.

Yet NOx also plays a role in acid rain and climate change, notes John Bachmann, associate director for science policy at the EPA's Office of Air Quality Planning and Standards. Thus, the agency's current approach to emissions trading isn't likely to change as a result of the Nashville study. He says, however, that the study is a valuable contribution to understanding the nature of the rural ozone problem and could help guide the agency when utilities apply for permits to build new power plants.

The US "is now developing a new energy policy," he says, "and we'll need new sources in the region. As we consider location and size, we have some choice on where to put these things."

(c) Copyright 2001. The Christian Science Monitor

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