Science paper suggests strategy to more effectively control harmful ozone

A report in the 27 April issue of the journal Science suggests that current U.S. Environmental Protection Agency (EPA) programs to reduce harmful ozone produced by electric utility power plants could be significantly improved by additionally considering power plant emission rates and geographic location.

"We think that our research represents an opportunity to build on current efforts to improve air quality," says Science lead author T.B. Ryerson of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado.

Ryerson and colleagues analyzed ozone production in chemical plumes emitted by coal-fired power plants in the rural United States, and found that ozone production depends in large part on the plant's emission rate and location. Current emission reduction programs "implicitly value all emissions equally, regardless of emission density or location," according to the study.

Tropospheric ozone (not to be confused with stratospheric ozone, which shields the Earth from ultraviolet radiation) is found in the atmospheric layer nearest to Earth's surface and is a prominent pollutant, especially during the summer months when elevated concentrations of ozone can harm human health and vegetation.

Ozone is formed in an atmospheric reaction involving NO_x and volatile organic compounds (VOCs). Electric utility power plants are a key source of NO_x emissions in the U.S., accounting for 25 percent of the nation's total human-produced NO_x emissions each year. As part of the Clean Air Act, the EPA has proposed emission reductions at 392 NO_x sources, primarily rural coal-fired power plants in the eastern and midwestern United States.

VOCs can come from anthropogenic sources such as car exhaust and petrochemical refining, but they are also emitted in large amounts by natural vegetation. In the Eastern U.S., for instance, the hydrocarbon isoprene from oak trees contributes a substantial amount of very reactive VOCs to the atmosphere, say the Science researchers.

To determine if all power plant emissions are equal culprits in ozone formation, the research team used data collected by aircraft flying through plant emission plumes to pinpoint the critical factors involved in ozone production within these plumes.

Focusing on two Tennessee power plants, Ryerson and colleagues found that power plant plumes with a lower concentration of NO_x emissions produced more ozone per NO_x emitted than plumes with a higher NO_x concentration, when all other atmospheric factors (such as wind speed) and VOC levels were equal. NO_x concentrations were determined by the power plant's emission rate.

The high NO_x environment in a plume from a large power plant promotes a series of reactions that don't form ozone, "but chew up the available NO_x," said Ryerson.

This doesn't mean that big power plants are necessarily better, however, because other factors such as combustion efficiency and the plant's age come into play, cautions Ryerson.

Focusing solely on emission rate or plant size also ignores another important aspect of ozone formation: location, location, location. The new study confirms that VOCs from natural sources can play a significant role in ozone creation within the power plant plumes.

The Science researchers compared ozone formation in a Missouri plume in a region of relatively low isoprene concentration with ozone formation in plumes from the two Tennessee plants, located in a region of greater isoprene concentration. The relatively low amounts of biological VOCs in the Missouri plume led to lower levels of ozone production.

Ryerson and colleagues say that the new study is the clearest observation yet that differences in NO_x emission rate and plant location have a relatively significant effect on ozone formation.

"Best of all, these are two factors that are addressable within the existing emission program," says Ryerson.

Since their findings are relevant to current ozone control efforts, the researchers have shared their findings with the EPA. For instance, they suggest that ozone impacts would be lessened if rural power plant emissions were transferred away from regions with high isoprene emissions.

The other members of the research team include M. Trainer, J.S. Holloway, D.D. Parrish, D.T. Sueper, G.J. Frost, W.C. Kuster, P.D. Goldan, G. Hübler, J.F. Meagher, and F. C. Fehsenfeld at NOAA, Boulder, Colorado, L.G. Huey at the Georgia Institute of Technology, Atlanta, S.G. Donnelly, S. Schauffler, and E.L. Atlas at the National Center for Atmospheric Research, Boulder, Colorado. J.S. Holloway, D.T. Sueper, G.J. Frost, G. Hübler, and F.C. Fehsenfeld are also at the University of Colorado, Boulder.This research was supported in part by the NOAA Climate and Global Change Program.