Can a change in fire burning season affect air pollution?

Researchers examine the smoke from prescribed forest fires in the southeastern U.S.

Planned forest fires can save lives and millions of dollars in property by reducing the amount of biomass that is available for wildfires. Yet these prescribed fires emit significant amounts of air pollutants. What would happen if the managed fires occurred during different seasons or on days when the wind blows away from urban centers? Does the amount of time between each intentionally set fire affect pollution levels?

New research published in ES&T (DOI: 10.1021/es0711213) examines all of these questions in relation to data gathered in Georgia, where forests cover more than 66% of the state's total land. The researchers conclude that new changes in forest management practices can reduce the pollution levels from prescribed forest fires, including from smoldering ash. For example, fires in the spring and winter have a greater impact on PM_2.5 levels than equivalent fires in the summer and fall. Complicating matters, however, is the fact that ozone levels rise because of smoke from forest fires that occur during the spring and summer, they find.

The results have real implications for local air-quality planners striving to meet air-quality standards, says coauthor Di Tian, who was a graduate student at the School of Civil and Environmental Engineering at the Georgia Institute of Technology when the research was conducted. In the U.S., about 2 million acres per year of federal forests were burned by prescribed fires from 1998 to 2006. When air-quality impacts are considered together, prescribed fires and wildfires contribute about 20% of the PM_2.5 emissions in the U.S.

Forest managers do realize that planned fires have undesirable air-quality impacts, and practices such as choosing good dispersion days for burning have been implemented to minimize the bad impacts, Tian explains. "However, a systematic analysis of the fires' impacts on the environment has not been performed, as we have done for other industrial or mobile sources," she says. One question that needs to be addressed in such analysis is when and how frequently prescribed fires should be applied.

The researchers were inspired by a February 2007 prescribed fire located about 80 kilometers southeast of Atlanta. Once the fire's smoke plume hit Atlanta, several air-quality monitors located in various parts of the city recorded PM_2.5 levels as high as 145 micrograms per cubic meter (µg/m³) for several hours, much higher than the 35 µg/m³ average over a 24 hour period set by the National Ambient Air Quality Standards (NAAQS) for this pollutant. "In addition, as the plume hit, 1 hour average ozone concentrations increased markedly from 63 to 95 parts per billion at one monitor," the authors write.

Tian, who now works at the Georgia Department of Natural Resources, and colleagues quantified the air-quality impacts of changing burning seasons and frequencies, and of controlling smoldering, with what is known as source-oriented air-quality modeling. This model is used to determine the source of certain pollutants and is capable of predicting air quality under different emissions and meteorological conditions.

Using the air-quality models, the researchers first reproduced the air-quality conditions during January, March, May, and July 2002, each of which represents a different burning season. Fall is not considered because it's neither a naturally preferred season nor practical for prescribed fires. They examined the air-quality impacts from existing fires during these months by simulating the impacts with and without emissions from prescribed fires. They then modified emissions from planned forest fires to characterize the effects of various management practices.

The modeling shows that burning frequency is an important factor, too. "If prescribed fires are less frequent, the biofuel burned in each fire is more, leading to larger emissions and air-quality impacts per fire," Tian says. "However, the long-term regional impacts on air quality are reduced since the annual burned area is reduced. It has great implications for attaining annual and 24 hour PM_2.5 NAAQS."

Colin Hardy, the acting program director for fire, fuel, and smoke science with the Rocky Mountain Research Station of the U.S. Department of Agriculture Forest Service, says that the authors do not address how the reduction in fuel through prescribed burning affects air-pollutant emissions from wildfires.

"They model prescribed fires and wildfires separately; they don't describe the dynamics of fuel loss resulting from fires among prescribed burning options and how that directly impacts both wildfire behavior and air-pollution impacts," Hardy says. Without this data, he adds, the research "has much less certainty that it could have."

"In the Southeast, except under rare circumstances, we are trying to manage the air-quality impacts from prescribed fires, and wildfires have a minor impact," Tian says. But this problem can become crucial for prescribed fires in the western U.S. and should be thoroughly studied, as suggested by Hardy, she adds. —CATHERINE M. COONEY