Smog Components Sent Into Air by Test Fire

“Tremendous amounts” of pollutants from automobile exhaust, deposited over decades in brush around Los Angeles, may become airborne during brush fires, according to scientists reporting the first results of a December experimental fire set in the Angeles National Forest.

Smoke samples gathered by an instrument-laden aircraft above that fire reveal that concentrations of some nitrogen- and sulfur-containing compounds were as much as 300 times the levels previously measured above isolated forest fires in other parts of the country, including the Pacific Northwest, Lawrence Radke, a professor of atmospheric science at the University of Washington, said Tuesday.

These compounds, including nitrogen oxides and sulfur dioxide, are major components of Los Angeles’s smog and can form acids that may contribute to acid rain.

The pollution data were the first results to emerge from the December fire, when 300 scientists deployed a fleet of aircraft and scores of instruments to study a 300-acre brush fire set in Lodi Canyon in the rugged hills above San Dimas.

The project was the first large-scale effort to study the contribution of brush fires to air pollution and to study some aspects of “nuclear winter,” the controversial theory that smoke from fires burning after a nuclear war may block sunlight and chill the Earth.

On Tuesday, two dozen scientists representing agencies such as NASA, the U.S. Forest Service, and the Environmental Protection Agency discussed the preliminary findings at a meeting at a Los Angeles County Fire Department station in La Canada-Flintridge.

Scientists had long theorized that forest fires are a significant source of some of the pollutants that contribute to Los Angeles’ smog problem, said Philip Riggan, the U.S. Forest Service scientist who ran the Lodi research project. Previously, “spectacularly high” levels of nitrates--another type of nitrogen-containing pollutant--have been found in runoff from chaparral-covered slopes after rainstorms. But no one anticipated that such high levels would be found in smoke, he said.

The initial findings suggested that a source of the nitrogen compounds may have been the chapparal itself because chaparral, the unique mix of shrubbery indigenous to Southern California, is naturally rich in nitrogen. But Radke discounted that possibility, saying that burning chapparal cannot alone account for the huge amounts of nitrogen oxides.

Only one forest fire in the Pacific Northwest produced smoke with nitrogen oxide levels similar to those collected at Lodi, and that fire was only 15 miles downwind from a large coal-fired power plant, which--like automobile traffic--is a major source of such pollution, Radke said.

Other pollutants, including lead, were collected during small test burns of one-acre plots around Lodi Canyon before the large fire was set, said Darold Ward, a U.S. Forest Service atmospheric scientist. The lead probably also had come from deposited automobile exhaust, Ward said. He said the lead concentrations were four times those found in fires set away from urban areas.

The effect of the nitrogen and sulfur compounds could vary tremendously depending on the intensity of a fire, which would determine how high in the atmosphere the compounds would be “pumped,” he said. This could mean the difference between a bad day of smog locally, or subsequent acid rain hundreds of miles distant, Riggan and others said.