Biomass burning ignites concern.
Burning practices around the world pollute the atmosphere far more than scientists had assumed, leaving once-pristine tropical regions bathed in high levels of harmful ozone and acid precipitation, researchers reported last week in Williamsburg, Va., at the first international conference to address the issue of biomass burning.
Scientists once viewed air pollution as a problem primarily of smokestacks and tailpipes, but a growing body of evidence shows that intentionally lit fires over the globe also contribute significantly to pollution levels and could play a major role in changing Earth's climate. "[Fires related to] agricultural practices and land-use conversion have a very large impact on the overall chemistry of the atmosphere. That is something that has come as a surprise," says Paul J. Crutzen, an atmospheric chemist from the Max Planck Institute for Chemistry in Mainz, West Germany.
Fires cover about 2 to 5 percent of the Earth's land areas each year, and scientists at the conference estimate that humans light more than 95 percent of them, says Joel Levine of the NASA Langley Research Center in Hampton, Va., who organized the meeting. The fires burn primarily in tall-grass savannas, in tropical rain forests and on farms around the world.
Concern over the effects of biomass burning has grown since the late 1970s, when Crutzen first alerted scientists to the widespread emission of gaseous pollutants from fires. In the last few years, researchers have carried out studies in Africa, South America, the United States and elsewhere to measure how biomass burning affects the local and global environment.
Among the results reported at the meeting:
* Levels of ozone in the troposphere (lower atmosphere) over certain regions of tropical Africa are approaching values shown to be toxic to plants, says Meinrat Andreae of the Max Planck Institute. Tropospheric ozone, which irritates the eyes and lungs as well as harms vegetation, forms when sunlight energizes chemical reactions between nitric oxide, hydrocarbons and carbon monoxide -- three kinds of chemicals emitted during combustion. It also adds to the greenhouse effect.
In field experiments in the Congo, researchers from Germany, France and the Congo measured monthly average ozone levels as high as 40 parts per billion at ground level and 100 parts per billion in air during the dry season, when fires are worst.
The ground measurements verify satellite data collected over the past few years, which suggest that levels of tropospheric ozone over a wide region of the tropics are much higher than researchers believed. Without confirmation from ground-based studies, many scientists questioned the high values measured by satellite.
The satellite data indicate that ozone over broad regions of unindustrialized West Africa reaches levels comparable to those over the heavily industrialized eastern United States, says Jack Fishman of NASA Langley. Satellite measurements also show high ozone levels over Indonesia and South America, particularly Brazil. Moreover, Fishman reports, aircraft and satellite data collected last year suggest that ozone from fires in Africa travels clear across the Atlantic and can be measured in easternmost Brazil.
* Measurements taken over the last several years in the Ivory Coast and Congo reveal unexpectedly high acidity in the rainwater. "Our data show that acidity fluxes in regions of the tropics are of the same order of magnitude certainly as those we get in the eastern United States," says Andreae. Other researchers have found high acidity levels in rain falling over the Amazon.
Much of the acidity comes from gases emitted by fires, says Levine. Combustion of vegetation produces nitric oxide and other gases that get converted in the atmosphere into nitric acid and organic acids. Crutzen adds that researchers don't really know how the acidic precipitation and high ozone levels affect tropical ecosystems. Future research must address this question, he says.
Levine says scientists expect pollution from fires in tropical regions to worsen as the rapidly growing population places greater stress on the surrounding environment.
* New studies highlight the importance of carbon monoxide emissions from fires. Carbon monoxide tends to lower the concentration of hydroxyl radicals, so-called "detergent" molecules that clean the atmosphere by reacting with pollutants. The reduction in hydroxyl concentrations also contributes indirectly to the greenhouse effect by lengthening the tropospheric life spans of ozone and methane, another greenhouse gas.
From test fires, Crutzen estimates that biomass burning contributes about half the carbon monoxide present in the atmosphere; other recent studies suggest it contributes about one-fourth the carbon monoxide. Scientists have estimated that the carbon monoxide level itself has been growing at about 1 percent per year.
* Until the past few years, researchers studying biomass burning focused on fires associated with the clearing of tropical rain forests. But recent findings have revealed the importance of two other practices that appear to burn as much material as land-clearing fires: the worldwide burning of agricultural waste left over after harvest, and the annual burning of grasslands in Africa, South America, Australia and elsewhere to improve grazing.
Scientists at the meeting also discussed the importance of cooking/heating fires in heavily populated regions of Asia, and a huge boreal forest fire in 1987 in China and the Soviet Union. Satellite measurements indicate the 1987 blaze may have been the single largest fire in the last 500 to 1,000 years, says Levine.
As a result of the recent findings, researchers have boosted their estimates of the amount of biomass burned by fires each year. Only a few years ago, Crutzen says, experts thought biomass fires burned about 2.5 billion tons of carbon annually; now they are speaking in a range of 3 billion to 4 billion tons each year.
Using current estimates of biomass burning, two research groups calculate that fires and the process of deforestation contribute about 15 percent of the warming expected from the buildup in greenhouse gases. This means policies attempting to slow global warming will have to limit biomass burning, insists Daniel Lashof of the Natural Resources Defense Council in Washington, D.C.
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|Date:||Mar 31, 1990|
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