Fresh smoke lowers nitrous oxide estimate.

Fresh smoke lowers nitrous oxide estimate

Biomass burning -- the combustion of organic matter in forest fires, wood stoves and "slash and burn" land clearing -- may contribute far less nitrous oxide to the atmosphere than previously thought, a team of U.S. and Canadian scientists reports. Atmospheric concentrations of this gas -- which not only contribute to the greenhouse effect but also destroy stratospheric ozone -- have been growing in recent years. If confirmed, the new findings would seem to indicate that major sources of this worrisome air pollutant have either been seriously under-estimated or ignored.

Wesley R. Cofer III of NASA's Langley Research Center in Hampton, Va., and his colleagues sampled air from above a forest fire near Morley Lake in Ontario, Canada. Cofer not only analyzed combustion-gas levels in the air during the fire -- using a new helicopter-mounted gas chromatograph he developed -- but also collected additional air samples in bottles for periodic analysis over the next 21 days. And in the Feb. 21 NATURE, these researchers report finding that chemical reactions within the stored samples generated nitrous oxide ([N.sub.2.O]). Quantifiable changes, first detected within 4 to 8 hours, reached significant increases--on the order of 20 percent -- within 10 to 21 days.

Because chemists have traditionally regarded nitrous oxide as inert in collection jars and because it often takes hours or days to transport samples from a burn site to a laboratory, pollutant-assaying delays commonly occur, observes Joel S. Levine, an atmospheric scientist at Langley and co-author of the study. The new data now suggest that previous estimates of nitrous-oxide emissions -- largely based on such delayed analyses -- have seriously exaggerated biomass burning's role in growing atmospheric levels of this pollutant, Levine says.

The new biomass-burning data parallel findings two years ago showing that fossil-fuel combustion gases can react within collection jars to form additional nitrous oxide (SN: 11/26/88, p.340).

Previously, researchers have estimated that combustion of biomass and fossil fuel together account for 40 to 50 percent of the annual atmospheric increase of nitrous oxide. Levine now estimates that biomass burning yields no more than 7 percent of the global production of nitrous oxide. As a result, he says, these new estimates fail to account for about 30 percent of the nitrous oxide emitted worldwide.

Scientists should confirm Cofer's results by performing equivalent experiments in the tropics, the site of most large-scale biomass fires, says F. Sherwood Rowland, an atmospheric chemist at the University of California, Irvine. The new results should also prompt scientists to "look even harder for other new sources of nitrous oxide," and to re-examine each known source of the gas in order to "balance the nitrous-oxide equation," says Mark H. Thiemens, a chemist at the University of California, San Diego.

Thiemens and his San Diego colleague, William C. Trogler, recently reported that the production of nylon may account for up to 10 percent of the annual increase in nitrous oxide levels (SN: 2/23/91, p.117). Recent studies by Levine indicate that soil bacteria feeding on ammonium compounds in the ash of biomass fires may also excrete significant amounts of nitrous oxide.