Biogenic Emissions Higher than Expected over African Savanna
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Air-pollution-related hydrocarbon emissions from vegetation are much higher than expected over the African savanna (flat tropical grasslands), while those coming from the rain forests are somewhat lower than prior estimates, according to scientists. The National Science Foundation (NSF)-funded research team is mapping natural and human-caused trace gas emissions across the African continent in a project called EXPRESSO, the Experiment for Regional Sources and Sinks of Oxidants.
Because of biomass burning, smog levels over Africa often approach those of a high-pollution day in a major city, says Alex Guenther, a scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Scientists have gathered and analyzed data from previously unsampled remote regions to gain a better understanding of Africa's influence on the composition of the earth's atmosphere.
"Africa's impact on tropical and global air chemistry is considerable, but we haven't had good measurements until now," explains Guenther. "Because the study area includes terrain representative of sub-Saharan Africa, we can map emissions over the entire continent for the first time."
Huge stretches of African savanna and rain forest are burned each fall and winter for agricultural and territorial purposes. The fires produce large amounts of hydrocarbons and nitrogen oxides (NOx). These react in the presence of sunlight to produce low-altitude ozone and other smog-like products. Satellite pictures show that the plumes of ozone stretch, at times, as far as South America. While stratospheric ozone shields us from ultraviolet radiation, closer to the earth this molecule damages forests and crops; destroys nylon, rubber, and other materials; and injures or kills living tissue. Ozone is a particular threat to people who work or exercise outdoors or who suffer from respiratory problems.
The team is also studying the role of isoprene and other volatile organic compounds (VOCs) released by plants into the atmosphere. Isoprene plays a role in low-altitude ozone formation and in the atmospheric lifetime of methane--a greenhouse gas. Isoprene fluxes from the African forest are lower than those reported for a tropical rain forest in the Amazon basin.
EXPRESSO gathered data during the dry season in November and December 1996 along a 800-kilometer (500-mile) band from the savannas of the Central African Republic in the north to the tropical rain forests of the Republic of Congo in the south. A French ARAT research aircraft measured winds, temperatures, water vapor, and radiation from the visible and invisible spectrum, in addition to NOx, ozone, and several other chemical compounds. Volatile organic compounds were measured by aircraft and ground instruments. A 60-meter (200-foot) tower installed by NCAR in collaboration with the University of Brazzaville on the edge of the Nouabale-Ndoki National Park in the Republic of Congo gave researchers access to the rain forest canopy and the layer of air above it. The team also used satellite data to map fire distribution. During one 24-hour period, fires were burning over 25,000 square kilometers (15,500 square miles).
EXPRESSO results will next be used to evaluate and improve computer models of air chemistry on regional and global scales.
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