NSF PR 97-75 - December 9, 1997
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Biogenic Emissions Higher than Expected over African
Savanna
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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