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EXPRESSO: Sampling a tropical atmospheric brew

by Robert Henson
UCAR Communications

Scientists from NCAR are working with an international group of collaborators on a major field study of the chemical and meteorological interactions between Africa's rain forests and savannas and the atmosphere above them. The Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) will include two intensive study periods in 1996-97 that will last up to four weeks each.

Why Africa? The continent exerts a powerful influence on global air chemistry, says Patrick Zimmerman of NCAR's Atmospheric Chemistry Division (ACD). Vast sections of grassland and forest are burned each year, pumping hydrocarbons and oxides of nitrogen (NOX) into the air. These react in the presence of sunlight to produce ozone and other smoglike products. Satellite images sometimes show a bridge of tropospheric ozone that extends from Africa to South America. Ozone levels beneath these plumes can reach 100 parts per billion, roughly the same as on a high-pollution day in a major city.

Humans have been burning the African savanna for centuries to clear land for crops or a variety of other reasons. Though the burning isn't new, the atmospheric effects--when combined with those of industrial processes--could be changing. The burning of the savanna has been shown to exert a dominant influence on the ecology and the atmospheric chemistry of most of the tropics through much of the year, say project planners. Because of the vast extent of the tropics, any understanding of global air chemistry has to come to grips with the chemical fluxes in and above the region. But that understanding has come slowly, if only because mounting a research project in a place like equatorial Africa is not easy.

EXPRESSO will perform an intensive survey of the atmosphere along Africa's savanna-to-forest transition zone between 8 and 2N. One of the first goals of the experiment was to choose a cross-sectional study area that would be politically and logistically feasible as well as representative of the larger surroundings. The Central African Republic (CAR) was a natural choice, in part because it is a former French colony, where the French have already done a lot of atmospheric chemistry research and where there's a long history of collaboration between them and NCAR. Zimmerman and Lee Klinger, also of ACD, are serving as EXPRESSO's U.S. coordinator and manager, respectively, in tandem with two French scientists, Robert Delmas and Jean-Pierre Lacaux of the Laboratory of Meteorology at Paul Sabatier University in Toulouse. Cooperative work between French and ACD scientists began in the 1980s with the Dynamics and Chemistry of the Atmosphere in Equatorial Forests experiment (whose French acronym is DECAFE).

EXPRESSO will build on the findings of DECAFE and other work in the area. "There have been a few studies done in the continental tropics to measure ambient chemical concentrations and extrapolate those to other areas," Zimmerman notes, "but we still need to nail down the sources and sinks of important chemical species which may affect the regional and global oxidant balance."

To pave the way for EXPRESSO field work, Zimmerman and ACD scientist Alex Guenther went to Africa to scout potential study regions in January 1994. Klinger met with two French colleagues in the CAR and Congo later that year and has made two more trips to the republic this year, accompanied by other principals such as ACD deputy director Paul Sperry and logistics expert Karyn Sawyer (UCAR's Joint International Climate Project/Planning Office). In January, the visitors covered over 2,000 kilometers in surveying the region.

The trips also have served as "pre-EXPRESSO" experiments, helping the researchers to focus the science for the intensive observing periods. On his most recent visit, Klinger and his colleagues collected volatile organic carbon emissions from a wide range of plant species and sampled ambient air concentrations.

The study region finally chosen extends from the northeast part of the CAR southward into the rain forests of the Congo. Across this band, about 800 kilometers long, average annual rainfall jumps from 80 to 180 centimeters per year. The north-south wobbling of the equatorial convergence zone produces wet conditions at the north end of the study area while the south is dry, with rains to the south when the north is dry. EXPRESSO plans to hold two intensives separated by about six months to capture the dynamics and chemistry of both climatic modes.

Fixed towers and tethered balloons at each of the four major field sites for EXPRESSO (listed below with the collaborators at each site) will allow for intensive sampling over areas ranging from the size of several football fields to several square kilometers. Collaboration with local CAR and Congolese scientists will take place at each location. The sites are

Several students hope to glean material from the experiment for their doctoral dissertations. Lee Vierling, a NASA fellow and doctoral student at the University of Colorado, is working on characterization of remote sensing over tropical regions. Susan Canney (Oxford University, England) plans to do her dissertation on the effects of land-use change in central Africa.


Atmospheric chemist Lee Vierling samples gases emitted by plants in the Congo in preliminary studies to help plan the intensive field phases of EXPRESSO. (Photo by Lee Klinger.)

Meteorological groundwork for EXPRESSO is being overseen by Greg Jenkins, a former postdoctoral fellow in the NCAR Advanced Study Program who is now at Pennsylvania State University. Jenkins plans to run the Penn State/NCAR mesoscale model, version 5 (MM5), for the study area before the intensives begin. The picture of local climate thus gained will help guide the deployment of radiosondes and ground-based stations.

"There's never been a program in this region that has combined all the elements of photochemistry, biochemistry, and meteorology," says Zimmerman. "This could really be a landmark study for the atmospheric chemistry community."

EXPRESSO details are available on the World Wide Web from the ACD home page, http://acd.ucar.edu/ To view a CAR-produced video spotlighting the nation, contact Zimmerman (303-497-1406 or zimmer@ucar.edu). The July 1995 issue of National Geographic featured the Congolese national park that includes the Bomassa field site.


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