UCAR > Communications > Staff Notes > July 1995 Search

ACD Samples a Tropical Atmospheric Brew with EXPRESSO

Taking it slowly and surely, the Atmospheric Chemistry Division is gearing up for what looks to be its key field program of the latter 1990s. The Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) is a study of the chemical and meteorological interactions between Africa's rain forests and savannas and the atmosphere above them. EXPRESSO will involve aircraft and ground crews from NCAR, the French government, and numerous other collaborators for 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 Pat Zimmerman, head of ACD's Biogeochemistry Group. 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 smog-like products. "In some cases," says Pat, "you can see a bridge of tropospheric ozone, as derived from satellite imagery, that extends from Africa to South America." Ozone levels beneath these plumes can reach 100 parts per billion, roughly the same as on a bad air day in Denver. Along with detrimental effects on creatures and plants, the low-altitude ozone affects the cycling rate of other chemical reactions.

Susan Canney, second from left (Oxford University) joins local youth at Boali Chutes, Central African Republic, during a recent visit for EXPRESSO. (Photo by Lee Klinger.)

Humans have been burning the African savanna for centuries, says Pat. The reasons are many: to improve forage quality, to clear land for crops, or simply to uphold tradition. "The grass can be two or three meters high in places. I think part of why they burn it is simply to be able to see. It's also a way to show ownership-if you don't burn it, you don't own it." A breakdown in tribal governments and associated constraints has produced what ACD's Lee Klinger calls "essentially a deregulation of burning activities."

Though the burning isn't new, the atmospheric effects-when combined with industrial processes-could be changing over time. According to the EXPRESSO draft plan, the burning of 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." 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 places like equatorial Africa is not easy.

"It's hot and humid," notes Pat, "and that's hard on people and instruments alike. If it were easy, more work would already have been done there."

Pat Zimmerman and Lee Klinger. (Photo by Carlye Calvin.)

The Lay of the Land

EXPRESSO will perform an intensive survey of the atmosphere along Africa's savanna-to-forest transition zone between 8°ree; and 2°ree;N. One of the first goals of the experiment's planning team 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 of its status as a former French colony.

"The French have done a lot of atmospheric chemistry research there," says Pat. He and Lee 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 (whose French acronym is DECAFE).

"There were several things the French couldn't do in DECAFE that ACD could do," says Pat. This included measurement of hydrocarbon fluxes and airborne measurements of the hydroxyl radical carried out by Fred Eisele and colleagues. 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. But we still need to nail down the sources and sinks of important chemical species which may affect the regional and global oxidant balance."

ACD already has begun paving the way for EXPRESSO field work. Pat went to Africa with ACD's Alex Guenther to scout potential study regions in January 1994. Lee met with two French colleagues in the CAR and Congo later in 1994 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. Each of these visits, according to Pat, has helped to cement relations with officials in Africa and provided reassurance that the experiment, though arduous, is indeed doable.

The trips also have served as "pre-EXPRESSO experiments. We've planned and operated them to help us focus the science for the intensives." On his most recent visit, Lee and his colleagues collected volatile organic carbon emissions from a wide range of plant species and sampled ambient air concentrations, helping to give ACD an idea of the range to be expected during the intensives.

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.

Two aircraft are expected to be on hand: the C-130 from NCAR (pending approval by the NSF's facility board) and the French Arrat. "The C-130 would do long-range trajectory flights, along with vertical flights, to characterize the chemical climatology of the region and to trace the smoke plumes as they progress," says Pat. "The Arrat will look more closely at fluxes and characterize the region and the surface study sites to ensure they're representative of larger regions."

Who and Where

ACD's commitment to EXPRESSO is broad. According to Pat, as many as 25 staff from nearly every major ACD program could be going into the field or providing support in Boulder. This includes the divisional groups for global and stratospheric studies, regional and process studies, and stratospheric/tropospheric measurements, as well as teams for analytical, optical, and tunable laser-based measurements. Chris Cantrell (ACD) will serve as aircraft coordinator, overseeing flights out of the CAR's capital city of Bangui.

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 use the experiment to glean material for their doctoral dissertations. ACD's 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 (Green College, Oxford University, England) plans to do her dissertation on the effects of land-use change in central Africa. "In addition to participating in the scientific research on our last trip, Susan served as our interpreter and did a terrific job," says Lee.

Meteorological groundwork for EXPRESSO is being overseen by Greg Jenkins, a former postdoctoral fellow in the 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.

The EXPRESSO study area extends from north-central parts of the Central African Republic to the northern Congo. The locations of the four EXPRESSO field sites and the area's ecological characteristics are depicted below. (Illustration by Mike Shibao.)

Life in the CAR

One of Pat and Lee's primary concerns for now is to reassure supporters and potential participants that the experiment isn't as risky as it might seem. The CAR ended 14 years of military dictatorship in 1993 but is now relatively stable, says Pat. Because of their previous work in the area, he adds, "the French are almost nonchalant about this project."

Aircraft operations will be based in Bangui, a contemporary city where "people water-ski down the Ubangui River," according to Pat. Staff involved in this part of EXPRESSO would be going "mainly from the hotel to the airport and back." As for the four field sites, Pat says that "they're a little more remote. This is one of the least populated areas in the world, with fewer than two people per square kilometer, fewer than in the Sahara." Despite the isolation, Lee found the back country reasonably nonthreatening during his recent visits. For instance, he says that "the small population helps minimize the risk of disease."

Among the hundreds of Peace Corps volunteers who have served in rural parts of the CAR is Elizabeth Sulzman. She's now at NCAR's Climate Systems Modeling Program, which is collaborating with ACD on EXPRESSO data analysis. Elizabeth notes that life in the rural CAR isn't exactly cushy: "My home was a mud hut with no electricity, no running water, an outdoor hole in the ground for a bathroom, and plenty of bugs of all sorts to keep me company." Still, she adds, "I had an amazing experience. The village where I lived, which had a hundred or so residents, practically adopted me. They'd bring by a huge ream of bananas-far more than I could ever eat-and say, `Here, we have a present for you.' If you make even the slightest amount of effort to speak their language, they just love you for it. I found them to be a really open and warm group of people."

To keep all potential collaborators informed, and to gain perspective from previous overseas projects, ACD has been working closely with ATD director Dave Carlson, Brigitte Baeuerle (TOGA COARE International Project Office), and the NCAR committee on field deployment safety. "Americans don't have much experience doing research in Africa," says Pat. "We are taking all safety concerns very seriously; we don't want to gloss over them. We are trying to be as thorough and objective as possible." Should the project come off as planned, he adds, the benefits will have been well worth the sweat. "There's never been a program in this region that has combined all the elements of photochemistry, biochemistry, and meteorology. This could really be a landmark study for the atmospheric chemistry community." --BH

EXPRESSO details are available from ACD on the World Wide Web from the ACD home page, http://acd.ucar.edu/ To view a CAR-produced video spotlighting the nation, contact Pat (ext. 1406, zimmer@ucar.edu). National Geographic will feature the Congolese national park that includes the Bomassa field site in an issue to be published this summer.

Lee Klinger (left) holds an enclosure containing local vegetation while Jim Greenberg (right) draws air from the enclosure. Samples taken during this and other preliminary EXPRESSO visits are being analyzed in Boulder to help guide the experiment's field phases to come. (Photo by Lee Vierling.)


The folks who go to rural Africa for EXPRESSO will get a rare glimpse of some of the continent's most treasured and unusual wildlife. Lee Klinger's most recent visits are testimony. "I've seen several gorillas-lowland, silverback, and forest-bongo antelope, alligators, six-meter pythons, water buffalo, parrots, and monkeys." Lee says the ubiquitous monkeys "sort of follow along in the trees and chatter at you. They don't bother you, but they're definitely curious." Then there's the giant bush pig, which Pat says looks "kinda like a huge guinea pig. It's really a strange-looking critter."

CAR Notebook

Population: 3.2 million
Location: Central Africa; south of Chad and Sudan, north of Zaire and the Congo, and east of Cameroon
Total area: 622, 980 square kilometers, roughly the size of Texas
Official language: French
Ethnic makeup: Baya, 34%; Banda, 27%; Mandija, 21%; Sara, 10%; other, 8%. Although Sango make up a tiny fraction of the populace, theirs is the primary trading language.
Politics: Democratic elections were restored in 1993. Women have held high elective office: Elizabeth Domitien was prime minister in the 1970s and Ruth Rolland ran for president in 1993.
Gross national product per capita: $405
Trade: The CAR is self-sufficient in food. Major exports include iron, diamonds, cotton, timber, and coffee.
Environment: Considerable rain forest destruction has been countered by recent conservationist policies. Elephant kills fell from 80,000 per year in the 1970s to 15,000 in 1987 following a hunting ban in 1985.

Source: The Dorling Kindersley World Reference Atlas, 1st American Edition (New York: Dorling Kindersley Limited, 1994)

In this issue...
Other issues of Staff Notes Monthly


Edited by Bob Henson, bhenson@ucar.edu
Last revised: Wed Mar 29 17:28:10 MST 2000