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ACD treks to Africa for EXPRESSO intensive

Some of the most important sampling to date of biospheric and atmospheric chemistry in the tropical rainforest is taking place this November and December. ACD's Bill Baugh, Jim Greenberg, Alex Guenther, Peter Harley, and Pat Zimmerman are collecting data from the heart of Africa, with support in Boulder from principal investigator Lee Klinger. They're being joined by Brad Baker and Lee Vierling (University of Colorado at Boulder) and other colleagues from the United States, the Central African Republic (CAR), the Congo, France, and Italy. The intensive field work, supported by NSF, is the culmination of EXPRESSO, the multiyear Experiment for Regional Sources and Sinks of Oxidants.

The NCAR scientists and their colleagues had to scramble to put the current intensive together on a shoestring. "Originally EXPRESSO was more ambitious than it turned out to be," says Lee. Field work began in 1995, but the budgets for fiscal 1996 and 1997 were cut substantially, forcing ACD to mount the past year's activities on a relatively scant $200,000. "We've saved hundreds of thousands of dollars by using surplus equipment," says Lee. Pacific Northwest Laboratories donated a 60-meter tower for the cost of shipping. The experiment is using an Air Force truck that once hauled missiles through Wyoming and a ten-year-old ACD generator adapted for the project. Only the trailer/laboratory is new.

EXPRESSO's motive is to document the biosphere-atmosphere chemical interactions taking place in and near the African tropics. Huge stretches of savanna are burned each fall and winter for agricultural and other purposes. The fires produce large amounts of hydrocarbons and oxides of nitrogen, which interact with sunlight to produce ozone and other smoglike products--often at levels approaching those of a high-pollution day in a major city. Satellite pictures show that the plumes of ozone stretch, at times, as far as South America.

Lee and other ACD staff made several trips to Africa in the past three years to take preliminary air samples, install instruments, and make arrangements for this fall's field campaign. These visits spanned the entire ecological gradient from savanna to rainforest, which runs from the northeast CAR (around 8 degrees N) to the northern Congo (around 2 degrees N).

In this map of the EXPRESSO study region, X's indicate the five regions where preliminary work was conducted over the past two years. The primary sites for this fall's intensive are the two southernmost ones, at Bomassa/Ndoki/Mbeli and Maboké. (Illustration by Peter Bockenthien.)
This fall's intensive, originally planned to take place at four sites along the transition zone, has been scaled back to two main sites: one in rainforest near the Congo's Nouabalé-Ndoki National Park and the other in the savanna-rainforest transition zone near Bangui, the CAR capital. The scientists will use data from the preliminary visits to help fill out the picture along the transition zone.

EXPRESSO is collecting chemical data at the two main sites--along with meteorological data throughout the study area--to explore how the vegetation and fires interact with the atmosphere. Ground-based and tower-mounted instruments and a research aircraft will support studies of biomass burning, rainforest-savanna dynamics, and the influence of tropical vegetation on global air chemistry. "There's never been a program in this region that has combined all the elements of photochemistry, biochemistry, and meteorology," says Pat.

The field work, which began on 10 November, involves NCAR; Paul Sabatier University, in Toulouse, France; the University of Brazzaville, in the Congo; and L'Institut Français de Recherche Scientifique pour le Développement en Coopération. On hand for the intensive, along with the five NCAR scientists and technicians, will be scientists from these institutions as well as the California Institute of Technology, Germany's Max Planck Institute for Chemistry (in Mainz), and the New York-based Wildlife Conservation Society.

Getting there is half the fun

The experiment's logistics were challenging. Early this year, Lee and African botanist David Kenfack met a truck and trailer that were shipped across the Atlantic, drove them across Cameroon, and then accompanied them on a barge up the Sangha River to the Congolese site, located near one of Africa's most remote and best-preserved rainforests. Along the way, they slid off muddy logging roads, shoveled sand out of the barge for several days prior to departure, and found themselves accompanied by crowds of riverine hitchhikers on the three-day barge trip. At night, the team sacked out on deck beneath the tropical canopy with the barge tied up along the edge of the Sangha.

Autumn in the tropics isn't all sunshine, mangoes, and greenery. "It's mostly evergreen," says Lee, "but even in the heart of the rainforest, some of the trees do drop their leaves." Temperatures under the jungle canopy range from around 22 to 30 degrees C, but occasionally dip below 20 degrees after a rainstorm. "We had sleeping bags in the jungle last time, and we were glad we did. You can definitely get chilled."

While on site, the EXPRESSO team will sample the African atmosphere using

• enclosures that measure atmospheric exchange from leaves and soil

• a 60-meter tower studded with meteorological instruments and air samplers

• a balloon tethered for brief periods at heights of up to a kilometer

• France's Arat research aircraft, flying out of Bangui

Though relying on many standard sensors, EXPRESSO will serve as the debut for some specialized equipment, including a new isoprene flux sampler built by Alan Hills (now in ACD) that is "the world's best," according to Lee. Isoprene, a fast-reacting chemical emitted by trees, plays an important role in the atmosphere's overall chemical balance. "Measuring isoprene is a key to this experiment," says Lee.

Once the intensive field work is done, some of the instruments for monitoring weather and air chemistry will be maintained by the European and African collaborators for longer-term EXPRESSO research. "These studies," says Alex, "will look at carbon storage, long-term ozone effects, and other things that don't require a lot of high-tech support." For the African scientists, EXPRESSO has provided a rare opportunity to join a major international field program on their home turf. According to Lee, "For some of them, it's the pinnacle of their scientific careers thus far."

The big picture

EXPRESSO may shed light on a global problem in biosphere-atmosphere chemistry. Plants take up vast amounts of carbon, particularly in the lush tropics, but plants and soils also release carbon, and the overall cycling in the tropics may be affected by the perennial fires. "One big question we're asking," says Alex, leader of the ACD contingent at this fall's intensive, "is whether the tropics serve as a net source or a net sink for carbon. Right now, we don't really know.

"One study won't answer the question, of course, but it will provide a starting point. Tropical forest landscapes are an important component of the global carbon cycle, so any imbalance there can have global consequences." •BH

Jean-Marie Bissi, of Bomassi, the Congo, joined the EXPRESSO crew for preliminary site work.

Participants in the EXPRESSO field intensive of spring 1996 gather for a group portrait. From left to right:
Front row--Jean-Claude Emil, Jean-Marie Bissi, Giscard Kobo, Lee Klinger
Second row--Dominique Serça, François Dougali, Clobite Bouka-Biona, Jules Loemba-Ndembi, Bill Flens, Franco Ngembo
Back row--Dan Wentworth, Alex Guenther, Peter Harley

Once the EXPRESSO trailer/laboratory reached its destination at the Congo, it had to be unloaded using specially tailored slats of wood--each of which took 15 people to lift.

Shipment of the EXPRESSO trailer/laboratory included a treacherous drive across unpaved logging roads in eastern Cameroon.

NCAR scientists Peter Harley (left) and Alex Guenther prepare instruments in the cramped quarters of the EXPRESSO trailer/laboratory.

Workmen prepare a section of the tower erected for EXPRESSO.

A piece of EXPRESSO's 60-meter instrumented tower is moved into place.

Instruments for measuring wind, temperature, and moisture studded the tower.

Several days of work were required to prepare a barge (below) for shipment of the EXPRESSO trailer/laboratory (above) up the Sangha River.

David Kenfack, a botanist for Cameroon's national herbarium, filters water.

Dominique Serca (Paul Sabatier University) and Clobite Bouka-Biona (University of Brazzaville) take a break while working on the EXPRESSO instrumented tower.

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Prepared by Jacque Marshall, jacque@ucar.edu, 303-497-8616