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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).
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.
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."
"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
