The most important sampling to date of biospheric and atmospheric chemistry in tropical rainforests will culminate in December, when National Science Foundation (NSF)-supported scientists complete a rare study of the African atmosphere. Ground-based and tower-mounted instruments and a research aircraft will support studies of biomass burning, rainforest-savanna boundaries and the influence of tropical vegetation on global air chemistry.
This fall's field work, which begins on Nov. 10, is the climax of a multiyear project called EXPRESSO, the Experiment for Regional Sources and Sinks of Oxidants. Leaders of the project are the NSF-supported National Center for Atmospheric Research (NCAR), in Boulder, Colorado; Paul Sabatier University, in Toulouse, France; the University of Brazzaville, in the Congo; and L'Institut Francais de Recherche Scientifique pour le Developpement en Cooperation in France.
"The EXPRESSO project is providing a comprehensive look at sources and sinks of ozone and other oxidants in the tropics," says Cliff Jacobs, program director in NSF's division of atmospheric sciences, which funds NCAR and EXPRESSO. "The central Africa region is one of several areas in the world that play an influential role in global chemistry cycles. EXPRESSO will help define the degree of influence central Africa has on these cycles."
Africa with its vast expanse of land near the equator, exerts a powerful influence on tropical and global air chemistry. Huge stretches of African savanna and rainforest are burned each fall and winter for agricultural and territorial 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. 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.
This fall's campaign is focused at two sites: one in a rainforest near the Congo's Nouabale-Ndoki National Park and the other in the rainforest-savanna transition zone near Bangui, the Central African Republic capital. EXPRESSO is collecting chemical data at these sites along with meteorological data throughout the study area to explore how the vegetation and fires interact with the atmosphere.
To sample the African atmosphere, EXPRESSO is 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
"There's never before been a program in this region that has combined all the elements of photochemistry, biochemistry, and meteorology," says NCAR's Pat Zimmerman.