IGAC

IGAC research in Tropical Africa

The International Global Atmospheric Chemistry Project (IGAC) has been created to develop a fundamental understanding of the natural and anthropogenic processes that determine the chemical composition of the atmosphere and the interactions between atmospheric composition and biospheric and climatic processes. A specific goal is to predict changes over the next century in the composition and chemistry in the global atmosphere. IGAC is structured as five regional foci and the focus 2 of IGAC addresses natural variability and anthropogenic perturbations of tropical atmospheric chemistry. Tropical Africa is certainly the region where drastic changes on the global atmospheric chemistry can occur in the next future :
- because urban, industrial and agricultural growth are increasing rapidly in the African tropics, bringing strong increases in emissions of anthropogenically-derived, reactive, atmospheric pollutants.
- because the large fluxes of solar UV radiation, high temperatures and high water vapour content of the tropical atmosphere promote intense photochemistry all year round in this part of the lower atmosphere. Anthropogenic perturbations to this very active chemical system have the potential to increasingly affect human and plant health, atmospheric acidity, the levels of important greenhouse species and other climatically active atmospheric constituents (e.g., aerosols and clouds) as emissions grow.
- because deep convection in African tropical regions can provide a rapid vertical transport path that causes surface emissions to be lifted efficiently into the upper troposphere and lower stratosphere, so that atmospheric chemical effects of emissions are not simply limited to the lower atmosphere but have a more widespread influence.

Three of the four activities of focus 2 are currently underway within the study of the atmospheric chemistry in Tropical Africa: BIBEX (Biomass Burning Experiment), BATGE (Biosphere-Atmosphere Trace Gas Exchange in the Tropics), DEBITS (Deposition of Biogeochemically Important Trace Species).

Most of the recent international initiatives in the IGAC framework were related to trace gas emissions from the African savannas and more generally from biomass burning. DECAFE and SAFARI (Southern African Fire Atmospheric Research Initiative) programmes have provided new results. The main atmospheric effects observed during these experiments in Africa have the same order of magnitude and the same nature as those of industrial countries of the northern hemisphere. In particular, we can note in the African Tropics an increase of the tropospheric ozone, an acidification of the ecosystems by dry and wet deposition and also an increase of radiatively active trace gases.
In the conceptual scheme of biosphere-atmosphere interactions in tropical Africa, the presence of the savannas all in one block in the two hemispheres of Africa, the convergence of the trade winds towards the meteorological equator, the time-lag between the dry seasons and thus the burning in the two hemispheres contribute to a general pollution of the equatorial area by the products of biomass burning associated with the biogenic emissions of savanna and forest ecosystems.

The BIBEX research initiatives in Africa are addressed through a variety of research projects as for example :
- FOS/DECAFE 91 (Fire of Savannas), the first multidisciplinary experiment organized in Africa to study the gas and particle emissions from savanna fires,
- SAFARI 92; a cooperative international campaign with ground and airborne chemical and meteorological measurements in the source or near-source regions of South Africa. SAFARI 94 took place over southern Africa in wet season to characterize the seasonal change in atmospheric composition.

May be, the most exciting project in Tropical Africa is EXPRESSO (Experiment for Regional Sources ans Sinks of Oxidants - see MEDIAS Newsletter nr 5). This project is planned jointly with the BATGE and DEBITS activities for a 1996-1997 time frame in savannas of Central Africa and forest of Congo, with one field intensive in dry season with the study of the tropospheric chemistry produced by the biomass and natural products of savannas and forest and one field intensive in wet season to assess the chemistry with biogenic compounds only.

In 1994, DEBITS has launched a new project in AFRICA, called IDAF (IGAC DEBITS AFRICA), to assess in the long term the dry and wet atmospheric deposition at stations representative of the regional scale.