Introduction

Biospheric processes on the Earth's land masses have profound effects on the composition of the entire global atmosphere. In this context a key role is played by the tropical belt which accounts for about 40% of the planet's biomass and 60% of net primary production per year. Biomass burning is one of the factors which influences these processes and is mainly related to human landuse and land management (deforestation, agricultural residue burning, hunting, fire wood use, etc.). Within the tropical belt, the African continent, with large areas of savanna, contributes significantly to the total global amount of biomass burned. In Africa, there are two main periods of burning. In the northern hemisphere, the fire season generally runs from November to March, while in the southern hemisphere, it runs from May to October (Koffi et al., 1996).

Several studies have developed different methodologies to assess the quantity of burned biomass since the problem was first approached by Seiler and Crutzen (1980). Estimates of burned area in the Tropics based on statistical surveys have been used to produce a quantitative assessment of the phenomenon for the different sources contributing to the total amount of biomass burned, and gases and aerosols emitted (Seiler and Crutzen, 1980; Hao et al., 1990; Crutzen and Andreae, 1990; Andreae, 1991; Hao and Liu, 1994). Remote sensing techniques developed in the past decades have enabled a more detailed knowledge of the spatial and temporal distribution of fire occurring in the biomes of the world. Vegetation fire monitoring using satellite images has been performed with two different methodologies: active fires and burned-area detection. The first relies on measuring the thermal emission of the combustion process, hence enabling the mapping of fires occurring when the satellite passes overhead, while the second is based on fire-induced changes in the spectral characteristics of the land cover. Several algorithms have been developed using the available sensors in order to detect both active fires (Dwyer et al., 1998; Stroppiana et al., 1998; Koffi et al., 1996; Eva and Flasse, 1996; Elvidge et al., 1996; Prins and Menzel, 1992) and burned areas (Barbosa et al., 1998c; Eva and Lambin, 1998; Eva et al., 1998; Kasischke and French, 1995; Kasischke et al., 1993) from regional to global levels.

In this work, spatial and temporal distributions of active fires for the African continent were derived from the AVHRR (Advanced Very High Resolution Radiometer) sensors on the NOAA (National Oceanic and Atmospheric Administration) satellite series. A quantitative assessment of burned biomass and gas and aerosol emissions was produced for the period November 1992-October 1993 on a monthly basis using a temporal extract of the GFP (Global Fire Product) daily fire maps (Dwyer et al., 1998; Stroppiana et al, 1998). This data set, which covers a 21 month period from April 1992 to December 1993 was derived from AVHRR-HRPT 1.1 km (High Resolution Picture Transmission) imagery, in the frame work of the IGBP-DIS (International Geosphere-Biosphere Programme Data and Information System) project.

Seasonal and inter-annual variability of the same parameters were analysed for the four year period November 1984-October 1988 using monthly active fire maps derived from AVHRR-GAC (Global Area Coverage) 4.5 km images (Koffi et al. 1996).

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