BIOGENIC EMISSIONS AND BIOMASS BURNING INFLUENCES ON THE CHEMISTRY OF THE FOGWATER AND STRATIFORM PRECIPITATIONS IN THE AFRICAN EQUATORIAL FOREST

An automatic wet-only precipitation collector and a fogwater collector were operated in the coastal forest of equatorial Congo (Dimonika), for a complete seasonal cycle (November 1986-September 1987). Inorganic (Na+, K+, NH4+, Ca2+, NO3-, Cl-, SO42-) and organic (HCOO-, CH3COO-) ions were determined in 33 stratiform rain events and nine fog events. With the raindrop size distributions, measured over a 1 year period (June 1988-June 1989) at the site of Enyele in the Equatorial forest of Congo, were established the relationship between the liquid water content (LWC in g m-3) and the rate of rainfall (R in mm h-1) for the stratiform rains: LWC = 0.055 x R0.871 with a correlation coefficient of 0.98. Taking into account the dilution effect due to LWC, ionic concentrations of fogwater and stratiform precipitation are enriched during the dry season. In particular, K+, NO3-, SO42- and Ca2+ are considerably enriched indicating the seasonal influence of the biomass burning due to savanna fires and terrigenous source from deserts of the Southern Hemisphere. Comparison of the chemical contents of fogwater-which mainly represents the local emission of the forest-and stratiform precipitations-which represents the air chemical content of the planetary boundary layer-during the dry season enabled us to show the following. Fog and rain with comparable chemical contents in mineral elements indicate a generalized contamination of the boundary layer by marine (Na+, Cl-), terrigenous (Ca2+) and above all by biomass burning (K+, NO3-, SO42-) sources. The organic content (HCOO-, CH3COO-) higher for the fogs than for rains, unexplainable by the dilution effect, has its source at a local level in the forest ecosystem. The estimation, from the organic content of fog and rain, of the gaseous concentrations of formic and acetic acids confirm the production of carboxylic acids measured in Amazonia during ABLE (for HCOOH: 510 ppt at canopy level and 170 ppt in free troposphere and for CH3COOH: 410 ppt and 210 ppt, respectively).