Biogenic volatile organic compound emissions in central Africa during the Experiment for the Regional Sources and Sinks of Oxidants (EXPRESSO) biomass burning season

The recent aircraft and ground-based Experiment for the Regional Sources an d Sinks of Oxidants (EXPRESSO) campaign in central Africa studied atmospher ic trace gases and aerosols during the biomass burning season. Isoprene, em itted from vegetation, was the most abundant nonmethane hydrocarbon observe d over the forest and savanna, even though intense biomass burning activity was occurring several hundred kilometers to the north. The isoprene flux, measured directly from midmorning to noon by a relaxed eddy accumulation te chnique, was approximately 890 mu g isoprene m(-2) h(-1) from the tropical rain forest and semideciduous forest landscapes and 570 mu g isoprene m(-2) h(-1) from transitional and degraded woodland landscapes. Model estimates derived from satellite landscape characterization coupled with leaf enclosu re emission measurements conducted during EXPRESSO compared well with these measured fluxes. Isoprene concentrations and fluxes were used to determine the oxidant balance over the forest and savanna. Radiative transfer calcul ations indicate that the observed strong vertical gradient of the NO2 photo lysis rate coefficient could be explained by the presence of substantial am ounts of absorbing aerosols, probably from biomass burning. Chemical (box) model simulations of the planetary boundary layer (PBL), constrained by mea sured isoprene emission fluxes and concentrations, show that this suppressi on of photolytic radiation lowers OH concentrations by about a factor of 2 relative to aerosol-free conditions. Consequently, the direct contribution of PBL photochemistry to ozone production, especially from biogenic isopren e, is small.