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 and Sinks of Oxidants (EXPRESSO) campaign in central Africa studied atmospheric trace gases and aerosols during the biomass burning season. Isoprene, emitted from vegetation, was the most abundant nonmethane hydrocarbon observed 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 technique, was approximately 890 μg isoprene m−2 h−1 from the tropical rain forest and semideciduous forest landscapes and 570 μg isoprene m−2 h−1 from transitional and degraded woodland landscapes. Model estimates derived from satellite landscape characterization coupled with leaf enclosure 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 calculations indicate that the observed strong vertical gradient of the NO2 photolysis rate coefficient could be explained by the presence of substantial amounts of absorbing aerosols, probably from biomass burning. Chemical (box) model simulations of the planetary boundary layer (PBL), constrained by measured isoprene emission fluxes and concentrations, show that this suppression 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 isoprene, is small.