Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based and airborne measurements, and implications on the atmospheric chemistry and the local carbon budget U. Kuhn1, M. O. Andreae1, C. Ammann2, A. C. Araújo3, E. Brancaleoni4, P. Ciccioli4, T. Dindorf1, M. Frattoni4, L. V. Gatti5, L. Ganzeveld6, B. Kruijt7, J. Lelieveld6, J. Lloyd8,*, F. X. Meixner1, A. D. Nobre3, U. Pöschl1, C. Spirig2, P. Stefani9, A. Thielmann1, R. Valentini9, and J. Kesselmeier1 1Max Planck Institute for Chemistry, Biogeochemistry Dept., Mainz, Germany 2Federal Research Station for Agroecology and Agriculture, Zürich, Switzerland 3Instituto Nacional de Pesquisas da Amazônia (INPA), Manaus, Brazil 4Istituto di Metodologie Chimiche, Area delle Ricerca di Roma, Monterot. Scalo, Italy 5Instituto de Pesquisas Energeticas e Nucleares (IPEN), São Paulo, Brazil 6Max Planck Institute for Chemistry, Atmospheric Chemistry Dept., Mainz, Germany 7Alterra, Wageningen University and Research Centre, Wageningen, Netherlands 8Max Planck Institute for Biogeochemistry, Jena, Germany 9University of Tuscia, Department of Forest Science and Environment, Viterbo, Italy *now at: Earth and Biosphere Institute, School of Geography, University of Leeds, UK
Abstract. We estimated the isoprene and monoterpene source strengths of a pristine
tropical forest north of Manaus in the central Amazon Basin using three
different micrometeorological flux measurement approaches. During the early
dry season campaign of the Cooperative LBA Airborne Regional Experiment
(LBA-CLAIRE-2001), a tower-based surface layer gradient (SLG) technique was
applied simultaneously with a relaxed eddy accumulation (REA) system.
Airborne measurements of vertical profiles within and above the convective
boundary layer (CBL) were used to estimate fluxes on a landscape scale by
application of the mixed layer gradient (MLG) technique. The mean daytime
fluxes of organic carbon measured by REA were 2.1 mg C m−2 h−1 for
isoprene, 0.20 mg C m−2 h−1 for α-pinene, and 0.39 mg C m−2 h−1 for the sum of monoterpenes. These values are in
reasonable agreement with fluxes determined with the SLG approach, which
exhibited a higher scatter, as expected for the complex terrain
investigated. The observed VOC fluxes are in good agreement with simulations
using a single-column chemistry and climate model (SCM).
In contrast, the model-derived mixing ratios of VOCs were by far higher than
observed, indicating that chemical processes may not be adequately
represented in the model. The observed vertical gradients of isoprene and
its primary degradation products methyl vinyl ketone (MVK) and methacrolein
(MACR) suggest that the oxidation capacity in the tropical CBL is much
higher than previously assumed. A simple chemical kinetics model was used to
infer OH radical concentrations from the vertical gradients of
(MVK+MACR)/isoprene. The estimated range of OH concentrations during the
daytime was 3–8×106 molecules cm−3, i.e., an order of magnitude
higher than is estimated for the tropical CBL by current state-of-the-art
atmospheric chemistry and transport models. The remarkably high OH
concentrations were also supported by results of a simple budget analysis,
based on the flux-to-lifetime relationship of isoprene within the CBL.
Furthermore, VOC fluxes determined with the airborne MLG approach were only
in reasonable agreement with those of the tower-based REA and SLG approaches
after correction for chemical decay by OH radicals, applying a best estimate
OH concentration of 5.5×106 molecules cm−3. The SCM model
calculations support relatively high OH concentration estimates after
specifically being constrained by the mixing ratios of chemical constituents
observed during the campaign.
The relevance of the VOC fluxes for the local carbon budget of the tropical
rainforest site during the measurements campaign was assessed by comparison
with the concurrent CO2 fluxes, estimated by three different methods
(eddy correlation, Lagrangian dispersion, and mass budget approach).
Depending on the CO2 flux estimate, 1–6% or more of the carbon
gained by net ecosystem productivity appeared to be re-emitted through VOC
Citation: Kuhn, U., Andreae, M. O., Ammann, C., Araújo, A. C., Brancaleoni, E., Ciccioli, P., Dindorf, T., Frattoni, M., Gatti, L. V., Ganzeveld, L., Kruijt, B., Lelieveld, J., Lloyd, J., Meixner, F. X., Nobre, A. D., Pöschl, U., Spirig, C., Stefani, P., Thielmann, A., Valentini, R., and Kesselmeier, J.: Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based and airborne measurements, and implications on the atmospheric chemistry and the local carbon budget, Atmos. Chem. Phys., 7, 2855-2879, doi:10.5194/acp-7-2855-2007, 2007.