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

[摘要] We estimated the isoprene and monoterpene source strengths of a pristinetropical forest north of Manaus in the central Amazon Basin using threedifferent micrometeorological flux measurement approaches. During the earlydry season campaign of the Cooperative LBA Airborne Regional Experiment(LBA-CLAIRE-2001), a tower-based surface layer gradient (SLG) technique wasapplied simultaneously with a relaxed eddy accumulation (REA) system.Airborne measurements of vertical profiles within and above the convectiveboundary layer (CBL) were used to estimate fluxes on a landscape scale byapplication of the mixed layer gradient (MLG) technique. The mean daytimefluxes of organic carbon measured by REA were 2.1 mg C m⁻² h⁻¹ forisoprene, 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 inreasonable agreement with fluxes determined with the SLG approach, whichexhibited a higher scatter, as expected for the complex terraininvestigated. The observed VOC fluxes are in good agreement with simulationsusing a single-column chemistry and climate model (SCM).

In contrast, the model-derived mixing ratios of VOCs were by far higher thanobserved, indicating that chemical processes may not be adequatelyrepresented in the model. The observed vertical gradients of isoprene andits primary degradation products methyl vinyl ketone (MVK) and methacrolein(MACR) suggest that the oxidation capacity in the tropical CBL is muchhigher than previously assumed. A simple chemical kinetics model was used toinfer OH radical concentrations from the vertical gradients of(MVK+MACR)/isoprene. The estimated range of OH concentrations during thedaytime was 3–8×10⁶ molecules cm⁻³, i.e., an order of magnitudehigher than is estimated for the tropical CBL by current state-of-the-artatmospheric chemistry and transport models. The remarkably high OHconcentrations 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 onlyin reasonable agreement with those of the tower-based REA and SLG approachesafter correction for chemical decay by OH radicals, applying a best estimateOH concentration of 5.5×10⁶ molecules cm⁻³. The SCM modelcalculations support relatively high OH concentration estimates afterspecifically being constrained by the mixing ratios of chemical constituentsobserved during the campaign.

The relevance of the VOC fluxes for the local carbon budget of the tropicalrainforest site during the measurements campaign was assessed by comparisonwith 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 carbongained by net ecosystem productivity appeared to be re-emitted through VOCemissions.

[学科分类] 大气科学