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

Abstract

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 towerbased 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 VOCsCorrespondence to: U. Kuhn (kuhn@mpch-mainz.mpg.de)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 3-8×10 6 molecules cm −3 , i.e., an order of magnitude higher than is estimated for the tropical CBL by current state-of-theart 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 5.5×10 6 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.Published by Copernicus Publications on behalf of the European Geosciences Union. 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 CO 2 fluxes, estimated by three different methods (eddy correlation, Lagrangian dispersion, and mass budget approach). Depending on the CO 2 flux estimate, 1-6% or more of the carbon gained by net ecosystem productivity appeared to be re-...

Isoprene photochemistry over the Amazon rainforest

“…The products MVK and MACR have been studied in many atmospheric environments both by proton transfer reaction mass spectrometry (PTR-MS) and gas chromatography (GC) (21). Large data sets are available (7,8,21,22). By comparison, ambient measurements of ISOPOOH isomers are sparse, and available data sets are limited to temperate regions (23,24).…”

“…Isoprene photooxidation products are also important sources of atmospheric organic particulate matter (4)(5)(6). Isoprene is mostly oxidized in the atmospheric mixed layer, although entrainment and reaction in the free troposphere can also be important (7,8).…”

Are plant emissions green?

“…Recent field studies in the remote Amazon basin have provided indirect evidence that isoprene has a considerably smaller effect on • OH than was previously thought 3,4 . Lelieveld et al 1 now confirm this with direct measurements of • OH over the tropical rainforest in Suriname, Guyana and French Guiana.…”

Biomass burning emissions disturbances on the isoprene oxidation in a tropical forest

“…During the day, the isoprene chemistry is affected mainly by the distance from the emission source (transport time), photochemical degradation and availability of OH, which react with isoprene to produce (among other chemical species) MVK, MACR and ISOPOOH (Kuhn et al, 2007;Liu et al, 290 2016). During SAMBBA, the mean isoprene mixing ratio in BG was 2.8 ppbv and 1.5 ppbv for the boundary layer and cloud layer, respectively.…”

“…Motivated by the discrepancies between model and observation of OH production in the atmosphere and the influence of the biomass burning plumes in the isoprene reactivity with OH during the day, we propose in this study a refinement in the OH estimation method that has been applied by several previous studies (Apel, 2002;Karl et al, 2007;Kuhn et al, 2007;Stroud et al, 2001). …”