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 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 regional 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 emissions.

The shift in plant species composition in a subarctic mountain birch forest floor due to climate change would modify the biogenic volatile organic compound emission profile

“…However, Bäckstrand et al (2008) measured a higher proportion of carbon emitted as BVOCs reaching 5% of the total carbon uptake of a subarctic peatland. The proportion of the carbon emitted as BVOCs of the CO 2 exchanges varies between 0.07% and 12.4% in Mediterranean and tropical ecosystems (Kesselmeier et al 2002;Kuhn et al 2007), which is similar and up to two orders of magnitude higher than on the present study site. Several factors could explain these differences between ecosystems such as the vegetation and soil types and growth conditions (e.g.…”

Non-methane biogenic volatile organic compound emissions from boreal peatland microcosms under warming and water table drawdown

“…However, on a subarctic peatland in northern Sweden, Bäckstrand et al (2008) report that the proportion of carbon emitted as BVOCs is as much as 5% of the CO 2 -C uptake. In Mediterranean and tropical ecosystems, proportions of carbon emitted as BVOCs of the CO 2 -C uptake vary between 0.07 and 12.4%, which is the same range or up to two orders of magnitude higher than in the present experiment (Kesselmeier et al 2002;Kuhn et al 2007). Carbon emitted as BVOCs can be up to 10% of the carbon uptake, or even more, under stressful conditions (Peñuelas and Llusià 2003).…”

Temperature and rainfall interact to control carbon cycling in tropical forests

“…Aboveground NPP also includes C losses via herbivory, leaching, volatile organic compound emissions and in situ decomposition (Clark et al 2001). However, since they are rarely measured and are not thought to account for a large proportion of ANPP (Greenberg et al 2004;Kuhn et al 2007), we did not attempt to estimate these fluxes.…”