Coupled carbon-water exchange of the Amazon rain forest, II. Comparison of predicted and observed seasonal exchange of energy, CO<sub>2</sub>, isoprene and ozone at a remote site in Rondônia

Abstract

Abstract.A one-dimensional multi-layer scheme describing the coupled exchange of energy and CO 2 , the emission of isoprene and the dry deposition of ozone is applied to a rain forest canopy in southwest Amazonia. The model was constrained using mean diel cycles of micrometeorological quantities observed during two periods in the wet and dry season 1999. Calculated net fluxes and concentration profiles for both seasonal periods are compared to observations made at two nearby towers.The modeled day-and nighttime thermal stratification of the canopy layer is consistent with observations in dense canopies. The observed and modeled net fluxes above and H 2 O and CO 2 concentration profiles within the canopy show a good agreement. The predicted net carbon sink decreases from 2.5 t C ha −1 yr −1 for wet season conditions to 1 t C ha −1 yr −1 for dry season conditions, whereas observed and modeled midday Bowen ratio increases from 0.5 to 0.8. The evaluation results confirmed a seasonal variability of leaf physiological parameters, as already suggested in a companion study. The calculated midday canopy net flux of isoprene increased from 7.1 mg C m −2 h −1 during the wet season to 11.4 mg C m −2 h −1 during the late dry season. Applying a constant emission capacity in all canopy layers, resulted in a disagreement between observed and simulated profiles of isoprene concentrations, suggesting a smaller emission capacity of shade adapted leaves and deposition to the soil or leaf surfaces. Assuming a strong light acclimation of emission capacity, equivalent to a 66% reduction of the standard emission factor for leaves in the lower canopy, resulted in a better agreement of observed and modeled concentration profiles and a 30% reduction of the canopy net flux compared to model calculations with a constant emisCorrespondence to: E. Simon (simon@mpch-mainz.mpg.de) sion factor. The mean calculated ozone flux for dry season conditions at noontime was ≈12 nmol m −2 s −1 , agreeing well with observed values. The corresponding deposition velocity increased from 0.8 cm s −1 to >1.6 cm s −1 in the wet season, which can not be explained by increased stomatal uptake. Considering reasonable physiological changes in stomatal regulation, the modeled value was not larger than 1.05 cm s −1 . Instead, the observed fluxes could be explained with the model by decreasing the cuticular resistance to ozone deposition from 5000 to 1000 s m −1 .

Nine years of global hydrocarbon emissions based on source inversion of OMI formaldehyde observations

“…S3). MEGAN-MACC (Sindelarova et al, 2014) relies on the MEGANv2.1 model for biogenic volatile organic compounds (BVOC) and is based on the MERRA reanalysis fields (Rienecker et al, 2011). The emissions are provided at 0.5 • × 0.5 • resolution and on a monthly basis from 1980 through 2010.…”

“…The global annual flux is estimated at ca. 1000 Tg VOC, with isoprene accounting for half of this emission (Guenther et al, 1995(Guenther et al, , 2012Sindelarova et al, 2014). Despite a general consensus on the isoprene emission patterns, including their dependence on temperature and light density responsible for their marked diurnal and seasonal variations, these emission estimates come, however, with large uncertainties, associated with the strong variability of emission factors and the extrapolation of sparse measurements to larger scales.…”

“…Despite a general consensus on the isoprene emission patterns, including their dependence on temperature and light density responsible for their marked diurnal and seasonal variations, these emission estimates come, however, with large uncertainties, associated with the strong variability of emission factors and the extrapolation of sparse measurements to larger scales. An uncertainty of a factor of 2 in global and regional isoprene fluxes was reported based on a compilation of numerous literature studies (Sindelarova et al, 2014), whereas emission models were found to be strongly sensitive to choices of input variables, leading to even wider uncertainty, of ca. 200-1000 Tg C year −1 globally (Arneth et al, 2011).…”

“…Besides the emission dependence on leaf temperature, photosynthetically active radiation (PAR), leaf area, and leaf age, the model accounts for the inhibition of isoprene emissions in very dry soil conditions through a dimensionless soil moisture activity factor (γ SM ) expressed as a function of volumetric soil moisture content (Guenther et al, 2006) obtained from the ERA-Interim reanalysis. The parameterization of γ SM bears large uncertainties, as it is based on scarce (and sometimes contradictory) field data, and its implementation can lead to very different results depending on the choice of database for soil moisture data (Müller et al, 2008;Sindelarova et al, 2014;Marais et al, 2012). It reduces the emissions by ca.…”

“…The assumed error in the a priori biogenic and pyrogenic fluxes is factor of 3. This choice reflects the high variability of the biomass burning emission source and the strong uncertainties associated with the biogenic emissions, as demonstrated by the large range of literature emission estimates (Arneth et al, 2011;Sindelarova et al, 2014). The spatiotemporal correlations among the a priori errors in the flux parameters are defined as in Stavrakou et al (2009b).…”