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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 23 | Copyright
Biogeosciences, 14, 5571-5594, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 08 Dec 2017

Research article | 08 Dec 2017

Isoprene emission potentials from European oak forests derived from canopy flux measurements: an assessment of uncertainties and inter-algorithm variability

Ben Langford1, James Cash1,2, W. Joe F. Acton3, Amy C. Valach3,a, C. Nicholas Hewitt3, Silvano Fares4, Ignacio Goded5, Carsten Gruening5, Emily House1,2,3, Athina-Cerise Kalogridis6,b, Valérie Gros6, Richard Schafers1,2, Rick Thomas7, Mark Broadmeadow8, and Eiko Nemitz1 Ben Langford et al.
  • 1Centre for Ecology & Hydrology, Edinburgh, EH26 0QB, UK
  • 2School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK
  • 3Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK
  • 4Council for Agricultural Research and Economics – Research Centre for Forestry and Wood (CREA-FL), Arezzo, Italy
  • 5European Commission, Joint Research Centre, Ispra, Italy
  • 6Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL), Unite Mixte CEA-CNRS-UVSQ (Commissariat a l'Energie Atomique, Centre National de la Recherche Scientifique, Universite de Versailles Saint-Quentin-en-Yvelines), 91198 Gif-sur-Yvette, France
  • 7School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
  • 8Forestry Commission, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK
  • anow at: British Antarctic Survey, Cambridge, UK
  • bnow at: N.C.S.R. “Demokritos”, Institute of Nuclear and Radiological Sciences & Technology, Energy & Safety, 15341 Agia Paraskevi, Attiki, Greece

Abstract. Biogenic emission algorithms predict that oak forests account for ∼ 70% of the total European isoprene budget. Yet the isoprene emission potentials (IEPs) that underpin these model estimates are calculated from a very limited number of leaf-level observations and hence are highly uncertain. Increasingly, micrometeorological techniques such as eddy covariance are used to measure whole-canopy fluxes directly, from which isoprene emission potentials can be calculated. Here, we review five observational datasets of isoprene fluxes from a range of oak forests in the UK, Italy and France. We outline procedures to correct the measured net fluxes for losses from deposition and chemical flux divergence, which were found to be on the order of 5–8 and 4–5%, respectively. The corrected observational data were used to derive isoprene emission potentials at each site in a two-step process. Firstly, six commonly used emission algorithms were inverted to back out time series of isoprene emission potential, and then an average isoprene emission potential was calculated for each site with an associated uncertainty. We used these data to assess how the derived emission potentials change depending upon the specific emission algorithm used and, importantly, on the particular approach adopted to derive an average site-specific emission potential. Our results show that isoprene emission potentials can vary by up to a factor of 4 depending on the specific algorithm used and whether or not it is used in a big-leaf or canopy environment (CE) model format. When using the same algorithm, the calculated average isoprene emission potential was found to vary by as much as 34% depending on how the average was derived. Using a consistent approach with version 2.1 of the Model for Emissions of Gases and Aerosols from Nature (MEGAN), we derive new ecosystem-scale isoprene emission potentials for the five measurement sites: Alice Holt, UK (10500±2500µgm−2h−1); Bosco Fontana, Italy (1610±420µgm−2h−1); Castelporziano, Italy (121±15µgm−2h−1); Ispra, Italy (7590±1070µgm−2h−1); and the Observatoire de Haute Provence, France (7990±1010µgm−2h−1). Ecosystem-scale isoprene emission potentials were then extrapolated to the leaf-level and compared to previous leaf-level measurements for Quercus robur and Quercus pubescens, two species thought to account for 50% of the total European isoprene budget. The literature values agreed closely with emission potentials calculated using the G93 algorithm, which were 85±75 and 78±25µgg−1h−1 for Q. robur and Q. pubescens, respectively. By contrast, emission potentials calculated using the G06 algorithm, the same algorithm used in a previous study to derive the European budget, were significantly lower, which we attribute to the influence of past light and temperature conditions. Adopting these new G06 specific emission potentials for Q. robur (55±24µgg−1h−1) and Q. pubescens (47±16µgg−1h−1) reduced the projected European budget by ∼ 17%. Our findings demonstrate that calculated isoprene emission potentials vary considerably depending upon the specific approach used in their calculation. Therefore, it is our recommendation that the community now adopt a standardised approach to the way in which micrometeorological flux measurements are corrected and used to derive isoprene, and other biogenic volatile organic compounds, emission potentials.

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Short summary
Isoprene flux measurements made above five European oak forests were reviewed to generate new emission potentials. Six variations of the Guenther algorithms were inverted to back out time series of isoprene emission potential, and then an “average” emission potential was determined using one of four commonly used approaches. Our results show that emission potentials can vary by up to a factor of 4 and highlight the need for the community to now harmonize their approach to reduce uncertainty.
Isoprene flux measurements made above five European oak forests were reviewed to generate new...