Articles | Volume 16, issue 11
Atmos. Chem. Phys., 16, 7149–7170, 2016
Atmos. Chem. Phys., 16, 7149–7170, 2016

Research article 10 Jun 2016

Research article | 10 Jun 2016

Canopy-scale flux measurements and bottom-up emission estimates of volatile organic compounds from a mixed oak and hornbeam forest in northern Italy

W. Joe F. Acton1, Simon Schallhart2, Ben Langford3, Amy Valach1,3, Pekka Rantala2, Silvano Fares4, Giulia Carriero5, Ralf Tillmann6, Sam J. Tomlinson3, Ulrike Dragosits3, Damiano Gianelle7,8, C. Nicholas Hewitt1, and Eiko Nemitz3 W. Joe F. Acton et al.
  • 1Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK
  • 2Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2, 00560 Helsinki, Finland
  • 3Centre for Ecology & Hydrology, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK
  • 4Research Centre for Soil-Plant System, Council for Agricultural Research and Economics, Rome, Italy
  • 5Institute for Plant Protection CNR Research Area – Building E, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
  • 6Institute of Energy and Climate Research, IEK-8: Troposphere, Research Centre Jülich, 52425 Jülich, Germany
  • 7Sustainable Agro-ecosystems and Bioresources Department, Research and Innovation Centre, Fondazione Edmund Mach, 38010 S. Michele all'Adige, Italy
  • 8Foxlab Joint CNR-FEM Initiative, Via E. Mach 1, 38010 San Michele all'Adige, Italy

Abstract. This paper reports the fluxes and mixing ratios of biogenically emitted volatile organic compounds (BVOCs) 4 m above a mixed oak and hornbeam forest in northern Italy. Fluxes of methanol, acetaldehyde, isoprene, methyl vinyl ketone + methacrolein, methyl ethyl ketone and monoterpenes were obtained using both a proton-transfer-reaction mass spectrometer (PTR-MS) and a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) together with the methods of virtual disjunct eddy covariance (using PTR-MS) and eddy covariance (using PTR-ToF-MS). Isoprene was the dominant emitted compound with a mean daytime flux of 1.9 mg m−2 h−1. Mixing ratios, recorded 4 m above the canopy, were dominated by methanol with a mean value of 6.2 ppbv over the 28-day measurement period. Comparison of isoprene fluxes calculated using the PTR-MS and PTR-ToF-MS showed very good agreement while comparison of the monoterpene fluxes suggested a slight over estimation of the flux by the PTR-MS. A basal isoprene emission rate for the forest of 1.7 mg m−2 h−1 was calculated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) isoprene emission algorithms (Guenther et al., 2006). A detailed tree-species distribution map for the site enabled the leaf-level emission of isoprene and monoterpenes recorded using gas-chromatography mass spectrometry (GC–MS) to be scaled up to produce a bottom-up canopy-scale flux. This was compared with the top-down canopy-scale flux obtained by measurements. For monoterpenes, the two estimates were closely correlated and this correlation improved when the plant-species composition in the individual flux footprint was taken into account. However, the bottom-up approach significantly underestimated the isoprene flux, compared with the top-down measurements, suggesting that the leaf-level measurements were not representative of actual emission rates.

Short summary
Volatile organic compounds (VOCs) represent a large source of reactive carbon in the atmosphere and hence have a significant impact on air quality. It is therefore important that we can accurately quantify their emission. In this paper we use three methods to determine the fluxes of reactive VOCs from a woodland canopy. We show that two different canopy-scale measurement methods give good agreement, whereas estimates based on leaf-level-based emission underestimate isoprene fluxes.
Final-revised paper