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 Nemitz3W. Joe F. Acton et al.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
Received: 24 Sep 2015 – Discussion started: 27 Oct 2015 – Revised: 05 Apr 2016 – Accepted: 16 May 2016 – Published: 10 Jun 2016
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.
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.
Volatile organic compounds (VOCs) represent a large source of reactive carbon in the atmosphere...
