Abstract
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.
Highlights
The term volatile organic compound (VOC) describes a broad range of chemical species emitted from natural and anthropogenic sources into the atmosphere
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
Direct above-canopy fluxes of methanol, acetaldehyde, acetic acid, isoprene, MVK + MACR, MEK and monoterpenes were calculated using the method of virtual disjunct eddy covariance from mixing ratio data obtained with a PTRMS above a semi-natural mixed oak and hornbeam forest in northern Italy from 13 June to 11 July 2012
Summary
The term volatile organic compound (VOC) describes a broad range of chemical species emitted from natural and anthropogenic sources into the atmosphere. Of the BVOCs, isoprene is almost certainly the dominant species globally with an estimated annual emission of 535–578 × 1012 g C (Arneth et al, 2008; Guenther et al, 2012). Mixing ratios by volume were calculated from data generated using the PTR-MS using a programme written in LabVIEW (National Instruments, Austin, Texas, USA). Mixing ratios by volume (χVOC) were calculated from the raw PTR-MS data (in counts per second, cps) using a method based on those of Taipale et al (2008) and Tani et al (2004). Where Snorm is the normalized sensitivity and I (RH+)norm represents the background corrected normalized count rate (ncps) for the protonated compound R, which was calculated as shown below.
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