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

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.

Characterization of total ecosystem-scale biogenic VOC exchange at a Mediterranean oak–hornbeam forest

“…The measurement tower was 42 m high and located in the southwestern part of the nature reserve (45.20 • N, 10.74 • E). Figure 1 shows a satellite image of the area, with the position of the tower and the mean 80 % footprint (Acton et al, 2016). The climatological mean annual temperature is 13.3 • C and the mean annual precipitation is 834 mm (Willmott and Matsuura, 2012a, b).…”

“…A companion paper (Acton et al, 2016) compares the PTR-ToF-MS measurements with simultaneous measurements by PTR-QMS and a bottom-up estimate of the canopy flux scaled up from leaf-level emission measurements, and also derives emission factors for the use in emissions models.…”

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

“…By contrast, Rantala et al (2016) chose to determine the emission potential as the gradient in a least-squares regression between F iso and γ . The latter approach, which we hereafter term the LSR method, has gained in popularity (Acton et al, 2016;Rantala et al, 2016), with some choosing to set the intercept to zero (Kalogridis et al, 2014;Acton et al, 2016) and others leaving it to be determined by the fit ). Yet, the application of this approach is often questionable, because the relationship between flux and γ is sometimes non-linear and thus violates the assumptions of the least-squares approach.…”

“…A detailed description of the instrument setup, calibration procedures and sensitivities are presented by Acton et al (2016).…”

“…Olofsson et al, 2005) and eddy covariance (EC) (Karl et al, 2004;Rinne et al, 2007;Davison et al, 2009;Ruuskanen et al, 2011;Potosnak et al, 2013;Park et al, 2013;Kalogridis et al, 2014;Acton et al, 2016;Rantala et al, 2016) have been used to determine canopy-scale emissions directly. This "top-down" approach is, in principle, favourable because the flux measurements are integrated over a wide source area (the flux footprint), giving an emission potential that is representative of an ecosystem as a whole.…”