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

Abstract

Abstract. Recently, the number and amount of biogenically emitted volatile organic compounds (VOCs) has been discussed in great detail. Depending on the ecosystem, the published number varies between a dozen and several hundred compounds. We present ecosystem exchange fluxes from a mixed oak-hornbeam forest in the Po Valley, Italy. The fluxes were measured by a proton transfer reaction-timeof-flight (PTR-ToF) mass spectrometer and calculated using the eddy covariance (EC) method. Detectable fluxes were observed for up to 29 compounds, dominated by isoprene, which comprised over 60 % of the total upward flux (on a molar basis). The daily average of the total VOC upward flux was 10.4 nmol m −2 s −1 . Methanol had the highest concentration and accounted for the largest downward flux. Methanol seemed to be deposited to dew, as the downward flux happened in the early morning, right after the calculated surface temperature came closest to the calculated dew point temperature.We estimated that up to 30 % of the upward flux of methyl vinyl ketone (MVK) and methacrolein (MACR) originated from atmospheric oxidation of isoprene. A comparison between two methods for the flux detection (manual and automated) was made. Their respective advantages and disadvantages were discussed and the differences in their results shown. Both provide comparable results.

Temporal variation of VOC fluxes measured with PTR-TOF above a boreal forest

“…The average sensitivities for the different compound groups were 11.4 ± 2.5 ncps ppb −1 for C x H y , 18.6 ± 3.1 ncps ppb −1 for C x H y O z and 17.7 ± 2.0 ncps ppb −1 for C x H y N z . Overall the sensitivities are comparable with Schallhart et al (2016); only the standard deviation increased due to the longer time period of the measurements. The setup for background and calibration measurements is described in more detail in Schallhart et al (2016).…”

“…For uncalibrated compounds the sensitivities were categorized into three groups C x H y (calculated from isoprene, benzene, toluene, o-xylene, trimethylbenzene, naphthalene, α-pinene combined with C 6 H 9 fragment), C x H y O z (based on acetaldehyde, acrolein, acetone, 2-butanone) and C x H y N z (set to acetonitrile) similar to the setup Schallhart et al (2016). The average sensitivities for the different compound groups were 11.4 ± 2.5 ncps ppb −1 for C x H y , 18.6 ± 3.1 ncps ppb −1 for C x H y O z and 17.7 ± 2.0 ncps ppb −1 for C x H y N z .…”

“…The VOC flux measurements at the station have consisted of different instruments and techniques: disjunct eddy covariance (DEC; e.g., Rinne et al, 2007), the surface layer profile (SLP) method (e.g., Rantala et al, 2014) using a proton transfer reaction quadrupole (PTR-Quad) mass spectrometer and a gas chromatograph mass spectrometer using the gradient method (e.g., Rinne et al, 2000). In recent years, proton transfer reaction time-of-flight (PTR-TOF) mass spectrometers have been used to measure EC fluxes (Ruuskanen et al, 2011;Kaser et al, 2013a;Park et al, 2013;Brilli et al, 2016;Schallhart et al, 2016), but the number of these studies is still low. With this new measurement setup, it is possible to identify the elemental composition of the compounds with detectable fluxes.…”

Direct measurement of NO<sub>3</sub> reactivity in a boreal forest

“…The instrument measured total monoterpenes at m/z = 137 and isoprene at m/z = 69, which were calibrated with a gas standard (Apel Riemer Environmental Inc., USA) containing isoprene and α-pinene. The calibration set up and routine are described in detail in Schallhart et al (2016). The campaign average limit of detection LOD (3 σ, 10 minute time resolution) was 5.5 pptv and 3.2 pptv for isoprene and monoterpenes, respectively.…”

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

“…OH, O 3 and NO 3 ) and inversely proportional to the turbulent velocity scale which determines the rate of transport through the turbulent boundary layer, as well as measurement height. Schallhart et al (2016) and Kalogridis et al (2014) directly estimated the chemical loss of isoprene between canopy and measurement height to be 4 and 5 % at the Bosco Fontana and O3HP sites, respectively. For the remaining sites we assume a 5 % chemical loss of isoprene which is also consistent with model simulations by Stroud et al (2005), who predict canopy escape efficiencies for isoprene to be typically greater than 0.9.…”