Characterization of total ecosystem-scale biogenic VOC exchange at a Mediterranean oak–hornbeam forest
- 1Department of Physics, University of Helsinki, Helsinki, Finland
- 2Centre for Ecology & Hydrology (CEH), Penicuik, UK
- 3Department of Plant Physiology, Estonian University of Life Sciences, Tartu, Estonia
- 4Department of Forest Sciences, University of Helsinki, Helsinki, Finland
- 5Forschungszentrum Jülich GmbH, Jülich, Germany
- 6National Institute of Agronomic Research UMR ECOSYS INRA, AgroParisTech, Université Paris Saclay, Saint-Aubin, France
- 7Department of Mathematics and Physics, Catholic University of Brescia, Brescia, Italy
- 8Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland
- 9Finnish Meteorological Institute, Helsinki, Finland
- 10Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden
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-time-of-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.