1Institute of Ion Physics and Applied Physics, University of Innsbruck, Austria
2Ionicon Analytik, Innsbruck, Austria
3Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307, USA
4Institute of Ecology, University of Innsbruck, Innsbruck, Austria
*current address: Division of Atmospheric Sciences, Department of Physics, University of Helsinki, Finland
**current address: UMR 5256, IRCELYON CNRS, Université Lyon 1, Lyon, France
***current address: Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA
Abstract. Eddy covariance (EC) is the preferable technique for flux measurements since it is the only direct flux determination method. It requires a continuum of high time resolution measurements (e.g. 5–20 Hz). For volatile organic compounds (VOC) soft ionization via proton transfer reaction has proven to be a quantitative method for real time mass spectrometry; here we use a proton transfer reaction time of flight mass spectrometer (PTR-TOF) for 10 Hz EC measurements of full mass spectra up to m/z 315. The mass resolution of the PTR-TOF enabled the identification of chemical formulas and separation of oxygenated and hydrocarbon species exhibiting the same nominal mass. We determined 481 ion mass peaks from ambient air concentration above a managed, temperate mountain grassland in Neustift, Stubai Valley, Austria. During harvesting we found significant fluxes of 18 compounds distributed over 43 ions, including protonated parent compounds, as well as their isotopes and fragments and VOC-H+-water clusters. The dominant BVOC fluxes were methanol, acetaldehyde, ethanol, hexenal and other C6 leaf wound compounds, acetone, acetic acid, monoterpenes and sequiterpenes.
The smallest reliable fluxes we determined were less than 0.1 nmol m−2 s−1, as in the case of sesquiterpene emissions from freshly cut grass. Terpenoids, including mono- and sesquiterpenes, were also deposited to the grassland before and after the harvesting. During cutting, total VOC emission fluxes up to 200 nmol C m−2 s−1 were measured. Methanol emissions accounted for half of the emissions of oxygenated VOCs and a third of the carbon of all measured VOC emissions during harvesting.