Turbulent exchange and segregation of HOx radicals and volatile organic compounds above a deciduous forest
1Arbeitsgruppe Atmosphärische Prozesse (AGAP), München, Germany
2Institut für Chemie und Dynamik der Geosphäre 2 (ICG-2): Troposphäre, Forschungszentrum Jülich, Jülich, Germany
3Leopold Franzens – Universität, Institut für Ionenphysik und Angewandte Physik (IAP), Innsbruck, Austria
4Fachgruppe Physik, Fachbereich Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, Wuppertal, Germany
5Geophysical Institute, University of Alaska, Fairbanks, USA
6Geschäftsbereich Sicherheit, Forschungszentrum Jülich, Jülich, Germany
Abstract. The eddy covariance method was applied for the first time to estimate fluxes of OH and HO2 together with fluxes of isoprene, the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) and the sum of monoterpenes above a mixed deciduous forest. Highly sensitive measurements of OH and HO2 were performed by laser induced fluorescence (LIF), and biogenic volatile organic compounds (BVOCs) were measured by Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) at a time resolution of 5 s, each. Wind speed was measured by a sonic anemometer at 10 Hz. The one-day feasibility study was conducted at a total height of 37 m, about 7 m above forest canopy, during the ECHO 2003 intensive field study in July 2003. The daytime measurements yielded statistically significant OH fluxes that indicate downward transport of OH into the direction of the canopy and HO2 fluxes mainly upward out of the canopy. This hints towards a significant chemical sink of OH by reaction with BVOCs and conversion of OH to HO2 at the canopy. In addition, the highly time-resolved trace gas measurements were used to calculate the intensity of segregation of OH and BVOCs, demonstrating that the effective reaction rate of isoprene and OH was slowed down as much as 15% due to inhomogeneous mixing of the reactants. The paper describes the applied methods and provides a detailed analysis of possible systematic errors of the covariance products.