ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-6215-2010 Turbulent exchange and segregation of HO<sub>x</sub> radicals and volatile organic compounds above a deciduous forest Dlugi R. 1 Berger M. 1 Zelger M. 1 Hofzumahaus A. 2 Siese M. 2 Holland F. 2 Wisthaler A. 3 Grabmer W. 3 Hansel A. 3 Koppmann R. 2 4 Kramm G. 5 Möllmann-Coers M. 6 Knaps A. 6 Arbeitsgruppe Atmosphärische Prozesse (AGAP), München, Germany Institut für Chemie und Dynamik der Geosphäre 2 (ICG-2): Troposphäre, Forschungszentrum Jülich, Jülich, Germany Leopold Franzens – Universität, Institut für Ionenphysik und Angewandte Physik (IAP), Innsbruck, Austria Fachgruppe Physik, Fachbereich Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, Wuppertal, Germany Geophysical Institute, University of Alaska, Fairbanks, USA Geschäftsbereich Sicherheit, Forschungszentrum Jülich, Jülich, Germany 09 07 2010 10 13 6215 6235 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/6215/2010/acp-10-6215-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/6215/2010/acp-10-6215-2010.pdf

The eddy covariance method was applied for the first time to estimate fluxes of OH and HO<sub>2</sub> 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 HO<sub>2</sub> 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 (Emission and CHemical transformation of biogenic volatile Organic compounds) intensive field study in July 2003. The daytime measurements yielded statistically significant OH fluxes directed downward into the direction of the canopy and HO<sub>2</sub> fluxes mainly upward out of the canopy. This hints towards a significant local chemical sink of OH by reactions with BVOCs, other organic and inorganic compounds and conversion of OH to HO<sub>2</sub> above the canopy. For OH the measured flux is locally balanced by chemical sources and sinks and direct transport of OH plays no important role for the local chemical OH budget at the measurement height, as expected from the short OH lifetime (<1 s). For HO<sub>2</sub> the chemical lifetime (20 s) is in the range of the turbulent transport time for transfer between the top of the canopy and the measuring point. In this case, the radical balance is significantly influenced by both chemistry and transport processes. 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 results, the applied methods and provides a detailed analysis of possible systematic errors of the covariance products.

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