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
Received: 30 Sep 2009 – Published in Atmos. Chem. Phys. Discuss.: 16 Nov 2009 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 (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
HO2 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 HO2 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 HO2 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.
Revised: 11 Jun 2010 – Accepted: 23 Jun 2010 – Published: 09 Jul 2010
Citation: Dlugi, R., Berger, M., Zelger, M., Hofzumahaus, A., Siese, M., Holland, F., Wisthaler, A., Grabmer, W., Hansel, A., Koppmann, R., Kramm, G., Möllmann-Coers, M., and Knaps, A.: Turbulent exchange and segregation of HOx radicals and volatile organic compounds above a deciduous forest, Atmos. Chem. Phys., 10, 6215-6235, doi:10.5194/acp-10-6215-2010, 2010.