Abstract. The eddy covariance method was applied for the first time to estimate fluxes of OH and HO 2 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 2 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 2 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 2 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 2 the chemical lifetime (20 s) is in the range of the turbulent transport time for transfer between the top of the canopy and the Correspondence to: R. Dlugi (rdlugi@gmx.de) 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.

An inhomogeneous mixing of reactants causes a reduction of their chemical removal compared to the homogeneously mixed case in turbulent atmospheric flows. This can be described by the intensity of segregation I S being the covariance of the mixing ratios of two species divided by the product of their means. Both terms appear in the balance equation of the mixing ratio and are discussed for the reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest. For most of these data, I S is negatively correlated with the fraction of mean OH mixing ratio reacting with isoprene. I S is also negatively correlated with the isoprene standard deviation. Both findings agree with model results discussed by Patton et al. (2001) and others. The correlation coefficient between OH and isoprene and, therefore, I S increases with increasing mean reaction rate. In addition, the balance equation of the covariance between isoprene and OH is applied as the theoretical framework for the analysis of the same field data. The storage term is small, and, therefore, a diagnostic equation for this covariance can be derived. The chemical reaction term R ij is dominated by the variance of isoprene times the quotient of mixing ratios of OH and isoprene. Based on these findings a new diagnostic equation for I S is formulated. Comparing different terms of this equation, I S and R ij show a relation also to the normalised isoprene standard deviation. It is shown that not only chemistry but also turbulent and convective mixing and advection -considered in a residual term -influence I S . Despite this finding, a detection of the influence of coherent eddy transport above the forest according to Katul et al. (1997) on I S fails, but a relation to the turbulent and advective transport of isoprene variance is determined.The largest values of I S are found for most unstable conditions with increasing buoyant production, confirming qualitatively model predictions by Ouwersloot et al. (2011).

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