Turbulent exchange and segregation of HOx radicals and volatile organic compounds above a deciduous forest

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 fluxes of the radicals and BVOCs. 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. 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 above canopy as much as 15% due to inhomogeneous mixing of the reactants.