Turbulent exchange and segregation of HO sub(x) radicals and volatile organic compounds above a deciduous forest

• Published in: Atmospheric chemistry and physics discussions Vol. 9; no. 6; pp. 24423 - 24476
• Main Authors: Dlugi, R, Berger, M, Zelger, M, Hofzumahaus, A, Siese, M, Holland, F, Wisthaler, A, Grabmer, W, Hansel, A, Koppmann, R, Kramm, G, Moellmann-Coers, M, Knaps, A
• Format: Journal Article
• Language: English
• Published: 16.11.2009
• ISSN: 1680-7367; 1680-7375

The eddy covariance method was applied for the first time to estimate fluxes of OH and HO sub(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 sub(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 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 HO sub(2) 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 HO sub(2) 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.