Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 4 2008 10.5194/acpd-8-15491-2008 http://www.atmos-chem-phys-discuss.net/8/15491/2008/ http://www.atmos-chem-phys-discuss.net/8/15491/2008/acpd-8-15491-2008.html http://www.atmos-chem-phys-discuss.net/8/15491/2008/acpd-8-15491-2008.pdf 15491 15536 2008-08-15 Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: airborne measurements M. Martinez martinez@mpch-mainz.mpg.de H. Harder D. Kubistin M. Rudolf H. Bozem G. Eerdekens H. Fischer C. Gurk T. Klüpfel R. Königstedt U. Parchatka C. L. Schiller A. Stickler J. Williams J. Lelieveld Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany Department of Chemistry, York University, Toronto, Canada now at: Department of Biology, University of Antwerp, Belgium now at: Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland Direct measurements of OH and HO₂ over a tropical rainforest were made for the first time during the GABRIEL campaign in October 2005, deploying the custom-built HORUS instrument (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy), adapted to fly in a Learjet wingpod. Biogenic hydrocarbon emissions were expected to strongly reduce the OH and HO₂ mixing ratios as the air is transported from the ocean over the forest. However, surprisingly high mixing ratios of both OH and HO₂ were encountered in the boundary layer over the rainforest. <br><br> The HORUS instrumentation and calibration methods are described in detail and the measurement results obtained are discussed. The extensive dataset collected during GABRIEL, including measurements of many other trace gases and photolysis frequencies, has been used to quantify the main sources and sinks of OH. Comparison of these measurement-derived formation and loss rates of OH indicates strong previously overlooked recycling of OH in the boundary layer over the tropical rainforest, occurring in chorus with isoprene emission. Butler, T., Taraborrelli, D., Brühl, C., Fischer, H., Harder, H., Martinez, M., Williams, J., Lawrence, M., and Lelieveld, J.: Improved simulation of isoprene oxidation chemistry with the ECHAM5/MESSy chemistry-climate model: Lessons from the GABRIEL airborne field campaign, Atmos. Chem. Phys., 8(16), 4529–4546, 2008. Di Carlo, P., Brune, W. H., Martinez, M., Harder, H., Lesher, R., Ren, X., Thornberry, T., Carroll, M. A., Young, V., Shepson, P. 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