ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-12233-2010 Technical Note: Formal blind intercomparison of HO<sub>2</sub> measurements in the atmosphere simulation chamber SAPHIR during the HOxComp campaign Fuchs H. 1 Brauers T. 1 Dorn H.-P. 1 Harder H. 2 Häseler R. 1 Hofzumahaus A. 1 Holland F. 1 Kanaya Y. 3 Kajii Y. 4 Kubistin D. 2 Lou S. 1 5 Martinez M. 2 Miyamoto K. 4 Nishida S. 4 Rudolf M. 2 Schlosser E. 1 7 Wahner A. 1 Yoshino A. 4 Schurath U. 6 Forschungszentrum Jülich, Institut für Energie- und Klimaforschung: Troposphäre (IEK-8), 52428 Jülich, Germany Max Planck Institute for Chemistry, Atmospheric Chemistry Dept., 55020 Mainz, Germany Frontier Research Center for Global Change (currently Research Institute for Global Change), Japan Agency for Marine-Earth Science and Technology, Yokohama 236-0001, Japan Tokyo Metropolitan University, Department of Applied Chemistry, Tokyo 192-0397, Japan Shanghai Jiatong University, School of Environmental Science and Technology, Shanghai, China Karlsruhe Institute of Technology (KIT), IMK-AAF, 76021 Karlsruhe, Germany now at: Karlsruhe Institute of Technology (KIT), IMK-AAF, 76021 Karlsruhe, Germany 23 12 2010 10 24 12233 12250 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/12233/2010/acp-10-12233-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/12233/2010/acp-10-12233-2010.pdf

Hydroperoxy radical (HO<sub>2</sub>) concentrations were measured during the formal blind intercomparison campaign HOxComp carried out in Jülich, Germany, in 2005. Three instruments detected HO<sub>2</sub> via chemical conversion to hydroxyl radicals (OH) and subsequent detection of the sum of OH and HO<sub>2</sub> by laser induced fluorescence (LIF). All instruments were based on the same detection and calibration scheme. Because measurements by a MIESR instrument failed during the campaign, no absolute reference measurement was available, so that the accuracy of individual instruments could not be addressed. Instruments sampled ambient air for three days and were attached to the atmosphere simulation chamber SAPHIR during the second part of the campaign. Six experiments of one day each were conducted in SAPHIR, where air masses are homogeneously mixed, in order to investigate the performance of instruments and to determine potential interferences of measurements under well-controlled conditions. Linear correlation coefficients (<i>R</i><sup>2</sup>) between measurements of the LIF instruments are generally high and range from 0.82 to 0.98. However, the agreement between measurements is variable. The regression analysis of the entire data set of measurements in SAPHIR yields slopes between 0.69 to 1.26 and intercepts are smaller than typical atmospheric daytime concentrations (less than 1 pptv). The quality of fit parameters improves significantly, when data are grouped into data subsets of similar water vapor concentrations. Because measurements of LIF instruments were corrected for a well-characterized water dependence of their sensitivities, this indicates that an unknown factor related to water vapor affected measurements in SAPHIR. Measurements in ambient air are also well-correlated, but regression parameters differ from results obtained from SAPHIR experiments. This could have been caused by differences in HO<sub>2</sub> concentrations in the sampled air at the slightly different locations of instruments.

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