Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 5 2009 10.5194/acp-9-1665-2009 http://www.atmos-chem-phys.net/9/1665/2009/ http://www.atmos-chem-phys.net/9/1665/2009/acp-9-1665-2009.html http://www.atmos-chem-phys.net/9/1665/2009/acp-9-1665-2009.pdf 1665 1685 2009-03-05 Measurements of OH and HO₂ concentrations during the MCMA-2006 field campaign – Part 1: Deployment of the Indiana University laser-induced fluorescence instrument S. Dusanter sdusante@indiana.edu D. Vimal P. S. Stevens R. Volkamer L. T. Molina Center for Research in Environmental Science, School of Public and Environmental Affairs, and Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Massachusetts Institute of Technology, Cambridge MA, USA University of California at San Diego, La Jolla CA, USA University of Colorado at Boulder and CIRES, Boulder CO, USA Molina Center for Energy & the Environment, La Jolla CA, USA now at: Molecular Physics Laboratory, SRI International, Menlo Park, CA, USA Measurements of tropospheric hydroxyl (OH) and hydroperoxy (HO₂) radicals were made during the MCMA (Mexico City Metropolitan Area) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These radicals were measured using a laser-induced fluorescence instrument developed at Indiana University. This new instrument takes advantage of the Fluorescence Assay by Gas Expansion technique (FAGE) together with direct excitation and detection of OH at 308 nm. HO₂ is indirectly measured as OH by titration with NO inside the fluorescence cell. At this stage of development, IU-FAGE is capable of detecting 3.9&times;10⁵ molecule/cm³ of both OH and HO₂, with a signal to noise ratio of 1, an averaged laser power of 10-mW and an averaging time of 5-min. The calibration accuracies (1&sigma;) are &plusmn;17% for OH and &plusmn;18% for HO₂ using the water-vapor photolysis/O₂ actinometry calibration technique. <br><br> OH and HO₂ concentrations were successfully measured at an urban site in Mexico City, with observed concentrations comparable to those measured in other polluted environments. 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