Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 18 2009 10.5194/acp-9-6655-2009 http://www.atmos-chem-phys.net/9/6655/2009/ http://www.atmos-chem-phys.net/9/6655/2009/acp-9-6655-2009.html http://www.atmos-chem-phys.net/9/6655/2009/acp-9-6655-2009.pdf 6655 6675 2009-09-15 Measurements of OH and HO₂ concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget S. Dusanter sdusante@indiana.edu D. Vimal P. S. Stevens R. Volkamer L. T. Molina A. Baker S. Meinardi D. Blake P. Sheehy A. Merten R. Zhang J. Zheng E. C. Fortner W. Junkermann M. Dubey T. Rahn B. Eichinger P. Lewandowski J. Prueger H. Holder Center for Research in Environmental Science, School of Public and Environmental Affairs, and Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Department of Chemistry and Biochemistry, University of Colorado, USA Molina Center for Energy and the Environment USA Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USA University of California, Irvine, 92697 CA, USA Institute of Environmental Physics, University of Heidelberg, Germany Department of Atmospheric Sciences, Texas A&M University, USA Department of Chemistry, Montana State University, USA Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research, IMK-IFU, Garmisch-Partenkirchen, Germany Earth and Environmental Sciences Division, Los Alamos National Laboratory, USA IIHR-Hydroscience & Engineering, University of Iowa, Iowa City, IA, USA USDA National Soil Tilth Lab, Ames, IA, USA Duke University, Durham, NC, USA now at: SRI International, Menlo Park, CA, USA now at: Institute of Applied Photophysics, Dresden University of Technology, Germany now at: Aerodyne Research Incorporated, 45 Manning Road, Billerica, MA, USA Measurements of hydroxyl (OH) and hydroperoxy (HO₂) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric RO_x (OH + HO₂ + RO₂) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 <i>J</i>-values and the concentrations of 97 chemical species. Several issues related to the RO_x chemistry under polluted conditions are highlighted in this study: (i) Measured concentrations of both OH and HO₂ were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO₂/OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. 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