Articles | Volume 9, issue 18
Atmos. Chem. Phys., 9, 6655–6675, 2009

Special issue: MILAGRO/INTEX-B 2006

Atmos. Chem. Phys., 9, 6655–6675, 2009

  15 Sep 2009

15 Sep 2009

Measurements of OH and HO2 concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget

S. Dusanter1, D. Vimal1,*, P. S. Stevens1, R. Volkamer2, L. T. Molina3,4, A. Baker5, S. Meinardi5, D. Blake5, P. Sheehy3,4, A. Merten6,**, R. Zhang7, J. Zheng7, E. C. Fortner8,***, W. Junkermann9, M. Dubey10, T. Rahn10, B. Eichinger11, P. Lewandowski11, J. Prueger12, and H. Holder13 S. Dusanter et al.
  • 1Center for Research in Environmental Science, School of Public and Environmental Affairs, and Department of Chemistry, Indiana University, Bloomington, IN 47405, USA
  • 2Department of Chemistry and Biochemistry, University of Colorado, USA
  • 3Molina Center for Energy and the Environment USA
  • 4Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USA
  • 5University of California, Irvine, 92697 CA, USA
  • 6Institute of Environmental Physics, University of Heidelberg, Germany
  • 7Department of Atmospheric Sciences, Texas A&M University, USA
  • 8Department of Chemistry, Montana State University, USA
  • 9Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research, IMK-IFU, Garmisch-Partenkirchen, Germany
  • 10Earth and Environmental Sciences Division, Los Alamos National Laboratory, USA
  • 11IIHR-Hydroscience & Engineering, University of Iowa, Iowa City, IA, USA
  • 12USDA National Soil Tilth Lab, Ames, IA, USA
  • 13Duke 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

Abstract. 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 ROx (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 J-values and the concentrations of 97 chemical species. Several issues related to the ROx 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. This suggests that under high NOx conditions, the HO2 to OH propagation rate may be overestimated by the model or a process converting OH into HO2 may be missing from the chemical mechanism. On a daily basis (08:40 a.m.–06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O3-alkene reactions and dicarbonyls photolysis are the main radical sources. O3 photolysis contributes to less than 6% of the total radical production.

Special issue
Final-revised paper