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Atmos. Chem. Phys., 9, 6655-6675, 2009
www.atmos-chem-phys.net/9/6655/2009/ doi:10.5194/acp-9-6655-2009 © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License. |
Measurements of OH and HO2 concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget
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.
Citation: Dusanter, S., Vimal, D., Stevens, P. S., Volkamer, R., Molina, L. T., Baker, A., Meinardi, S., Blake, D., Sheehy, P., Merten, A., Zhang, R., Zheng, J., Fortner, E. C., Junkermann, W., Dubey, M., Rahn, T., Eichinger, B., Lewandowski, P., Prueger, J., and Holder, H.: Measurements of OH and HO2 concentrations during the MCMA-2006 field campaign – Part 2: Model comparison and radical budget, Atmos. Chem. Phys., 9, 6655-6675, doi:10.5194/acp-9-6655-2009, 2009.