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

  • Author(s): 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.
  • Holder, H.
  • et al.

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, O-3-alkene reactions and dicarbonyls photolysis are the main radical sources. O-3 photolysis contributes to less than 6% of the total radical production.

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