- 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.
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.