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

abstract

  • 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) median concentrations of both OH and HO2 were underpredicted during morning hours, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The predicted HO2/OH ratios were underestimated 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.

author list (cited authors)

  • Dusanter, S., Vimal, D., Stevens, P. S., Volkamer, R., Molina, L. T., Baker, A., ... Holder, H.

publication date

  • January 1, 2009 11:11 AM