Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 4 2005 10.5194/acpd-5-6041-2005 http://www.atmos-chem-phys-discuss.net/5/6041/2005/ http://www.atmos-chem-phys-discuss.net/5/6041/2005/acpd-5-6041-2005.html http://www.atmos-chem-phys-discuss.net/5/6041/2005/acpd-5-6041-2005.pdf 6041 6076 2005-08-17 Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003 T. R. Shirley W. H. Brune X. Ren J. Mao R. Lesher B. Cardenas R. Volkamer L. T. Molina M. J. Molina B. Lamb E. Velasco T. Jobson M. Alexander Dept. of Meteorology, Pennsylvania State University, University Park, Pennsylvania, USA General Direction of the National Center for Environmental Research and Training (CENICA), National Institute of Ecology (INE), Mexico City, Mexico Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Dept. of Civil and Environmental Engineering, Washington State University, Pullman, Washington, USA Atmospheric Sciences, Pacific Northwest National Laboratory, Richland, Washington, USA The Mexico City Metropolitan Area (MCMA) study in April 2003 had measurements of most atmospheric constituents including OH and HO₂. It provided a unique opportunity to examine atmospheric oxidation in a megacity that has more pollution than typical US and European cities. OH typically reached 0.35 pptv (~7×10⁶ cm^−3), comparable to amounts observed in US cities, but HO₂ reached 40 pptv in the early afternoon, more than observed in most US cities. A steady-state photochemical model simulated the measured OH and HO₂ for day and night to within combined measurement and modeling uncertainties for 2/3 of the results. For OH, measured = 0.65 (modeled) + 0.026 pptv, with R²=0.80. For HO₂, observed = 0.70 (modeled) + 3.4 pptv, with R²=0.64. Measurements tended to be higher during night and rush hour; the model was higher by ~30% during midday. With a large median measured OH reactivity of more than 120 s^−1 during morning rush hour, median ozone production from observed HO₂ reached 50 ppb hr^−1; RO₂ was calculated to have a similar ozone production rate. For both the HO₂/OH ratio and the ozone production, the measured values have the essentially same dependence on NO as the modeled values. This similarity is unlike other urban studies in which the NO-dependence of the measured HO₂/OH ratio was much less than the modeled ratio and the ozone production rate that was calculated from measured HO₂ unexpectedly appeared to increase as a function of NO with no obvious peak.