Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003

International audienceThe 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