PNNL

Abstract

The Mexico City Metropolitan Area (MCMA) study in April 2003 provided a unique opportunity to examine atmospheric oxidation in a megacity that has more pollution than US and European cities. Most atmospheric constituents that are important for atmospheric oxidation, including the free radicals hydroxyl (OH) and hydroperoxyl (HO2), were measured. OH typically reached 7x106 cm-3, comparable to amounts observed in US cities, but HO2 reached 40 pptv in the early afternoon, substantially more than observed in most US cities. A steady-state chemistry model was able to simulate the measured OH and HO2 to within well within measurement and model errors bars except for nighttime, when measured OH was 5 times modeled and measured HO2 was 2.5 times modeled, and during morning rush hour, when measured HO2 was ~5 times modeled. We observed similar comparisons in US cities. The agreement between the measured and calculated OH reactivity to within uncertainties indicates that the volatile organic compounds important for atmospheric oxidation are known. The high calculated instantaneous ozone production rate from HO2 measurements is consistent with the high ozone levels typically observed in MCMA.