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

The Mexico City Metropolitan Area (MCMA) study in April 2003 had measurements of many atmospheric constituents, including OH and HO₂. It provided the first opportunity to examine atmospheric oxidation in a megacity in a developing country that has more pollution than typical U.S. and European cities. At midday, OH typically reached 0.35 pptv (∼7 × 10⁶ cm^-3), comparable to amounts observed in U.S. cities, but HO ₂ reached 40 pptv, more than observed in most U.S. cities. The OH reactivity was also measured, even during the highly polluted morning rush hour. MCMA's OH reactivity was 25 s^-1 during most of the day and 120 s^-1 at morning rush hour, which was several times greater than has been measured in any U.S. city. Median measured and modeled OH and HO ₂ agreed to within combined uncertainties, although for OH, the model exceeded the measurement by more than 30% during midday. OH production and loss, which were calculated from measurements, were in balance to within uncertainties, although production exceeded loss during morning rush hour. This imbalance has been observed in other cities. The HO₂/OH ratio from measurements and steady-state analyses have essentially the same dependence on NO, except when NO was near 100 ppbv. This agreement is unlike other urban studies, in which HO₂/OH ratio decreased much less than expected as NO increased. As a result of the active photochemistry in MCMA 2003, the median calculated ozone production from measured HO₂ reached 50 ppb h ^-1, a much higher rate than observed in U.S. cities.