Characterizing ozone production in the Mexico City Metropolitan Area: a case study using a chemical transport model

An episodic simulation is conducted to characterize midday (12:00-17:00 CDT) ozone (O-3) photochemical production and to investigate its sensitivity to emission changes of ozone precursors in the Mexico City Metropolitan Area (MCMA) during an "O-3-South" meteorological episode using the Comprehensive Air Quality Model with extensions (CAMx). High O-x (O-3+NO2) photochemical production rates of 10-80 ppb/h are predicted due to the high reactivity of volatile organic compounds (VOCs) in which alkanes, alkenes, and aromatics exert comparable contributions. The predicted ozone production efficiency is between 4-10 O-3 molecules per NOx molecule oxidized, and increases with VOC-to-NO2 reactivity ratio. Process apportionment analyses indicate significant outflow of pollutants such as O-3 and peroxyacetyl nitrate (PAN) from the urban area to the surrounding regional environment. PAN is not in chemical-thermal equilibrium during the photochemically active periods. Sensitivity studies of O-3 production suggest that O-3 formation in the MCMA urban region with less chemical aging (NOz/NOy<0.3) is VOC-limited. Both the simulated behavior of O-3 production and its sensitivities to precursors suggest that midday O-3 formation during this episode is VOC-sensitive in the urban region on the basis of the current emissions inventory estimates, and current NOx levels depress the O-3 production.