Comparative assessment of the sensitivity of ozone to nitrogen oxides and volatile organic compounds in two dissimilar metropolitan areas of North America: Cincinnati, Ohio (United States) and Mexico City, DF (Mexico)

Because of the complexity of urban ozone (O3) formation, photochemical modeling and ambient data analysis are now needed to provide feedback regarding the effectiveness of O3 control strategies. However, procedures for diagnosing the O3 sensitivity to nitrogen oxides (NOx ) or volatile organic compounds (VOC) based on ambient data analysis still require further development and testing. This dissertation presents the results of an experimental investigation into the usefulness of using a combination of observational-driven techniques to determine the O3 -NOx-VOC sensitivity in urban areas. Afternoon O3/NO y, O3/NOz and NOy (where NOy represents the total oxidized nitrogen species and NOz represents the reaction products of NOx photochemical indicators were measured in two dissimilar cities: Cincinnati, Ohio (U.S.A.) and Mexico City, Federal District (Mexico). The evaluation the photochemical indicator analysis diagnosed VOC-sensitive conditions in both cities. The prevalence of these conditions, were confirmed with a complementary statistical analysis of the differences in average weekend/weekday O3 peak and morning NO emissions (WE/WD effect). The comparative assessment of the factors determining the O3-NOx-VOC sensitivity in Cincinnati and Mexico City revealed strong differences on population, urban characteristics, topography, precursors emissions densities, and magnitude of VOC emissions, but an apparent similarity in NOx emissions between these two urban areas. However, measured maximum O3 were high in Mexico City as compared to Cincinnati. This phenomenon could be explained as the result of the higher VOC emissions and stronger VOC reactivity in Mexico City than in Cincinnati. A comparative assessment between the photochemical indicators method and the VOC/NO x emission ratio to diagnose O3-NOx-VOC sensitivity demonstrated the consistency of the first approach to perform this identification under severe conditions of O3 formation. The O3 air quality management implications were, due to the predominating VOC-sensitive conditions, that additional reductions in NOx emissions, stipulated by previous model-based O3 control strategies now in progress in both areas, should be avoided or at least thoroughly reviewed. Otherwise, O3 levels might increase. This research demonstrates that the combination of the photochemical indicators method with the analyses of the WE/WD effect can be a useful tool in the development and evaluation of O3 air quality management strategies.