Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 6 2008 10.5194/acpd-8-20501-2008 http://www.atmos-chem-phys-discuss.net/8/20501/2008/ http://www.atmos-chem-phys-discuss.net/8/20501/2008/acpd-8-20501-2008.html http://www.atmos-chem-phys-discuss.net/8/20501/2008/acpd-8-20501-2008.pdf 20501 20536 2008-12-09 Reactive nitrogen in Mexico City and its relation to ozone-precursor sensitivity: results from photochemical models S. Sillman J. J. West University of Michigan, Ann Arbor, Michigan, USA University of North Carolina, Chapel Hill, North Carolina, USA We use results of a 3-D photochemistry/transport model for ozone formation in Mexico City during events in 1997 to investigate ambient concentrations of reactive nitrogen in relation to ozone-precursor sensitivity. Previous results from other locations suggest that ratios such as O₃/NO_y and H₂O₂/HNO₃ might provide measurement-based indicators for NO_x-sensitive or VOC-sensitive conditions. Mexico City presents a different environment due to its high concentrations of VOC and high level of pollutants in general. The model predicts a correlation between PAN and O₃ with relatively high PAN/O₃ (0.07), which is still lower than measured values. The model PAN is comparable with results from a model for Paris but much higher than were found in Nashville in both models and measurements. The difference can be explained by the lower temperature in Mexico City relative to Nashville. Model HNO₃ in Mexico City is unusually low for an urban area and PAN/HNO₃ is very high, probably due to the high ratio of reactivity-weighted VOC to NO_x. The model predicts that VOC-sensitive chemistry in Mexico is associated with high NO_x, NO_y and NO_x/NO_y and with low O₃/NO_y and H₂O₂/HNO₃, suggesting that these indicators work well for Mexico City. The relation between ozone-precursor sensitivity and either O₃/NO_z or O₃/HNO₃ is more ambiguous. VOC-sensitive conditions are associated with higher O₃/HNO₃ than would be found in NO_x-sensitive conditions, but model O₃/HNO₃ associated with both NO_x-sensitive and VOC-sensitive chemistry is higher in Mexico than in other cities. The model predicts mixed sensitivity to NO_x and VOC in Mexico City, with a tendency towards VOC-sensitive chemistry in the morning and NO_x-sensitive in the afternoon, in contrast to model results for more recent events that predicted strongly VOC-sensitive conditions. The difference in predicted ozone-precursor sensitivity is most likely due to changes in emission rates over time. 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