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26The Mexico City Metropolitan Area (MCMA) is one of the most polluted megacities in North 27 America. Therefore, it is an excellent benchmark city to understand atmospheric chemistry and 28 to implement pilot countermeasures. Air quality in the MCMA is not within acceptable levels, 29 mainly due to high ground levels of ozone (O 3 ). Tropospheric O 3 is a secondary pollutant formed 30 from the oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides and 31 sunlight. To gain a better understanding of O 3 formation in megacities, evaluate the effectiveness 32 of already-implemented countermeasures, and identify new cost-effective alternatives to reduce 33 tropospheric O 3 concentrations, researchers and environmental authorities require updated 34 concentrations for a broader range of VOCs. Moreover, in an effort to protect human health and 35 the environment, it is important to understand which VOCs exceed reference safe values or most 36 contribute to O 3 formation, as well as to identify the most probable emission sources of those 37 VOCs. In this work, 64 VOCs, including 36 toxic VOCs, were measured at four sites in the 38 MCMA during 2011-2012. VOCs related to liquefied petroleum gas leakages exhibited the 39 highest concentrations. Toxic VOCs with the highest average concentrations were acetone and 40 ethanol. The toxic VOC benzene represented the highest risk to Mexican citizens, and toluene 41 contributed the most to O 3 formation. Correlation analysis indicated that the measured VOCs 42 come from vehicular emissions and solvent-related industrial sources. 43 44 Keywords: Volatile organic compound, Toxic volatile organic compound, Ozone, Ozone 45 formation potential 46 47 48 CAPSULE ABSTRACT 49 VOC measurements revealed that compounds related to liquefied petroleum gas leakages are the 50 most abundant, the toxic VOC benzene represents the highest risk to citizens, and toluene is the 51 greatest VOC contributor to O 3 formation in Mexico City. 52 53 5

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