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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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Abstract:The use of ethanol in gasoline has become a worldwide tendency as an alternative to reduce net CO 2 emissions to the atmosphere, increasing gasoline octane rating and reducing dependence on petroleum products. However, recently environmental authorities in large urban centers have expressed their concerns on the true effect of using ethanol blends of up to 20% v/v in in-use vehicles without any modification in the setup of the engine control unit (ECU), and on the variations of these effects along the years of operation of these vehicles. Their main concern is the potential increase in the emissions of volatile organic compounds with high ozone formation potential. To address these concerns, we developed analytical and experimental work testing engines under steady-conditions. We also tested carbureted and fuel-injected vehicles every 10,000 km during their first 100,000 km of operation. We measured the effect of using ethanol-gasoline blends on the power and torque generated, the fuel consumption and CO 2 , CO, NOx and unburned hydrocarbon emissions, including volatile organic compounds (VOCs) such as acetaldehyde, formaldehyde, benzene and 1,3-butadiene which are considered important ozone precursors. The obtained results showed statistically no significant differences in these variables when vehicles operate with a blend of 20% v/v ethanol and 80% v/v gasoline (E20) instead of gasoline. Those results remained unchanged during the first 100,000 km of operation of the vehicles. We also observed that when the vehicles operated with E20 at high engine loads, they showed a tendency to operate with greater values of λ (ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio) when compared to their operation with gasoline. According to the Eco-Indicator-99, these results represent a minor reduction (<1.3%) on the impact to human health, and on the deterioration of the ecosystem. However, it implies a 12.9% deterioration of the natural resources. Thermal equilibrium analysis, at the tailpipe conditions (~100 • C), showed that ethane, formaldehyde, ethylene and ethanol are the most relevant VOCs in terms of the amount of mass emitted. The use of ethanol in the gasoline reduced 20-40% of those emissions. These reductions implied an average reduction of 17% in the ozone formation potential.

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