Volatile Organic Compound Emissions from Light-Duty Vehicles in Monterrey, Mexico: a Tunnel Study

A two-week tunnel study was conducted in Monterrey, Mexico during June 2009 to characterize volatile organic compound (VOC) emissions from the local vehicle fleet and estimate the corresponding emission factors (EFs). The Loma Larga Tunnel (LLT), a 532 meter-long structure that is mainly used by lightduty gasoline-powered vehicles was used as the experimental set-up. Ambient air samples (2-hour averages) were taken inside the LLT using 6 L SUMMA®-polished canisters. Samples collected in the canisters were analyzed for Total Non-Methane Hydrocarbons (TNMHC) and 53 individual VOCs using high-resolution GC-MS. Identified individual VOCs accounted for ~80% of the TNMHCs. The most abundant VOCs, on a molar basis, were ethene (13.8%), acetylene (9.0%), isopentane (7.1%), toluene (5.6%), and n-butane (5.5%). High correlations were obtained for known markers of vehicular emissions. Particularly, for the ethene/ acetylene ratio values between 1.1 and 2.4 were obtained (R2 = 0.95), indicating the presence of vehicles with a working catalytic converter. Estimated EFs were compared to values reported in similar studies elsewhere, including one in Mexico City. Results indicate that the vehicles in the LLT tend to emit lower molecular weight VOCs (in the C2 to C6 range). Reactivity of the emissions, expressed propylene-equivalent concentrations, was estimated resulting in an important contribution of the olefins (including isoprene) to the ozone-forming potential of the emitted mixtures.