A tunnel study to estimate emission factors from mobile sources in Monterrey, Mexico.

UNLABELLED: A six-day tunnel field study was conducted in the city of Monterrey, Mexico, during June 2009 to derive on-road emission factors (EFs)for trace gases and fine particulate matter from the local vehicle fleet. The Loma Larga Tunnel (LLT) is a 532-m-long structure that is mainly used by light-duty gasoline-powered vehicles. It is composed of two independent bores that have a semicircular cross section, 17 m in diameter with a 3.5% slope. During the study, a fleet of 108,569 vehicles with average speeds that ranged from 43 to 76 km/hr was sampled. Ambient air samples were taken inside each bore using 6-L SUMMA-polished canisters and low-volume samplers for the quantification of total nonmethane hydrocarbons (TNMHC) and PM2.5, respectively. The effect of road dust resuspension was considered in the computation of PM2.5 EFs. Additional equipment was used to measure real-time levels of CO2 and NO(x); CO EFs were estimated using NO(x) as a surrogate. TNMHC samples and NO(x) levels were obtained for 2-hr time periods, while PM2.5 samples and CO2 levels were obtained using 2.5-hr time periods, which included the time periods of the TNMHC and NO(x) measurements. Estimated EFs for TNMHC, CO, NO(x), and PM2.5 were 1.16 +/- 0.05, 4.83 +/- 2.9, and 0.11 +/- 0.07 g/km-veh (2-hr average) and 17.5 +/- 5.7 mg/veh-km (2.5-hr average), respectively, while CO2 EFs were 182.7 +/- 44 g/km-veh for the 2-hr time periods and 170 +/- 22 g/veh-km for the 2.5-hr time periods. The average fuel economy estimated from the field data was 12.3 +/- 2.3 km/L. The CO2 and TNMHC EFs (on a mass per distance basis) tended to be higher for traffic moving upslope, while the inverse occurred for the PM2.5 EFs. In comparison to other tunnel studies, the CO2 EFs obtained were similar the NO(x) and PM2.5 EFs were lower, and the CO and TNMHC EFs were higher. IMPLICATIONS: Mobile source emission factors (EFs) for Mexican cities other than Mexico City are scarce. In Monterrey, Mexico, one of the three major cities in the country, emissions inventories are constructed based on EFs from other locations. However, it is quite relevant to obtain local information to construct reliable inventories. We present what is, to our knowledge, the first tunnel study conducted in a Mexican city other than Mexico City to estimate fleet-average mobile source EFs. This is also the first study that reports PM2.5 EFs derived from a tunnel study in the country.