ABSTRACT
In this work, the particulate matter (PM) from three different monitoring stations in the Monterrey Metropolitan Area in Mexico were investigated for their compositional, morphological, and optical properties. The main aim of the research was to decipher the different sources of the particles. The methodology involved the ex situ sequential analysis of individual particles by three analytical techniques: scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), polarized light microscopy (PLM), and micro-Raman spectroscopy (MRS). The microanalysis was performed on samples of total suspended particles. Different morphologies were observed for particles rich in the same element, including prismatic, spherical, spheroidal, and irregular morphologies. The sequential microanalysis by SEM-EDS/PLM/MRS revealed that Fe-rich particles with spherical and irregular morphologies were derived from anthopogenic sources, such as emissions from the metallurgical industry and the wear of automobile parts, respectively. In contrast, Fe-rich particles with prismatic morphologies were associated with natural sources. In relation to carbon (C), the methodology was able to distinguish between the C-rich particles that came from different anthopogenic sources-such as the burning of fossil fuels, biomass, or charcoal-and the metallurgical industry. The optical properties of the Si-rich particles depended, to a greater extent, on their chemical composition than on their morphology, which made it possible to quickly and accurately differentiate aluminosilicates from quartz. The methodology demonstrated in this study was useful for performing the speciation of the particles rich in different elements. This differentiation helped to assign their possible emission sources.
ABSTRACT
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.
