Spatial differences in ambient coarse and fine particles in the Monterrey metropolitan area, Mexico: Implications for source contribution

Abstract

ABSTRACTThe ambient air of the Monterrey Metropolitan Area (MMA) in Mexico frequently exhibits high levels of PM10 and PM2.5. However, no information exists on the chemical composition of coarse particles (PMc = PM10 – PM2.5). A monitoring campaign was conducted during the summer of 2015, during which 24-hr average PM10 and PM2.5 samples were collected using high-volume filter-based instruments to chemically characterize the fine and coarse fractions of the PM. The collected samples were analyzed for anions (Cl–, NO3–, SO42–), cations (Na+, NH4+, K+), organic carbon (OC), elemental carbon (EC), and 35 trace elements (Al to Pb). During the campaign, the average PM2.5 concentrations did not showed significance differences among sampling sites, whereas the average PMc concentrations did. In addition, the PMc accounted for 75% to 90% of the PM10 across the MMA. The average contribution of the main chemical species to the total mass indicated that geological material including Ca, Fe, Si, and Al (45%) and sulfates (11%) were the principal components of PMc, whereas sulfates (54%) and organic matter (30%) were the principal components of PM2.5. The OC-to-EC ratio for PMc ranged from 4.4 to 13, whereas that for PM2.5 ranged from 3.97 to 6.08. The estimated contribution of Secondary Organic Aerosol (SOA) to the total mass of organic aerosol in PM2.5 was estimated to be around 70–80%; for PMc, the contribution was lower (20–50%). The enrichment factors (EF) for most of the trace elements exhibited high values for PM2.5 (EF: 10–1000) and low values for PMc (EF: 1–10). Given the high contribution of crustal elements and the high values of EFs, PMc is heavily influenced by soil resuspension and PM2.5 by anthropogenic sources. Finally, the airborne particles found in the eastern region of the MMA were chemically distinguishable from those in its western region.Implications: Concentration and chemical composition patterns of fine and coarse particles can vary significantly across the MMA. Public policy solutions have to be built based on these observations. There is clear evidence that the spatial variations in the MMA’s coarse fractions are influenced by clearly recognizable primary emission sources, while fine particles exhibit a homogeneous concentration field and a clear spatial pattern of increasing secondary contributions. Important reductions in the coarse fraction can come from primary particles’ emission controls; for fine particles, control of gaseous precursors—particularly sulfur-containing species and organic compounds—should be considered.