Chemical composition of size-resolved particulate matter at near-freeway and urban background sites in the greater Beirut area

Abstract

To characterize road-traffic emissions in the greater Beirut area, size-resolved PM10–2.5, PM2.5–0.25 and PM0.25 samples were concurrently collected at near-freeway and background sites. While particle mass levels were 1.3–2.6 times greater at the roadside than background location, PM10–2.5 and PM0.25 prevailed at both sites (36–43% of PM10). A chemical mass closure showed that PM10–2.5 was mostly composed of crustal material, contributing to 12–23% of its mass across sites. On the other hand, in PM2.5–0.25 and PM0.25, organic matter (46–56%) was dominant at the roadside location, while secondary ions (SI, 54–68%) were more abundant at the background site. In the vicinity of the freeway, organic carbon (OC) levels varied from 4 μg m−3 in PM10–2.5 to 10.1 μg m−3 in PM0.25, exceeding those at the background site by 4–7 times. In contrast, OC was largely water-soluble at the background location, with its water-soluble fraction displaying comparable PM2.5–0.25- and PM0.25-concentrations at both sites, corroborating its regional secondary origin. PM0.25-elemental carbon (EC), which dominated overall EC mass, was 7-fold greater at the near-freeway than background location, indicating a contribution from diesel or also smoking vehicles to road-traffic emissions. PM2.5-SI showed fairly uniform concentrations across sites, confirming their mainly regional source. Polycyclic aromatic hydrocarbons (PAHs) were undetected at the coastal background site, whereas their levels peaked at 11.5 ng m−3 in proximity to the freeway, with the majority confined to PM0.25. Compared to other roadways, organic compounds at the freeway location exhibited high hopanes plus steranes-to-total carbon ratios (0.16–1.1 ng μg−1), indicative of different engine configurations, fleet or also lubricating oil formulation. At both locations, PM0.25-bound n-alkanes (C19–C34) showed a predominance of high molecular weight homologues and no carbon number preference, suggesting their likely road dust source. Findings of this work indicate that ambient particles at the urban background site in Beirut are mainly of secondary origin and exist in significantly lower levels relative to PM at the roadside location. Particle emissions at the near-freeway site markedly exceed those measured at roadways in similar areas in the U.S. with comparable meteorology and geomorphology. Compared to I-710—a diesel-impacted freeway in Southern California—levels of potentially toxic vehicular tracers are 5 (EC) and 3.7 (PAHs) times greater at the freeway in this study.