Characterization of traffic-related particulate matter in an urban tunnel: Source apportionment of exhaust and non-exhaust particle number-size distribution emissions.

Emissions from urban traffic are the primary sources of particulate matter in metropolitan areas, contributing to environmental and human health issues. To identify sources and evaluate emission factors for PM2.5 and PM10 from vehicles, tunnel measurements were conducted in two seasons at the Niayesh Tunnel in Tehran. Sampling was performed using high-volume air samplers and PMS5003 low-cost sensors at tunnel entrance and exit. Results show that PM2.5 and PM10 concentrations were higher in the cold months. The chemical profile indicated that traffic-related metals were present at high concentrations. Specifically, in PM2.5, the incremental concentrations (μg/m3) of Al (3.2 ± 2), Fe (4.1 ± 0.6), Ca (2.3 ± 0.5), Zn (3.4 ± 0.7), B (4.6 ± 3), and Ba (4.2 ± 2) were measured. Furthermore, in PM10, the corresponding concentrations (μg/m3) were Al (3.9 ± 0.6), Fe (6.7 ± 0.9), Ca (2.5 ± 1.3), Zn (3.06 ± 1.06), B (6.5 ± 3.4), and Ba (5.6 ± 2.4). To determine source contributions, PMF was applied to particle number-size distribution, and the associated volumetric contributions were subsequently estimated. Vehicle exhaust was the dominant contributor to PM2.5 concentration, followed by non-exhaust-related emissions and road dust resuspension. In contrast, PM10 was primarily influenced by non-exhaust emissions, particularly resuspended road dust. Results indicated that the EF for PM2.5 and PM10 were significantly higher during the warm days (26 ± 7.7 for PM2.5, 40.4 ± 9.7 for PM10) than during the cold days (12.9 ± 1.3 for PM2.5 and 16 ± 1.14 for PM10). These findings highlight that seasonal variations in EFs are likely driven by resuspended road dust and PM concentration differences, although other factors may also contribute to the observed seasonal variability. Accordingly, the significant contribution of non-exhaust sources to PM emissions underscores the need to account for them in air pollution management strategies.