Particle size distributions of metal and non-metal elements in an urban near-highway environment

Determination of the size-resolved elemental composition of near-highway particulate matter (PM) is important due to the health and environmental risks it poses. In the current study, twelve 24 h PM samples were collected (in July–August 2006) using a low-pressure impactor positioned 20 m from a Raleigh, North Carolina interstate. The interstate supported ∼125,000 vehicles/d, the majority of which were light-duty gasoline passenger vehicles. The overall near-highway PM mass size distribution was trimodal with a major accumulation mode peak at 500–800 nm. PM mass levels reflected daily traffic activity, while mean near-highway PM10 (33 ± 7.5 μg m−3), PM2.5 (29 ± 6.8 μg m−3), and PM0.1 (1.4 ± 0.3 μg m−3) mass levels varied less than 24% over the two week sampling period. The elemental composition of the impactor-collected PM was investigated using magnetic sector inductively coupled plasma-mass spectroscopy (SF-ICPMS). Accumulation mode sulfur (7 ± 4% w/w) was the major inorganic constituent detected at the near-highway site followed by coarse mode group IA and IIA elements likely from re-suspension of crustal matter. As expected, elements regularly detected in asphalt, vehicle, catalyst (e.g., Pt, Rh, and Pd), brake, and tire wear (e.g., Cu and Sb) were also found in the near highway PM. Maximum concentrations of the platinum group, rare earth, and common brake and tire wear elements were observed at d50 = 1–2 μm, d50 = 1–4 μm, and d50 ≥ 1–4 μm, respectively. Ten of the eleven metals listed as EPA air toxics (Mn, Cr, Sb, Ni, Pb, As, Co, Cd, Se, and Be) were detected in each PM size fraction but were generally enriched in PM0.1. Several biologically antagonistic suites of metals (Cd, Cu, and V) were found in multiple PM size modes. Some of these metals showed erratic size distributions with daily changes in enrichment (e.g., Ni, Zn, Cd, As, and Cu) and particle size, suggesting a variety of emissions and metal exposure scenarios occurring in the near-highway environment.