Estimating light-duty vehicles' contributions to ambient PM2.5 and PM10 at a near-highway urban elementary school via elemental characterization emphasizing rhodium, palladium, and platinum
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The primary objective of this research is to accurately estimate light-duty vehicles' (LDVs') emissions of PM2.5 and PM10 over the course of a year within the property line of an inner-city school located adjacent to a heavily-trafficked interstate highway by measuring platinum group elements (PGEs - Rh, Pd, and Pt) along with 49 other major and trace elements. Amongst PGEs, ambient Pd concentrations were the highest, averaging 11 pg/m3 in PM10 and 4.0 pg/m3 in PM2.5 followed by Pt (3.5 pg/m3 in PM10 and 1.4 pg/m3 in PM2.5), and Rh (1.6 pg/m3 in PM10 and 0.52 pg/m3 in PM2.5). Simultaneous three-component variations in Rh, Pd, and Pt in both PM size classes at this surface site closely matched the composition of (i) a mixed random lot of recycled autocatalysts obtained from numerous LDVs and (ii) PM inside a proximal underwater tunnel open only to light-duty vehicles. Additionally, quantitative estimates of LDV contributions to ambient PM calculated by chemical mass balance modeling (CMB) were strongly correlated with PGE abundances. Therefore, PGEs predominantly originated from gasoline-driven motor vehicles validating them as unique LDV tracers. Further, CMB estimated that vehicles contributed 37% on average (12-67%) to PM10 and 49% on average (25-73%) to PM2.5. Evidence is also presented for a subset of other trace metals; i.e. Cu, As, Mo, Cd, and Sb to also be relatively strong LDV tracers. Results highlight the importance of measuring PGEs in addition to numerous other elements in PM to accurately apportion aerosols emanating from LDVs, which will better isolate public health and environmental impacts associated with the transportation sector.
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