Composition and size distribution of metals in diesel exhaust particulates

Abstract

This study characterizes the size distribution and composition of metals in diesel exhaust particulates (DEPs) emitting from four driving conditions. We quantified 17 metals in DEPs (34–1000 nm) with a total concentration ranging from 5.4–7.0 µg/m³. Depending on driving conditions, ultrafine (<100 nm) and accumulation-mode DEPs carried up to 41% and 75% of the quantified metals, respectively. The size distribution of individual metals consistently indicates that under a medium (60%) engine load, more than three fourths of quantified metals partitioned in accumulation-mode DEPs, indicating prominent heterogeneous condensation. Enhancing the engine load up to 100% significantly increased metals in ultrafine DEPs around 1.8 times, particularly in DEP < 66 nm, suggesting enhanced metal nucleation. Under the maximum engine load, metals and elemental carbon showed an opposite trend in size distribution, providing tailpipe evidence that metals may catalyze oxidation of DEPs during combustion. Among the identified metals, Fe (2.3–3.9 µg/m³) was most abundant contributing to more than 43% of quantified metals, followed by Li, V, and Pb. Although As and Cd together contributed to less than 2% of the total quantified metals in DEP (<1 µm), their concentrations peaked in ultrafine DEPs under the maximum engine load, indicating that a decrease in engine loads can reduce amounts and toxicity of ultrafine DEPs.