Effects of vehicle speed and engine load on diesel exhaust particulates

Abstract

To better understand diesel exhaust particulates (DEPs) as one of major primary emission sources affecting air quality, size distribution and composition of metals and organic compounds in DEPs emitting from four driving conditions were characterized. Eighteen metals in DEPs (34-1000 nm) were quantified with a total concentration ranging from 6.1-7.7 micro-gram per cubic meter. Depending on driving conditions, ultrafine (<100 nm) and accumulation-mode DEPs carried up to 43% and 75% of the quantified metals, respectively. Under an engine load of 60%, metals favorably partitioned in accumulation-mode DEPs. Enhancing the engine load up to 100% substantially increased metals in ultrafine DEPs (from 1.5-3.1 micro-gram per cubic meter) and peaked at DEP of <66 nm. Under the maximum engine load, metals and elemental carbon showed an opposite trend in size distribution, providing tailpipe evidence that metals may reduce DEPs during combustion. Among the identified metals, Fe (2.3-3.9 micro-gram per cubic meter) is most abundant contributing to more than 38% of quantified metals, followed by Li, Cr, V, and Pb. A Cr-to-Fe ratio (0.08-0.29) can be employed as a fingerprint differentiating diesel- vs. gasoline-origin particulates at locations under heavy traffic influences.For all four driving conditions, concentration of identifiable organic compounds in DEPs ultrafine (34-94 nm) and accumulation (94-1000 nm) modes ranged from 2.9-5.7 micro-gram per cubic meter and 9.5-16.4 micro-gram per cubic meter, respectively. As a function of driving conditions, the non-oxygen-containing organics exhibited a reversed concentration trend to the oxygen-containing organics. The identified organic compounds were classified into eleven classes: alkanes, alkenes, alkynes, aromatic hydrocarbons, carboxylic acids, esters, ketones, alcohols, ethers, nitrogen-containing compounds, and sulfur-containing compounds. For all driving conditions, alkane class consistently showed the highest concentration (8.3 micro-gram per cubic meter to 18.0 micro-gram per cubic meter) followed by carboxylic acid, esters, ketones and alcohols. Twelve polycyclic aromatic hydrocarbons (PAHs) were identified with a total concentration ranging from 37.9-174.8 nano-gram per cubic meter. In addition, nine nitrogen-containing polycyclic aromatic compounds (NPACs) were identified with a total concentration ranging from 7.0-10.3 nano-gram per cubic meter. The most abundant PAH (phenanthrene) and NPACs (7,8-benzoquinoline and 3-nitrophenanthrene) comprise a similar molecular (3 aromatic-ring) structure under the highest engine speed and engine load.