Relationship between Chemical Composition and Oxidative Potentials of Diesel Exhaust Particles Collected During Inhalation Exposure Studies

SEUNG-HYUN CHO (1,2,3), William P. Linak (1), Arthur K. Cho (4), Q. Todd Krantz (1), Charly J. King (1), William T. Preston (5), M. Ian Gilmour (1)

(1) U.S. Environmental Protection Agency, NC, (2) Oak Ridge Institute for Science and Education, TN, (3) RTI International, NC, (4) University of California, Los Angeles, CA, (5) ARCADIS U.S.,Inc., NC

     Abstract Number: 2477
     Last modified: February 4, 2010

Abstract
Diesel exhaust, and especially diesel exhaust particles (DEP), are a health concern because of their complex chemistry, small size, and ubiquitous presence in urban environments. Seven different DEP samples (CDEP1-7) were collected from a 4 cyl. 31 kW diesel engine equipped with an air compressor during a series of inhalation exposure studies. The samples were analyzed for organic and elemental carbon (OC-EC), non-EC inorganic elements, and 130 single organic components, including alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes, steranes, and alkanoic and benzoic organic acids. Dichloromethane (DCM)-extracted organic material (EOM) was determined gravimetrically. The same samples were examined for partitioning properties of redox, Fenton, and electrophilic activities in water-soluble (WS) and water-insoluble (WI) fractions, and DCM-EOM by three chemical assays using dithiothreitol (DTT), 2,3- and 2,5-dihydroxybenzoic acids (DHBA), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Induction of heme oxygenase-1 protein (HO-1) in macrophages exposed to EOM was also measured for cellular oxidative stress. Significantly correlated chemical component groups were extracted as factors by principle component/factor analysis, and the association between each factor and oxidative stress potential was investigated by regression analysis. OC/EC ratio varied between 0.2-0.8. At least 84% of the organics identified (<3% of DEP mass) were alkanes and organic acids. EOM comprised 10-40% of the DEP mass. Factor 1 was associated with fuel component, indicative of poor combustion, showing a high correlation among low molecular weight (mw) normal alkanes (nCL, carbon number 10-24), PAHs of mw 178-202, and methyl-PAHs; factor 2 indicated more fully oxidized fuel and engine oil contribution with high correlation among benzoic acid, high mw normal alkanes (nCH, carbon number 25), and various inorganic elements; and factor 3 accounted for soot and pyrogenic organic condensates, showing high correlation among elemental carbon, EOM, PAHs of mw 228-276, hopanes, and steranes. First, using aqueous media, DTT and DHBA assay data showed WI DEP fractions contributed at least 90% of oxidative stress potential in association with factors 2 and 3. This indicated that much of redox and Fenton reactions were induced by particle-bound components of unburned fuel and engine oil and large mw pyrogenic material. However, GAPDH showed higher electrophile activity in WS (76%) compared with WI (24%) in association with unburned fuel and engine oil (factors 1 and 2). Second, for the EOM fraction, the DTT assay showed the oxidative potentials associated with factors 1, 2, and 3. GAPDH showed an association with factor 1. HO-1 was induced by factors 1 and 3 chemicals, whereas DHBA showed non-measurable Fenton reaction. These observations indicate that the DEP organic extracts contained oxygen-transferring material and electrophiles but not inorganic elements, which were isolated together with particles during the extraction process. In summary, different fractions of DEP have potentials to induce different levels of oxidative stress through two distinct exposure pathways. Particle-bound material, associated with metals in unburned engine oil and pyrogenic components, would directly interact with cells, whereas soluble components of oxygen-transferring materials and electrophiles would cross the cell membrane as individual molecular species. [This abstract does not reflect EPA policy.]