Emission Characteristics and Cancer Risks of Polycyclic Aromatic Hydrocarbon Emissions from Diesel-fueled Vehicles Complying with Recent Regulations

Abstract

Polycyclic aromatic hydrocarbon (PAH) emissions from diesel vehicles have been reduced by recent regulations further dropping the permissible levels of regulated substances. We analyzed emissions of 13 PAHs from cold- or hot-start test cycles in three diesel vehicles complying with these stringent regulations, and we estimated cancer risk in terms of toxic equivalency factors (TEFs). Two vehicles were equipped with oxidation catalysts and one with a urea-selective catalytic reduction (SCR) system. Most PAH emissions were lower from the compliant vehicles than from other diesel vehicles with no aftertreatment devices. For the three vehicles, naphthalene (Naph) was emitted at the highest rate (2.92-376 μg/km); by mass it constituted 51.1-84.8% (mean 73.0%, S.D.±12.2%) of all PAH emissions. However, in the SCR system, Naph emissions probably decomposed during collection, because the percentage recoveries of surrogate were low, suggesting the presence of specific reactive substances in the SCR system exhaust. The cancer risk of PAH emissions was reduced by application of the emission-control devices. Most benzo[a]pyrene-equivalent (B[a]P_eq) emissions of two-ring PAHs (Naph) or total emissions of five-ring PAHs like benzo[a]pyrene were greater than those of other-ring PAHs. Although Naph has a low TEF (0.001), it constituted a high proportion of the total B[a]P_eq (26-74%; mean 54%, S.D.±15%). The contents of particulate five-ring PAHs, which pose high cancer risks, are decreasing because of improvements in emissions-reduction technology. Hence, the relative contribution of Naph as a gaseous PAH to the TEF-determined risk of carcinogenesis is increasing.