Thermodynamic behavior of stable carbon isotopic compositions of individual polycyclic aromatic hydrocarbons derived from automobiles

Concentrations, molecular compositions, and compound-specific stable carbon isotopic compositions (δ¹³C) of polycyclic aromatic hydrocarbons (PAHs) in gasoline exhaust particles (GEPs) and diesel exhaust particles (DEPs) were investigated in this study. δ¹³C of PAHs in GEPs ranged from -13.3‰ to -26.8‰, and that in DEPs ranged from -21.7‰ to -26.3‰. The interspecies δ¹³C variations in each sample were 5.3 ± 2.2‰ in GEPs and 2.6 ± 1.3‰ in DEPs. PAHs in GEPs show larger interspecies δ¹³C variation than those in DEPs; hence, a degree of carbon isotopic fractionation during the conversion from fuel to PAH seems to be larger in gasoline engines than that in diesel engines. Pyrene series PAHs, which consist of only hexagonal rings, in almost all GEP samples show strong negative correlation between the H/C ratio and δ¹³C whereas fluoranthene series PAHs, which contain a pentagonal ring, show less systematic isotopic behavior in GEP samples. A kinetic isotope effect in thermal cracking of organic macromolecules may be minor for PAH formation in vehicle engines. We suggest that the isotopic trend of pyrene series in GEPs can be explained by a thermodynamic isotope effect, and that disturbance from isotopic equilibrium may cause a weak correlation between the isotopic behavior and the H/C ratio among the interspecies PAHs.