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 (delta(13)C) of polycyclic aromatic hydrocarbons (PAHs) in gasoline exhaust particles (GEPs) and diesel exhaust particles (DEPs) were investigated in this study. delta(13)C of PAHs in GEPs ranged from -13.3parts per thousand to -26.8parts per thousand, and that in DEPs ranged from -21.7parts per thousand to -26.3parts per thousand. The interspecies 8130 variations in each sample were 5.3 +/- 2.2% in GEPs and 2.6 +/- 1.3parts per thousand in DEPs. PAHs in GEPs show later interspecies delta(13)C variation than those in DEPs; hence, a degree of carbon isotopic fractionation during the conversion from fuel to PAH seems to be later in gasoline engines than that in diesel engines. Pytene series PAHs, which consist of only hexagonal rings, in almost all GEP samples show strong negative correlation between the H/C ratio and delta(13)C 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 pyene 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.