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

摘要

Concentrations, molecular com0ositions, and compound-specific stable carbon isotopic comositions (delta~13C) of polycyclic aromatic hydrocarons (PAHs) in gasoline exhaust particles (GEPs) and diesel exhaust particles (DEPs) were investigated in this study. delta~13C of PAHs in GEPs ranged from -13.3 per thousand to -26.8 per thousand, and that in DEPs ranged from -21.7per thousand to -26.3per thousand. The interspecies delta~13C variations in each sample were 5.3 (+-) 2.2 per thousand in GEPs and 2.6(+-)1.3 per thousand in DEPs. PAHs in GEPs show larger ingerspecies delta~13C 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 corelation between the H/C ratio and delta~13C whereas fluoranthene series PAHs, which contain a pentagonal ring, show less systematic isotopic behavior in GEP samples. A kinetic isotope effect in thermal craking 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 themodynamic isotope effect, and that disturbance from isotopic equilibrium may cause a weak correlating between the isotopic behavior and the H/C ratio among the interspecies PAHs.