SURVIVAL OF POLYCYCLIC AROMATIC-HYDROCARBONS DURING DIESEL COMBUSTION

The application of a radiotracer technique to investigate the sources of polycyclic aromatic hydrocarbons (PAH) in diesel exhaust emissions is described.; In separate experiments, C-14-radiolabeled naphthalene, 2-methylnaphthalene (2-MeNp), fluorene, pyrene, and benzo[a]pyrene (B[a]P) were each added to diesel fuel, which was combusted in a 2-L direct injection Perkins Prima diesel engine. Exhaust samples were collected using a novel exhaust gas sampling device designed for sampling organic species in automobile exhaust emissions. Survivals for these PAH were 0.87% for fluorene, 0.54% far 2-MeNp, 0.47% for naphthalene, 0.17% for pyrene, and 0.04% for B[a]P. A linear relationship was observed between the extent to which individual PAH survived combustion and the energy level of the lowest unoccupied molecular orbital (LUMO) of the molecule. LUMO energy levels far each molecule were calculated from Huckel molecular orbital theory. The relationship observed in the current experiment suggests that, for these PAH and under steady-state engine conditions, it is the chemical kinetics of reactions occurring in the combustion chamber as opposed to thermodynamic stabilities that determine the extent of PAH survival during diesel combustion. Using this relationship, it should be possible to predict the extent of PAH survival in diesel emissions from a knowledge of the PAH composition of the fuel.