Environ. Sci. Technol. 1995, 29, 2871-2876
Sunrival of Polycyclic Aromatic Hydrocarbons burin1 - Diesel Combustion PAUL J. TANCELL,* MICHAEL M. RHEAD, ROBIN D. PEMBERTON, AND JIM BRAVEN Department of Environmental Sciences, University of Plymouth, Devon, PL4 8AA, U.K.
The application of a radiotracer technique to investigate the sources of polycyclic aromatic hydrocarbons (PAH) in diesel exhaust emissions is described. In separate experiments, 14C-radiolabeled naphthalene, 2-methylnaphthalene (2-MeNp), fluorene, pyrene, and benzoialpyrene (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% for 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) ofthe molecule. LUMO energy levels for 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.
Introduction Europe as a whole has experienced a substantial increase in the proportion of new diesel cars sold during the last decade (1). The greater fuel economy of diesel-powered vehicles when compared with their petrol engine equivalents offers significant financial benefits to the motorist and is the primary reason why this trend is set to continue (2). The increased market penetration of diesels is of concern because of the possible risk to human health posed by diesel exhaust emissions. The International Agency for Research on Cancer (IARC) has classified whole diesel exhaust as probably carcinogenic to humans (3). Epidemiological studies have linked diesel emissions with an
0013-936)(/95/0929-2871$09.00/0 Q 1995 American Chemical Society
increased incidence of lung cancer in occupationally exposed workers (4, 5). Of especial concern are diesel emissions of polycyclic aromatic hydrocarbons (PAH), which are sigmficantly greater than PAH emissions from comparable catalyst-equipped petrol engines (2). Several PAH have been classified as probable human carcinogens (3). An increase in PAH emissions, particularly in urban areas, may have implications for public health. Diesel fuel contains significant quantities of PAH (61, and it has been proposed that PAH in diesel emissions arise primarily from diesel fuel PAH surviving the combustion process (6, 7). There is also evidence to suggest that PAH in diesel emissions may be synthesized during the combustion process (8, 9). Future emissions control strategies will focus, in part, on improving engine design and on the continuing development of “low emissions” diesel fuels. Diesel fuels with a low aromatic and sulfur content have been marketed in Sweden since 1991 (10). Determining the origin of PAH in diesel emissions is of fundamental importance if the effectiveness of these strategiesis to maximized. A large volume of experimental data is available concerningtotal PAH emissionrates from diesel engines (11-15). However, little published data is available regarding the extent to which diesel fuel PAH survive the combustionprocess, reflecting the experimental difficulties associated with distinguishing between combustion-formed PAH and fuel PAH that have survived combustion. Recent research (13,14) has utilised multivariate statistical analysis to distinguish between these two sources of PAH. The radiolabeling technique developed at Plymouth (16,171is to our knowledge the only technique that is able to distinguish unambiguously by direct measurement between the relative PAH contribution from the various sources (18). This paper presents results obtained from the combustion of five 14C-radiolabeledPAH: naphthalene, 2-methylnaphthalene,fluorene, pyrene, and B[alP. These PAH are all present in commercially available diesel fuels at levels of between 300 ppm (naphthalene) and
