Particle size distribution of polycyclic aromatic hydrocarbons in motorcycle exhaust emissions

The size distribution of polycyclic aromatic hydrocarbons (PAHs) in emission of a two-stroke carburetor motorcycle was studied. The exhaust gas from the test motorcycle was passed to a dilution tunnel and collected using a 10 cascade micro-orifice uniform deposit impactor (MOUDI) of 0.056-10 μm aerodynamic diameter fitted with aluminum substrates. All MOUDI substrates were analyzed for particulate mass and for PAHs by GC/MS. Most of the 21 analyzed PAHs have two significant modes that peak at <0.1 and 0.18-0.32 μm. For some PAHs, a third peak appears around 1.8 μm. MOUDI impactor samples show that 88.9% particulate and 89.6% PAH mass distributed smaller than 2.5 μm. Mass median diameters of PAHs are about 0.2 μm. Total benzo[a]pyrene toxic equivalency emission factor was 440 ± 13.8 ng/km for the test motorcycle. An average of 90.3% of carcinogenicity is observed in particulate smaller than 1.0 μm. The results suggest that submicron particulates predominate in the exhaust from motorcycle and exhibit high carcinogenic potency for these particulate.