Size Distribution of Particulate Polycyclic Aromatic Hydrocarbons in the Diluted Four-stroke Motorcycle Exhausts

Twenty-one polycyclic aromatic hydrocarbons (PAHs) from a four-stroke motorcycle were collected with a micro-orifice uniform deposit impactor and measured in size-segregated particulate samples of 0.056– 18 m aerodynamic diameter. The motorcycle exhausts were passed through a dilution tunnel to simulate reactions between ambient air and exhaust. The PAHs were analyzed using Gas Chromatography/Mass Spectrometry (GC/MS). The size distribution of particulates was unimodal with a peak at 0.1– 0.18 m. Lower molecular weight PAHs predominate in motorcycle exhaust. The emission factors increase as the particle diameter decreases for PAHs with low molecular weights. Two peaks appear at 0.056– 0.1  m and 0.18– 0.32  m for most middle and higher molecular weight PAHs. For PAHs, the cumulative fractions attributable to diameters less than 1.0 and 2.5 m were 77.0% and 84.3% for the test motorcycle. The mass median diameter of total PAHs was 0.45 ± 0.11  m. The BaPeq emission factor for 0.056– 18 m aerodynamic diameter is 70.8 ± 38.4 ng/km for the test motorcycle. High molecular weight PAHs have the highest BaPeq emission factor, indicating that higher molecular weight PAHs are more carcinogenic.