Size distribution and dry deposition of road dust PAHs

Abstract

The size distribution of polycyclic aromatic hydrocarbons (PAHs) for road dust and for the engine exhaust of both gasoline-powered cars and motorcycles was investigated. In addition, by using the measured size distribution data, monitoring and modeling the PAH dry deposition, the contribution fraction of road dust on the dry deposition materials was also studied. Twenty-one PAHs were analyzed primarily by using a gas chromatograph/mass spectrometer (GC/MS). The mass median diameters (MMDs) of 21 individual PAHs for resuspendable road dust (cut size <100 μm) ranged between 63.4 μm and 65.5 μm. However, the MMDs of total-PAH size distributions for the engine exhaust of both gasoline-powered cars and four-stroke motorcycles averaged 0.45 μm and 0.35 μm, respectively, which were near the MMDs of PAHs (average 0.50 μm) in the ambient air of traffic intersections. Suspended particle-phase total PAHs in the ambient air of traffic intersections were found to be more than 90% of the result of the automobile exhaust; that is, less than 10% of the amount was contributed by the road dust. However, the modeled MMDs of 21 individual PAHs on the dry deposition material were between 22.1 μm and 44.6 μm, and the contribution fraction of road dust on the PAH dry deposition was found to be more than 95%, even though the suspendable ambient-air PAHs were mainly from the mobile exhaust.