Emission factors and characteristics of criteria pollutants and volatile organic compounds (VOCs) in a freeway tunnel study

Carbon monoxide (CO), nitrogen oxide (NOx), hydrocarbon (HC), sulfur oxide (SO2), particulate matter < 10 μm (PM10), and 57 VOC species of emissions were confirmed in a freeway tunnel in southern Taiwan. Emission factors were 1.89 (CO), 0.73 (NOx), 0.46 (HC), 0.02 (SO2) and 0.06 (PM10) g/km-vehicle for all vehicle fleets. Heavy-duty truck and trailer vehicles contributed 20% of the emissions on workdays and 9.5% on weekends in this study. Paraffins and aromatics were the main VOC groups in the tunnel. Isopentane, toluene, n-pentane, isoprene, 2,3-dimethylbutane, acetone, 2-methylpentane, 1-hexene, 1,2,4-trimethybenzene, 1-butene and propene emissions were the major VOC species. Their emission factors were over 10 mg/km-vehicle. Rainfall and high humidity in the tunnel could have reduced the VOC concentrations and increased the portion of aromatics. In addition to paraffins, olefins, and aromatic compounds, oxygenated compounds (i.e., acetone) were found. The pollutant ratios between the inside center and the outside of the tunnel were about 2–3 for CO, SO2, and PM10 and 42 for NOx. In addition, the emission factors of the vehicles could reflect real-world vehicle emissions on the highway and be used as baseline information for development of a vehicle control strategy.