where, EFi, j (mg/km) is emission factor for i carbonyl and j type of vehicle; VMTj is the j type vehicle’s annual mileage; VPj is vehicle population of type j and αj rep resents diesel fraction within type j. Vehicle types include medium bus, large bus, light-duty truck, medium-duty truck and heavy-duty truck, which contain considerable amounts of diesel vehicle. Emission data of light-duty diesel truck was obtained from Tsai et al. (2012). Due to the lack of speed distribution, emission factors were overall averaged for calculation. VMT was statistically estimated according to this study and available literatures. VP and the fraction of diesel vehicle αj were available in the statistical yearbook (CATARC, 2012).