There were few studies about the difference of carbonyl emission factors between diesel vehicle types, literature data showed that total carbonyl emissions from heavy-duty vehicles were ca. 4 times higher than those from light-duty vehicles and the ratios varied between carbonyl species (Grosjean et al., 2001). In this study, carbonyl emission factors for medium bus, large bus, medium-duty truck and heavy-duty truck were 70.5 ± 54.6, 106.1 ± 98.4, 67.1 ± 49.3, 145.5 ± 28.2 mg/km respectively (Fig. 2a). However, on fuel consumed basis, carbonyl emissions from all tested vehicles were more or less similar, ranging from 375 to 419 mg/L. Ratios of fuel-based emission factors between different vehicles were not obvious in this study, while Grosjean et al. (2001) calculated that the HD/LD ratio was ca. 0.86