Fig. 4: Difference analysis and tracers screening of fleet vehicles.

a Volcano plot showing the fold-change (FC) and P-value from t-test comparing tunnel samples to ambient atmosphere. Red points represent species that meet FC > 1.5 and P-value < 0.05, with their size proportional to the EFs. b Diurnal variation of EFs, molecular structure, and P-value of selected species. Serial numbers and compound details are provided in Table S2. Bar colors correspond to the four sampling periods. Boxes represent the interquartile range (25–75th percentiles), the centerlines indicate the medians, and the whiskers denote the full data range.