Fig. 5: Screening and classification of potential marker compounds for tunnel fleet emissions.

a EFs of selected species measured in the tunnel; Serial numbers and detailed compound information were provided in Table S2. b The dendrogram of HCA. Colors represent different clusters, with blue indicating auxiliary species referenced from previous studies. The horizontal axis corresponds to individual compounds, and the vertical axis indicates the relative linkage distance (dimensionless). c Potential markers recommended for fleet vehicles in real world.