Particulate matters (PMs) emitted by automobile exhaust contribute to a significant fraction of the global PMs.Extractive atmospheric pressure chemical ionization mass spectrometry (EAPCI-MS) was developed to explore the molecular dependence of PMs collected from exhaust gases produced at different vehicle engine speeds.The mass spectral fingerprints of the organic compounds embedded in differentially sized PMs (e.g.,0.22-0.45,0.45-1.00,1.00-2.00,2.00-3.00,3.00-5.00,and 5.00-10.00μm) generated at different engine speeds (e.g.,1000,1500,2000,2500,and 3000 r/min) were chemically profiled in the mass range of mass to charge ratio (m/z) 50-800.Organic compounds,including alcohols,aldehydes,and esters,were detected in all the PMs tested,with varied concentration levels for each individual PM sample.At relatively low engine speeds (≤1500 r/min),the total amount of organic species embedded in PMs of 0.22-1.00 μm was greater than in PMs of other sizes,while more organic species were found in PMs of 5.00-10.00 μm at high engine speeds (≥3000 r/min),indicating that the organic compounds distributed in different sizes of PMs strongly correlated with the engine speed.The experimental data showed that the EAPCI-MS technique enables molecular characterization of PMs in exhaust,revealing the chemical dependence of PMs on the engine speeds (i.e.,the combustion conditions) of automobiles.