Molecular fingerprints reveal traffic PM_2.5 complexity beyond a controlled gasoline tailpipe molecular profile

Abstract. Controlled gasoline tailpipe profiles may not fully represent traffic particulate matter with aerodynamic diameter ≤2.5 μm (PM_2.5) under real road conditions. The molecular compositions of tunnel and controlled tailpipe PM_2.5 were compared using high-resolution mass spectrometry. Formulas common to both profiles were concentrated at low m/z and low to intermediate carbon numbers and were dominated by CHO and CHON. In contrast, molecular formulas unique to the tunnel showed greater formula richness and had higher proportions of CHOS and CHONS formulas than those unique to the controlled tailpipe profile. Tailpipe coverage ratios were high for CHO and CHON formulas in both ionization modes (0.77–0.82 and 0.84–0.92, respectively) but much lower for the combined CHOS and CHONS class (0.10–0.15). Hierarchical clustering resolved a dominant shared CHO/CHON baseline and two smaller domains concentrated in tunnel samples, characterized respectively by organosulfur formulas and by candidate formulas associated with reported tire wear chemicals together with a low molecular mass CHN response in ESI+. Thus, the controlled gasoline tailpipe profile captured the dominant shared CHO/CHON molecular baseline but incompletely represented sulfur-rich and other chemically distinct molecular domains retained in tunnel aerosol. Comprehensive molecular characterization of traffic PM_2.5 therefore requires controlled tailpipe measurements together with molecular profiles representing non-tailpipe vehicle materials and near-road processing.