Overlooked\nTrace Molecules in Organic Aerosol Revealed\nby Gas Chromatography–Orbitrap Mass Spectrometry

Molecular characterization of organic\naerosol (OA) is crucial for\nunderstanding its sources and atmospheric processes. However, the\nchemical components of OA remain not well constrained. This study\nused gas chromatography–Orbitrap mass spectrometry (GC-Orbitrap\nMS) and GC-Quadrupole MS (GC-qMS) to investigate the organic composition\nin PM<sub>2.5</sub> from Xi’an, Northwest China. GC-Orbitrap\nMS identified 335 organic tracers, including overlooked isomers and\nlow-concentration molecules, approximately 1.6 times more than GC-qMS.\nThe “molecular corridor” assessment shows the superior\ncapability of GC-Orbitrap MS in identifying an expansive range of\ncompounds with higher volatility and oxidation states, such as furanoses/pyranoses,\ndi/hydroxy/ketonic acids, di/poly alcohols, aldehydes/ketones, and\namines/amides. Seasonal variations in OA composition reflect diverse\nsources: increased di/poly alcohols in winter are derived from indoor\nemissions, furanoses/pyranoses and heterocyclics in spring and summer\nmight be from biogenic emissions and secondary formation, and amides\nin autumn are probably from biomass burning. Integrating partial least\nsquares discriminant analysis (PLS-DA) and potential source contribution\nfunction (PSCF) models, the source similarities and differences are\nfurther elucidated, highlighting the role of local emissions and transport\nfrom southern cities. This study offers new insights into the OA composition\naided by the high mass resolution and sensitivity of GC-Orbitrap MS.