Molecular-Level Characterization of Carbonaceous Aerosols via Coupled Thermal-Optical Carbon Analysis and Ultra-High-Resolution FT-ICR Mass Spectrometry

Comprehensive molecular-level characterization of carbonaceous aerosols is essential for understanding their environmental transformations, optical properties, and health effects. Here, we present a hyphenated analytical approach combining a thermal-optical carbon analyzer (TOCA) with atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI-FT-ICR-MS) for the direct, temperature-resolved molecular analysis of particulate matter (PM). The method enables the simultaneous quantification and chemical speciation of evolved organic and elemental carbon fractions, following the IMPROVE_A protocol, thereby distinguishing between organic and elemental carbon fractions. The system achieves subppm mass accuracy and resolving power exceeding 2.6 × 105 at m/z 400, ensuring reliable differentiation of critical mass splits such as C3 vs SH4 (3.4 mDa) and allowing the unambiguous assignment of thousands of sum formulas across complex aerosol matrices. Compared with resonance-enhanced multiphoton ionization time-of-flight MS, the APPI-FT-ICR-MS coupling expands the accessible chemical space toward oxygenated and sulfur-containing species while maintaining robust performance and reproducibility. Application to representative aerosol sources, including ship diesel emissions, residential wood combustion, and ambient PM from Beijing, demonstrated distinct molecular fingerprints. Ship emissions were dominated by CH- and CHS-class species with double bond equivalents (DBE) ≤20, indicative of alkylated polycyclic aromatics, whereas wood combustion particles contained more oxidized CHO- and CHNO-classes with enhanced aromaticity and nitrogen incorporation. Ambient aerosols exhibited mixed signatures with elevated OC/EC ratios (∼3–4) and high contributions from oxygenated and nitrate-bearing compounds, reflecting secondary formation and seasonal variability. This integrated TOCA-APPI-FT-ICR-MS approach provides unprecedented resolution of thermally evolved aerosol species, bridging operational OC/EC quantification with molecular-level speciation. By correlating volatility, aromaticity, and oxidation state across source types, it establishes a powerful framework for tracing aerosol origins, transformation processes, and their environmental and health relevance.