Molecular characteristics of water-soluble organic aerosols in a coal resource-based city revealed by FT-ICR MS: a significant role of sulfur- and phosphorus-containing compounds

Knowledge of the molecular characteristics of organic aerosols is essential for evaluating their atmospheric processes and associated environmental and health effects. However, little is known regarding the molecular characteristics of organic aerosols in coal resource-based cities. Herein, the molecular characteristics of water-soluble organic matter (WSOM) in wintertime PM2.5 during haze and heavy haze days in a typical coal resource-based city (Taiyuan, China) were analyzed using Fourier-transform ion cyclotron resonance mass spectrometry. A total of 5106 CcHhOoNnSs formulas were assigned, with m/z values predominantly concentrated in the range of 150–400 Da. The proportion of CHOS is higher than that in other cities, and a series of C7H6(CH2)0–8O5S formulas exhibited high intensities, most of which could be traced to coal combustion sources. The relative abundance of sulfur-containing organic molecules increased significantly on heavy haze days compared to haze days (30.4% vs. 25.0%) and was much higher than that observed in other cities. Additionally, CHO, CHON, and CHOS formulas consistently exhibited higher oxygen content on heavy haze days, likely due to atmospheric oxidation processes. Moreover, oxygen addition, methylation, and carboxylic acid reactions were identified as the primary possible pathways driving the transformation of primary organic aerosol into secondary organic aerosol under both haze and heavy haze conditions. Meanwhile, a total of 263 organophosphorus (OP) formulas were identified, which were predominantly distributed within the 180–550 Da range. 41.9% of assigned OP formulas contain −OPO3 or −RPO3 groups, and most OP formulas were oxidation-available with high environmental stability. Correlation analyses indicated that urban atmospheric OP may be emitted from biological sources. These results highlight the significant and distinct roles of both sulfur- and phosphorus-containing compounds in the complex atmospheric chemistry of coal resource-based urban environments.