Molecular Characterization of Size-Segregated Organic Aerosols in the Urban Boundary Layer in Wintertime Beijing by FT-ICR MS
Organic aerosols, a complicated mixture of organics, are important constituents of atmospheric particulate matter. However, little is known about the size distributions and vertical profiles of these compositions at a molecular level in the urban boundary layer. Here, we characterized molecular compositions of size-segregated samples collected simultaneously at two heights (8 m and 260 m above ground level) in urban Beijing during the winter of 2018. Subgroups of CHO, CHNO, CHOS, and CHNOS were characterized using a 15-T ultrahigh-resolution Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. We found that both their number and magnitudes increased with the decrease of particle size, especially for high molecular weight (HMW) compounds except CHNOS. CHNOS species also showed a increases in coarse modes presumably because alkalinity could inhibit their hydrolysis in the coarse mode. Compounds in small particles with higher O/C ratios and carbon oxidation state were possibly more aged, while the coarse particles with more lipids- and peptides-like compounds should be from fresh emissions. Moreover, as the oxidation state increases in small particles, functionalization enhanced for sulfur-containing compounds with a fracture of the benzene ring, while CHO, CHNO potentially dominated by demethylation with the ring-retaining products. It was worth noting that the common compounds with same molecular characteristics between 260 m and the ground level (8 m) accounted for more than 86% of total compounds, demonstrating that the aerosols were well mixed in urban bundary layer. Nonethless, the relative content of compounds was relatively higher at the ground-level with the impact of primary emissions, which inceased with the particles size. In addition, the compounds in the submicron particles were more oxidizing at 260 m, while that in the coarse mode was the opposite.
- This article is part of the themed collection: Air quality in megacities