Impact of fossil and non-fossil sources on the molecular compositions of water soluble humic-like substance in PM_2.5 at a suburb site of Yangtze River Delta, China

Abstract. Atmospheric humic-like substances (HULIS) affect global radiation balance due to its strong light absorption at the ultraviolet wavelength. The potential sources and molecular compositions of water soluble HULIS at a suburb site of Yangtze River Delta from 2017 to 2018 were discussed based on the radiocarbon (¹⁴C) analysis combining the Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) technique in this study. The ¹⁴C results showed that the averaged non-fossil source contributions to HULIS were 39 ± 8 % and 36 ± 6 % in summer and winter, respectively, indicating that both the fossil and non-fossil sources played important roles in the formation of HULIS. The Van Krevelen diagrams obtained from the FT-ICR MS results showed that the proportions of tannins-like and carbohydrates-like groups were higher in summer, suggesting significant contribution of HULIS from biogenic secondary organic aerosols (SOA). The higher proportions of condensed aromatic structures in winter suggested the increasing anthropogenic emissions. Molecular composition analysis on the CHO, CHON, CHOS, and CHONS subgroups showed the relatively higher intensities of high O-containing macromolecular oligomers in CHO compounds in summer, further indicating stronger biogenic SOA formation in summer. High-intensity phenolic substances and flavonoids which were related to biomass burning and polycyclic aromatic hydrocarbons (PAHs) derivatives indicating fossil fuel combustion emissions were found in winter CHO compounds. Besides, two high-intensity CHO compounds containing condensed aromatic ring structures (C₉H₆O₇ and C₁₀H₅O₈) identified in summer and winter samples were similar to those from off-road engine samples, indicating that traffic emission was one of the important fossil sources of HULIS at the study site. The CHON compounds were mainly composed of organonitrates or nitro compounds with significant higher intensities in winter, which was associated to enhanced formation of organonitrates due to high NO_x in winter. However, the high-intensity CHON molecular formulas in summer were referring to N-heterocyclic aromatic compounds, which were produced from the atmospheric secondary processes involving reduced N species (e.g., ammonium). The S-containing compounds were mainly composed of organosulfates (OSs) derived from biogenic precursors, long-chain alkane and aromatic hydrocarbon, further illustrating the mixed sources of HULIS and both important biogenic and anthropogenic source contributions to HULIS at the study site. These findings add to our understanding of the interaction between the sources and the molecular compositions of atmospheric HULIS.