Light-absorbing and molecular composition characteristics of brown carbon in urbantunnel

TAN Tan-tan; QIANG Kai-long; LIU Si-rui; HUANG Yu-qiu; LI Yan-peng

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China Environmental Science ›› 2026, Vol. 46 ›› Issue (1) : 109-119.

Air Pollution Control

Light-absorbing and molecular composition characteristics of brown carbon in urbantunnel

TAN Tan-tan¹, QIANG Kai-long¹, LIU Si-rui¹, HUANG Yu-qiu¹, LI Yan-peng^1,2

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Abstract

Taking the typical urban tunnel (Jinhua Tunnel) in Xi'an as the research object, the light-absorption characteristics, molecular composition, and impact on visibility of brown carbon (BrC)from vehicle emissionswere investigated by employing aerosol size distribution measurements, PM_2.5 chemical speciation, combined withUV-vis spectrophotometry, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that aerosols within the tunnel were predominantly ultrafine particles (0.3~0.6μm), with the highest number concentration (2.38×10⁵#/cm³) observed in the internal section, consistent with traffic flow patterns.OC (31.11μg/m³) and EC (7.97μg/m³)concentrations in PM_2.5 were significantly higher than those in the open atmosphere.Elevated levels of SO₄^2-and Ca²⁺ indicated the synergistic effect of primary vehicle emissions and secondary aerosol formation. Optical analysis revealed a high BrC absorption coefficient of 329.2 Mm^-1 in the UV region (200nm) during weekends. At 550nm, the absorption contribution to total extinction was 0.32%. However, this contribution might be underestimated because the full visible spectrum and particle size effects were not considered. At the molecular level, 12548 molecular formulas were identified, primarily categorized as CHO (57.7%), CHOS (19.6%), and CHON (17.3%) compounds. CHON compounds exhibited strong light absorption (34.5% with DBE/C>0.5), while CHONS compounds, characterized by extremely low volatility (91.3% identified as ELVOCs), were more likely to accumulate in the particle phase. This study elucidated the relationships between molecular conjugation, heteroatom interactions, and the light-absorbing capacity of BrC within the tunnel. The visibility effects were quantified, providing a data basis for the refined management of traffic-related BrC, and for improving tunnel air quality and optimizing ventilation and lighting conditions.

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tunnel / brown carbon / lightabsorbing / molecular composition / visibility

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TAN Tan-tan, QIANG Kai-long, LIU Si-rui, HUANG Yu-qiu, LI Yan-peng. Light-absorbing and molecular composition characteristics of brown carbon in urbantunnel[J]. China Environmental Science. 2026, 46(1): 109-119

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