The Sources and Atmospheric Processes of Strong Light‐Absorbing Components in Water Soluble Brown Carbon: Insights From a Multi‐Proxy Study of PM2.5in 10 Chinese Cities

Humic‐like substances (HULIS) are significant contributor to the light absorption of water‐soluble brown carbon (WSBrC), which contains certain strong light‐absorbing chemical components that are not well understood, impeding the assessment of WSBrC's climate impact. China as the hotspot regions with high loading of WSBrC characterized by high light‐absorbing capacity, here, we investigated the sources and atmospheric processes (δ13C–Δ14C), molecular composition (Fourier transform ion cyclotron resonance mass spectrometry), and light absorption properties (UV spectrophotometry) of HULIS in PM2.5from 10 Chinese cities. HULIS‐C was major contributor to the light absorption coefficient (70.5 ± 6.6%) of WSBrC at 365 nm, which was more enriched with fossil sources (48.0 ± 9.0% vs. 30.3 ± 13.9%) but depleted in 13C (δ13C: −25.6 ± 0.9‰ vs. −22.4 ± 1.0‰) relative to non‐HULIS‐C. This suggests that the fossil components in HULIS are more recalcitrant to oxidative aging and exhibit higher light‐absorbing capacity, while the non‐fossil organic carbon is more likely to be oxidatively bleached into small, colorless, and highly polar molecules (i.e., non‐HULIS). Aromatic components are the major strong light‐absorbing fossil components in HULIS, dominantly originating from coal combustion (>77%). Non‐negative matrix factorization model showed that aromatic molecules from coal combustion have higher molecular weight and lower oxidation levels than biomass burning, potentially making them to be photo‐recalcitrant compounds. Our finding that coal combustion‐derived BrC maybe more persistent in the atmosphere and has greater long‐term impact on climate than BrC derived from biomass burning is an important consideration in climate models and mitigation policies. Both the loading and light‐absorbing capacity of water‐soluble brown carbon (WSBrC) are high in China, but little is known about sources and atmospheric processes of strong light‐absorbing components in WSBrC. Humic‐like substances (HULIS) account for a significant fraction of light absorption by WSBrC, which may contain strong light‐absorbing components. Through dual carbon isotopic analysis, we discovered that HULIS‐C from 10 Chinese cities had relatively higher fossil sources contribution but lower δ13C values compared to non‐HULIS‐C. This indicates that the fossil components in HULIS not only possess a higher light‐absorbing capacity but are also more resistant to oxidative aging than non‐fossil components. The higher light‐absorbing capacity and resilience of fossil components can be attributed to the enrichment of aromatic compounds derived from coal combustion. Using the non‐negative matrix factorization model, we found that coal combustion tends to generate aromatic compounds with higher molecular weight and lower oxidation‐to‐carbon (O/C: 0.31 ± 0.03 vs. 0.50 ± 0.07) ratios than biomass burning. This characteristic may contribute to the recalcitrant property of coal combustion‐derived HULIS. Our findings highlight that coal combustion‐derived BrC may persists longer in the atmosphere and has a greater long‐term impact on climate than that derived from biomass burning. The aromatic components with strong light‐absorbing capacity in HULIS mainly originated from coal combustion, enhancing light‐absorbing capacity of HULIS‐C and making it more recalcitrant to oxidative aging.