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

Abstract 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 (δ 13 C–Δ 14 C), molecular composition (Fourier transform ion cyclotron resonance mass spectrometry), and light absorption properties (UV spectrophotometry) of HULIS in PM 2.5 from 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 13 C (δ 13 C: −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.