The Sources and Atmospheric Processes of Strong Light-Absorbing Components in Water Soluble Brown Carbon: Insights From a Multi-Proxy Study of PM_2.5 in 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 (delta C-13-Delta C-14), molecular composition (Fourier transform ion cyclotron resonance mass spectrometry), and light absorption properties (UV spectrophotometry) of HULIS in PM2.5 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 C-13 (delta C-13: -25.6 +/- 0.9 parts per thousand vs. -22.4 +/- 1.0 parts per thousand) 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.