Atmospheric Brown Carbon from Biofuel Pyrolysis: Comparative Analysis of Dung and Wood Sources

Abstract

Brown carbon (BrC), a class of light-absorbing organic compounds produced during biomass burning, plays an important role in atmospheric radiative transfer and air quality. However, accurate representation of BrC in atmospheric models remains limited by insufficient understanding of its complex molecular composition and variable optical properties. In this study, we present a comparative molecular-level characterization of BrC chromophores in laboratory-generated organic aerosol (OA) mixtures representing pyrolysis components of wood-burning (WBOA) and dung-burning (DBOA) emissions, corresponding to two commonly used categories of residential biomass fuels. Using a hyphenated high-performance liquid chromatography–photodiode array–high-resolution mass spectrometry (HPLC-PDA-HRMS) platform, we analyzed these mixtures alongside 100 BrC reference compounds and evaluated the composition, volatility, and light-absorbing properties of their constituent species. WBOA was found to be enriched in CHO-class chromophores primarily derived from lignin decomposition, while DBOA contained a higher abundance of CHON and CHN classes corresponding to reduced N-containing organic compounds (RNOCs). N-heterocyclic compound classes, such as pyrrole- and pyrazine-containing species, were plausibly detected in the DBOA mixture. Double bond equivalency analysis identified a substantial fraction of potential BrC chromophores in both mixtures, although their chemical classes, structural features, and optical properties differed significantly. Volatility basis set modeling revealed that WBOA components are less volatile and remain in the particle phase under a wider range of atmospheric conditions, while DBOA constituents partition more readily to the gas phase. These findings underscore the need for more detailed treatment of BrC variability in chemical transport models, especially in regions where dung is a dominant household fuel. This study advances molecular-level understanding of BrC and highlights the importance of fuel type in shaping its atmospheric behavior.