Fine Brown Carbon Aerosol Emitted by Wood Pyrolysis - New Harmful Tracers and Cytotoxicity in Human Lung Cells

Biomass burning (BB) is a major global source of light-absorbing organic aerosols, the so-called atmospheric brown carbon (BrC), which negatively affects human health. However, the interplay between the molecular composition and toxicity of BB emissions remains ambiguous. In this study, cytotoxicity analyses and molecular-level characterization of fine BrC generated by wood pyrolysis at 350 °C, representative of smoldering combustion during vegetation fires, were performed. Solvent-extractable BrC was analyzed using UHPLC-ESI-ToF-MS coupled with a nontarget analysis (NTA) workflow, and cytotoxicity was evaluated on the A549 (CCL-185) human lung carcinoma cell line. In the dissolved BrC, 776–1132 unique molecular features were detected. The aqueous fraction was dominated by lignin-derived phenols and aromatic acids, whereas organic solvents (methanol and acetone) preferentially extracted less oxidized, more hydrophobic compounds, such as terpenoid- and sterol-like species, including several emerging BB tracers. Furthermore, reduction in metabolic activity, cellular membrane damage, death phenotype, and morphology were investigated. Cytotoxicity of solvent-extractable BrC increased in the order water < methanol < acetone. Descriptor-based screening showed that cytotoxicity was likely associated with lipophilic compounds capable of membrane partitioning and redox-active moieties, contributing to oxidative stress. The harmful BrC tracers identified included benzoic and cinnamic acids, methoxyphenols, quinols, and natural compounds such as flavonoids, coumarins, indoles, and plant sterols. The highest contributors to baseline narcosis were 3-hydroxybenzoic, salicylic, p-coumaric acids, benzene-1,2,4-triol, methylxanthoxylin, methyl ester of heteropeucenin, coniferylaldehyde, and agelasine. These results demonstrate that chemical composition strongly governs BrC toxicity. Moreover, by combining comprehensive molecular-level characterization with simple descriptors, it is possible to identify potentially harmful compounds within complex BB emissions. The presented approach provides a framework for identifying potentially harmful components of BrC aerosols and supports improved assessment of their health impacts.