New water-soluble, toxic tracers of wood burning identified in fine brown carbon aerosol using a non-target approach

Abstract. The molecular composition of water-soluble fine (PM₃) brown carbon aerosol (BrC_aq) generated by combustion of wood was studied with ultra-performance liquid chromatography coupled with electrospray ionization time-of-flight mass spectrometry (UPLC-ESI-ToF/MS) using a non-target analysis (NTA) workflow. The NTA analysis workflow based on MS-DIAL and MS-FINDER showed the best performance of the five software tested. Structures of 361 out of the 420 water-soluble organics in BrC were tentatively identified for the first time. The total emission of fine, water-soluble BrC_aq was approx. 1 g per kg of wood burned, comparable with the emission factors of some semi-volatile organics from open biomass burning. Potential precursors of aqueous secondary organic aerosols (_aqSOAs) and toxic molecules were selected among the newly identified molecules.

The newly identified harmful tracers of fine BrC included plant and wood care products, alkaloids, and fungal metabolites. Fungal metabolites were also identified among the potential precursors of _aqSOAs with high Henry's law constants values, alongside natural compounds occurring in roots and leaves, diterpenoids, flavonoids, anthraquinones, and coumarins. The release of these natural and man-made compounds is possible during wildfires and domestic uses of biomass. The atmospheric lifetimes calculated for the newly identified precursors of _aqSOAs showed that natural dyes, bacterial and fungal metabolites, and (aromatic) glucosides can undergo aqueous OH oxidation in cloud water. Such molecules can produce low-volatility products without decomposing due to their large carbon backbones. Many new potential chromophores were also identified in BrC, including natural dyes and molecules with conjugated double bonds and aromatic rings.