Rapid Levoglucosan Oxidation as a Key Process in Biomass Burning Aerosol Aging

The globally increasing wildfires significantly impact air quality, public health, and the climate. Extensive studies have focused primarily on the chemical transformation of the volatile gases and secondary aerosol formation in wildfires, but organic aerosol evolution and its impacts on aged plumes remain poorly understood. Here, we performed laboratory multiphase oxidation of levoglucosan aerosol, the single largest primary organic aerosol component in wildfires, by hydroxyl radicals (^•OH), and we quantified the oxidation products using thermal desorption chemical ionization mass spectrometry coupled with hydrogen-deuterium exchange. We show that levoglucosan undergoes unexpectedly rapid oxidation under atmospherically relevant ^•OH concentrations, driven by a combination of condensed-phase bimolecular autoxidation, ring-breaking reactions, and α-OH-peroxy radical unimolecular reactions. This chemistry efficiently produces highly oxygenated products and gaseous formic acid in high yields. These products have important implications for aged biomass burning aerosol composition and properties. Aircraft observations of western U.S. wildfires confirm rapid levoglucosan decay with kinetics consistent with laboratory results. The aircraft data suggest that levoglucosan oxidation explains 15-46% of the observed formic acid formation in aged wildfire plumes, representing a major hitherto unrecognized source. These findings reveal critical gaps in the current understanding of wildfire chemistry and highlight the important role of multiphase aerosol oxidation.