Effect of Initial Particle Acidity on the Heterogeneous Oxidation of Levoglucosan

Abstract

Levoglucosan (LEV) has long been used as a tracer for biomass burning emissions due to its presumed atmospheric stability. However, recent studies suggest that its degradation rate varies significantly under different environmental conditions. This work investigates the heterogeneous oxidation of LEV by hydroxyl radicals (•OH) and the influence of initial particle acidity on its degradation. Experiments were conducted in a smog chamber using internally mixed LEV particles with either ammonium sulfate (AS) or ammonium bisulfate (ABS). The chemical composition of the particle was monitored using a high-resolution aerosol mass spectrometer (HR-AMS) during the experiment. The results show that low-initial pH particles accelerate LEV degradation by 7–8 times relative to high-initial pH particles, with mean second-order reaction rate constants of 4.46 × 10–11 and 6.03 × 10–12 cm3 molecule–1 s–1, respectively. Acidic conditions also favor the formation of highly oxygenated products, some of which likely fragment and evaporate into the gas phase. These findings imply that acidic particles significantly reduce LEV’s atmospheric lifetime (from 40 to 3 h) and its reliability as a biomass burning tracer, highlighting the need to incorporate acidity effects into atmospheric models.