Ambient Measurements of Daytime Decay Rates of Levoglucosan, Mannosan, and Galactosan

Quantifying atmospheric degradation of levoglucosan, a highly specific molecular marker for biomass burning (BB) aerosols, is essential in assessing impacts of BB on air quality and climate. However, the stability of levoglucosan in the atmosphere has only been evaluated through laboratory experiments or model simulations. Here, we investigated the atmospheric stability of levoglucosan and its two co-emitting isomers, mannosan and galactosan, utilizing bihourly measurement data at a suburban site in Hong Kong over a period of 4 months. The concentrations of the three saccharides, upon normalization by BB-derived potassium, displayed a distinct and consistent depletion during daytime hours from 08:00 to 16:00, which follows an exponential decay pattern. We calculated individual daytime decay rate constants of the three saccharides by utilizing their day-by-day decay kinetics. The decay rates of mannosan and galactosan were 34% and 40% higher than that of levoglucosan. The estimated lifetime of levoglucosan was 13 ± 7 hr, ranging from 3 to 46 hr under the atmospheric condition of temperature of 22 ± 5°C, and a relative humidity of 55 ± 14%. This work provides the first kinetic data of levoglucosan degradation derived from real ambient measurements. They are valuable for refining model predictions concerning BB aerosol impacts on local, regional, and global scales.