Degradation of anhydro-saccharides and the driving factors in real atmospheric conditions: a cross-city study in China

Abstract. Anhydro-saccharides (levoglucosan, mannosan, and galactosan), as important components of organic aerosol, have been widely used as molecular markers for biomass burning. Previous studies have shown that levoglucosan degrades in the atmosphere, but most of the results are derived from laboratory experiments, little is known about the decay rates and their driving factors in the real complex ambient environment. In this study, a Thermal Desorption Aerosol Gas Chromatography-Mass Spectrometry (TAG-GC/MS) was utilized to collect PM2.5-bound saccharides in three typical cities across the major city clusters in eastern China (Zibo, North China Plain; Changzhou, Yangtze River Delta; and Hong Kong, Pearl River Delta region) during the autumn and winter seasons, with bihourly time resolution. A total of 31, 21, and 69 valid decay days were identified for Zibo, Changzhou, and Hong Kong, respectively. With the relative rate constant method, we found the daytime (08:00–16:00 LST) decay rate of levoglucosan was fastest in Changzhou, reaching 0.13 ± 0.05 h−1 (with a range of 0.01–0.55 h−1), and the maximum decay rates of mannosan (0.14 ± 0.05 h−1, range: 0.04–0.29 h−1) and galactosan (0.15 ± 0.06 h−1, range: 0.04–0.33 h−1) were observed in Hong Kong. Results from the generalized additive model indicate that the daytime decay rate of anhydro-saccharides is primarily influenced by aerosol liquid water content, relative humidity, and atmospheric oxidation capacity, while temperature and solar surface radiation also contribute to an increase in the decay rates. This study provides valuable field data on the degradation rates of saccharides in real ambient environments and demonstrates that their degradation results are driven by the combined effects of multiple oxidation pathways.