Aqueous-phase photooxidation of levoglucosan – A mechanistic study using aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS)

  • Zhao R
  • Mungall E
  • Lee A
 et al. 
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Abstract

Levoglucosan (LG) is a widely employed tracer for biomass burning (BB). Recent studies have shown that LG can react rapidly with hydroxyl (OH) radicals in the aqueous phase despite many mass balance receptor mod-els assuming it to be inert during atmospheric transport. In the current study, aqueous-phase photooxidation of LG by OH radicals was performed in the laboratory. The reaction kinetics and products were monitored by aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS). Approximately 50 reaction products were detected by the Aerosol ToF-CIMS during the photooxidation experi-ments, representing one of the most detailed product studies yet performed. By following the evolution of mass defects of product peaks, unique trends of adding oxygen (+O) and removing hydrogen (−2H) were observed among the prod-ucts detected, providing useful information for determining potential reaction mechanisms and sequences. Additionally, bond-scission reactions take place, leading to reaction inter-mediates with lower carbon numbers. We introduce a data analysis framework where the average oxidation state (OS c) is plotted against a novel molecular property: double-bond-equivalence-to-carbon ratio (DBE/#C). The trajectory of LG photooxidation on this plot suggests formation of polycar-bonyl intermediates and their subsequent conversion to car-boxylic acids as a general reaction trend. We also determined the rate constant of LG with OH radicals at room tempera-ture to be 1.08 ± 0.16 × 10 9 M −1 s −1 . By coupling an aerosol mass spectrometer (AMS) to the system, we observed a rapid decay of the mass fraction of organic signals at mass-to-charge ratio 60 (f 60), corresponding closely to the LG de-cay monitored by the Aerosol ToF-CIMS. The trajectory of LG photooxidation on a f 44–f 60 correlation plot matched closely to literature field measurement data. This implies that aqueous-phase photooxidation might be partially contribut-ing to aging of BB particles in the ambient atmosphere.

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Authors

  • R. Zhao

  • E. L. Mungall

  • A. K Y Lee

  • D. Aljawhary

  • J. P D Abbatt

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