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

R Zhao,J P D Abbatt,D Aljawhary,E L Mungall,A K Y Lee

doi:10.5194/acp-14-9695-2014

Abstract

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 models 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 experiments, 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 products detected, providing useful information for determining potential reaction mechanisms and sequences. Additionally, bond-scission reactions take place, leading to reaction intermediates with lower carbon numbers. We introduce a data analysis framework where the average oxidation state (OSc) 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 polycarbonyl intermediates and their subsequent conversion to carboxylic acids as a general reaction trend. We also determined the rate constant of LG with OH radicals at room temperature to be 1.08 ± 0.16 × 109 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 (f60), corresponding closely to the LG decay monitored by the Aerosol ToF-CIMS. The trajectory of LG photooxidation on a f44–f60 correlation plot matched closely to literature field measurement data. This implies that aqueous-phase photooxidation might be partially contributing to aging of BB particles in the ambient atmosphere.

Highlights

Biomass burning (BB) is a major source of atmospheric particles and volatile organic compounds (VOCs)
We propose that multiple oxidized by hydroxyl (OH) oxidation eventually lead to a group of polycarbonyl intermediates that exhibit high double bond equivalence (DBE)-to-carbon ratio (DBE/#C) values
This study presents the first detailed study of levoglucosan (LG) oxidation by OH radicals in the aqueous phase by online mass spectrometry: aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-chemical ionization mass spectrometer (CIMS))

Summary

Introduction

Biomass burning (BB) is a major source of atmospheric particles and volatile organic compounds (VOCs). Emitted VOCs and primary organic aerosol (POA) are subject to subsequent atmospheric processing, leading to formation of secondary organic aerosol (SOA) (Jimenez et al, 2009). Apportionment of BB is commonly done using chemical tracers (Simoneit, 2002). Levoglucosan (LG) is a widely used particle-phase molecular tracer of BB (Simoneit et al, 1999), due to its source-specificity and abundance in BB aerosol. Stability is an important requirement for a molecular tracer as it is a major assumption made in chemical mass balance receptor models commonly employed for source apportionment (Schauer et al, 1996; Robinson et al, 2006).

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