Environ. Sci. Technol. 2006, 40, 4983-4989
Water-Soluble Oligomer Formation from Acid-Catalyzed Reactions of Levoglucosan in Proxies of Atmospheric Aqueous Aerosols BRYAN J. HOLMES AND GIUSEPPE A. PETRUCCI* Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125
Herein is reported the first laboratory observation of the oligomerization of levoglucosan studied under atmospherically relevant conditions. Oligomers up to 1458 Da (9-mer) were measured by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. A rational mechanism is proposed based on both the acid-catalyzed cationic ringopening of levoglucosan and nucleophilic attack of ROH from levoglucosan on the hemi-acetal carbon to produce pyranose oligomers through the formation of glycosidic bonds. Oligomer formation is further supported by attenuated total reflectance Fourier transform infrared spectroscopy. Levoglucosan is a viable tracer for biomass burning aerosols, and the observed products may serve as secondary tracers for these types of aerosols, possibly providing additional information to facilitate source apportionment and better understand atmospheric processing of the aerosol parcel. Also, the processes supported here may contribute to the saccharide character of humic-like substances, which are proposed to be formed through the atmospheric processing of biomass burning aerosols.
Introduction Biomass burning, a major source of organic carbon, soot, and particulates in the atmosphere (1,2), is a global phenomenon with over 80% of burning occurring in tropical regions such as the Amazon basin (3). Chemical processing of these emissions in the atmosphere impacts their subsequent roles in atmospheric processes by, for example, lowering activation barriers to form cloud droplets and significantly altering optical extinction properties. Emissions from biomass burning are also a potential source of humiclike substances (HULIS) in the atmosphere (4-8). HULIS are a class of macromolecular organic compounds that exist in atmospheric aerosols, cloudwater, and fogwater, which many studies have shown to possess a significant saccharide character (9). HULIS are important because they may affect aerosol properties such as hygroscopicity and light absorption (9). Recently, it was proposed that HULIS could be formed in cloudwater through aqueous-phase reactions of polar aromatic compounds with hydroxyl radicals (10). Because of its high reactivity, the hydroxyl radical is known to be the most important atmospheric oxidant as it governs the oxidation and removal of most trace gases (11) as well as various organic species in cloudwater (12). Within aqueous * Corresponding author phone: (802)656-0957; fax: (802)656-8705; e-mail:
[email protected]. 10.1021/es060646c CCC: $33.50 Published on Web 07/07/2006
2006 American Chemical Society
aerosols, the Fenton reaction may generate significant amounts of hydroxyl radicals through the reaction of hydrogen peroxide with Fe(II) under acidic conditions (12). To assess the effects of biomass burning emissions on climate, unique chemical tracers are necessary for source apportionment. One compound, levoglucosan (1,6-anhydroR-D-glucopyranose), is produced in large quantities directly from the pyrolysis and combustion of cellulose (13-16), potentially making it an ideal molecular marker for biomass burning aerosols (13, 17, 18). Due to its high water solubility and low vapor pressure (19), levoglucosan is a common component of smoke aerosols, which are hygroscopic at a young age (20-22). Because levoglucosan is a unique product of foliar fuel combustion, it is also a viable molecular tracer for biomass burning in urban airsheds (23, 24) and sediments (25). To be an effective marker for long-range transport, levoglucosan must be chemically stable on an atmospherically relevant time scale compared with its loss from aerosol deposition to the landscape. To date, chemical processes that may remove levoglucosan from smoke aerosol remain unknown; therefore, the absence of measurable levoglucosan in an aerosol parcel should not necessarily negate apportionment of the aerosol to a biomass burning source. It is therefore the focus of this study to understand the chemical transformations of levoglucosan under atmospherically relevant conditions to better evaluate levoglucosan as a useful marker to characterize biomass burning aerosols. In cases where there are significant chemical removal pathways of levoglucosan, reaction products are generated that, we suggest, may serve as secondary tracers for the aerosol parcel. Even in cases where levoglucosan is measured, the secondary tracers may provide additional information regarding the age of the aerosol parcel, its source, and the atmospheric processing that it has undergone. This laboratory study serves to elucidate upon chemical loss mechanisms for levoglucosan as well as to identify potential secondary tracers within simulated cloudwater. Oligomerization via acid-catalyzed processes, including aldol condensation and reaction through the dehydration of hemi-acetal functionalities, has been observed previously in laboratory studies on secondary organic aerosol formation (26-28); therefore, we hypothesize that, in addition to hydroxyl radical reactions, acid-catalyzed oligomerization may also be a significant removal pathway for levoglucosan in atmospheric aqueous aerosols. Removal of free levoglucosan by acid-catalyzed oligomerization is facilitated by the naturally acidic pH of atmospheric aqueous aerosols. In this study, matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) and attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy were used to measure the reactivity of levoglucosan and identify molecular products in solutions designed to serve as proxies for atmospheric aqueous aerosols (10), providing a qualitative description of possible atmospheric aerosol processes leading to the chemical removal of levoglucosan.
Experimental Section All reagents were used as supplied by the manufacturer. Initial reagent concentrations were the same for all experiments: levoglucosan (10-3 M) (99%, Alfa Aesar), hydrogen peroxide (10-4 M) (30% in water, Acros), anhydrous ferric chloride hexahydrate (5 × 10-6 M) (99%, Fisher), and sulfuric acid (to bring the solution pH to 4.5) (96.1%, Mallinckrodt). The concentration of hydrogen peroxide was based on experiVOL. 40, NO. 16, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of the Five Experimental Conditions Used to Investigate the Aqueous-Phase Chemistry of Levoglucosan in Bulka reaction
levoglucosan
Fe3+
H2O2
H2SO4
A B C D E
X X X
X
X
X X X
X
X X X X
X
a Reactant concentrations: levoglucosan, 1 × 10-3 M; FeCl ‚6H O, 5 3 2 × 10-6 M; H2O2, 1 × 10-4 M; H2SO4, pH ) 4.5.
mental measurements in cloudwater (29). The iron concentration was based on both experimental (30) and model studies (12) of cloudwater. The pH of cloud droplets ranges from 4 to 6; 4.5 is taken as a typical value (12). Reactions were performed on a 1 L scale in 18 MΩ water (Milli-Q, model gradient A10, TOC
