Can levoglucosan be used to characterize and quantify char/charcoal black carbon in environmental media?
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abstract
The heterogeneous nature of pyrogenic carbon, also known as black carbon (BC), still stands in the way of comprehensive determination of the total proportion of combustion derived carbon in environmental mixtures. Char and charcoal occur mostly in the low temperature region of the BC continuum and as such are characterized using specific chemical and physical properties that set them apart from the more condensed forms of high temperature BC species (i.e., soot). This study aimed to assess the feasibility of using the molecular marker levoglucosan, exclusively produced from hemicellulose/cellulose combustion, to characterize and quantify char BC in the environment. A second objective was to compare the results from the molecular analysis with those from acid dichromate oxidation in terms of char BC characterization. First, we examined levoglucosan levels in a suite of BC materials derived from biomass burning and fossil fuel combustion, as well as non-BC materials (coal and melanoidin). The results confirm that levoglucosan is exclusively in the char BC materials, derived from biomass combustion. However, a large variability in levoglucosan yield was observed among the different char BC materials. To understand the causes of such variability, we analyzed levoglucosan in three series of laboratory-produced char samples (n = 23) made from honey mesquite (Prosopis glandulosa Torr.), cordgrass (Spartina spartinae) and loblolly pine (Pinus taeda) under controlled combustion conditions (150-1050 C, 0.5-5 h). The results show that temperature is the most influential factor affecting levoglucosan yield in char. Combustion duration, on the other hand, had no influence on yield. Notably, levoglucosan was only detectable in low temperature char samples (150-350 C), with maximum yield obtained from samples produced at 250 C, regardless of plant species. Large differences in levoglucosan yield were also observed across species. This taxonomic difference could not be corrected simply on the basis of hemicellulose and cellulose content, indicating that the relationship between combustion and levoglucosan formation is a complex function of many factors. Our observations also demonstrate that levoglucosan can hardly be used quantitatively for the determination of char BC in environmental samples since such samples often represent mixtures of different chars characterized by variable levoglucosan yield. Acid dichromate oxidation, as opposed to the levoglucosan method, detected mid- to high temperature char BC (350 C) but not low temperature char ( 250 C). Our results indicate that the overlap in the analytical windows of the two methods is limited to the upper limit of the latter. The study demonstrates that levoglucosan can serve as a good qualitative indicator for the presence of char produced under low temperature conditions in soil, sediments, and ultrafine aerosols. Although levoglucosan is not a proper quantitative proxy for char BC content in the environment, this molecular marker has a unique role in the current BC methods continuum because it traces combustion material often missed with most oxidation-based methods. 2008 Elsevier Ltd. All rights reserved.
