Oxygen isotope analysis of levoglucosan, a tracer of wood burning, in experimental and ambient aerosol samples.

Abstract

Levoglucosan is formed from cellulose during biomass burning. It is therefore often used as a specific tracer to quantify the contribution of wood burning to the aerosol loading. The stable oxygen isotope composition (δ18 O value) of biomass is determined by the water cycle and varies regionally, and hence the δ18 O value of levoglucosan could help to identify source regions of organic aerosols. After solvent extraction of the organic fraction and concentration steps, a recently developed methylation derivatisation technique was applied on experimental (i.e. controlled wood-burning experiments) and on ambient aerosol samples from Switzerland and Lithuania. The method achieves sufficient compound separation for isotope analysis in atmospheric particulate matter, enabling δ18 O analysis of levoglucosan by gas chromatography/pyrolysis-isotope ratio mass spectrometry (GC/Pyr-IRMS), with a precision better than 1.0 ‰ and an accuracy of 0.3 ‰. The δ18 O value of the levoglucosan released during controlled wood-burning experiments was not significantly different from the cellulose δ18 O values, which implies very little or no isotope fractionation during wood burning under the given conditions. While the δ18 O values of levoglucosan in Swiss samples were as expected for the source region, those in Lithuania were 1-4 ‰ lower than expected. This may be due to differences in vegetation (grass vs wood) or burning conditions (high vs low temperatures). Low oxygen isotope fractionation between cellulose and levoglucosan and clear differences in levoglucosan δ18 O values between the Swiss and Lithuanian ambient samples demonstrate that our new method is useful for source appointment studies on wood-burning-derived aerosols.