Measurement of levoglucosan and its application in source apportionment of humic-like substances in PM_2.5 in the Pearl River Delta region

Abstract

Levoglucosan, mannosan, and galactosan are thermal degradation products of cellulose during biomass burning, therefore serving as molecular tracers for biomass burning aerosols. Among the three anhydrosugar species, levoglucosan is the most abundant. A method using high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) was established in this work to quantify levoglucosan, mannosan, and galactosan in aerosol samples collected on filter substrates. The method has a ppb-level limit of detection and provides reproducibility better than 3%. This method was applied to quantify the three sugar species in hundreds of ambient PM_2.5 samples in the Pearl River Delta (PRD) region. The temporal variation of levoglucosan was observed to be above 50 ng/m3 in winter (November to February) and lower in summer at all sites, reflecting the seasonal variation in biomass burning activities in this region. Significantly more biomass burning exists in the winter months primarily as a result of crop residue burning. Contribution to organic carbon from biomass burning was estimated using levoglucosan as the tracer and found to be usually below 10% in summer while in winter usually above 10%. This work clearly shows that biomass burning is a significant PM_2.5 source in PRD and controlling this source is needed to reduce PM_2.5 level. The mannosan/K⁺ vs levoglucosan/K⁺ ratio-ratio plot was used to examine the degradation of the organic tracer, levoglucosan, in the ambient samples. These ratios would decrease with increasing oxidation of levoglucosan as biomass burning particles transport away from their emission sources. The tracer measurement data showed that levoglucosan effective degradation rate is similar to the degradation rates with OH derived in chamber studies. This work provides the first field measurements-based evidence for atmospheric degradation of the organic tracer levoglucosan. The anhydrosugar tracers for the biomass burning source are used together with other aerosol constituents for source apportionment of HUmic-LIke Substances (HULIS), a mixture of organic species with characteristics similar to humic and fulvic acids and often accounting for a dominant part of water-soluble organic carbon in PM_2.5. The source analysis was carried out using a receptor modeling approach, namely positive matrix factorization (PMF), for samples collected at three sites in PRD throughout 2009. The results showed that biomass burning source was an important contributor to HULIS, accounting for 20-50% at the three monitoring sites in winter. Another significant source for HULIS is secondary formation processes. Further characterization of HULIS from biomass burning is suggested.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology