Burning of agricultural residues, such as rice straw, is a sizeable source of smoke aerosol particularly in Asia. During a field study on the subtropical island of Taiwan ambient aerosol was examined for seasonal patterns in terms of size-resolved chemical composition. Ambient concentrations, size distributions and diagnostic ratios of three anhydrosugars (levoglucosan, mannosan, and galactosan), used as molecular tracers for biomass burning, are reported here. Levoglucosan concentrations showed a strong seasonal dependence in the ambient levels as well as size distributions. During the wet (summer monsoon) season levoglucosan was observed with average concentrations of 437, 882 and 1116 ng m(-3) in PM2.5, PM10 and TSP, respectively. In contrast, winter/dry season levoglucosan levels were lower with 436, 448, and 559 ng m(-3) on average in PM2.5, PM10 and TSP, respectively. The particle-size distributions were characterized by a drastically increased coarse (PM2.5-10) fraction in the summer season, which was 36 times higher than during winter, likely due to higher fuel moisture content as well as enhanced condensation, coagulation and hygroscopic growth of the relatively fresh smoke particles derived from rice straw burning. Furthermore, the relative abundance of the three anhydrosugars was examined for characteristic signatures in respect to different types of biomass. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.