To better understand the size-segregated chemical composition of aged organic aerosols in the western North Pacific rim, day- and night-time aerosol samples were collected in Sapporo, Japan during summer 2005 using an Andersen impactor sampler with 5 size bins: D-p < 1.1, 1.1-2.0, 2.0-3.3, 3.3-7.0, > 7.0 mu m. Samples were analyzed for the molecular composition of dicarboxylic acids, ketoacids, alpha-dicarbonyls, and sugars, together with water-soluble organic carbon (WSOC), organic carbon (OC), elemental carbon (EC) and inorganic ions. Based on the analyses of backward trajectories and chemical tracers, we found that during the campaign, air masses arrived from Siberia (a biomass burning source region) on 8-9 August, from China (an anthropogenic source region) on 9-10 August, and from the East China Sea/Sea of Japan (a mixed source receptor region) on 10-11 August. Most of the diacids, ketoacids, dicarbonyls, levoglucosan, WSOC, and inorganic ions (i.e., SO42-, NH4+ and K+) were enriched in fine particles (PM1.1) whereas Ca2+, Mg2+ and Cl- peaked in coarse sizes (> 1.1 mu m). Interestingly, OC, most sugar compounds and NO3- showed bimodal distributions in fine and coarse modes. In PM1.1, diacids in biomass burning-influenced aerosols transported from Siberia (mean: 252 ng m(-3)) were more abundant than those in the aerosols originating from China (209 ng m(-3)) and ocean (142 ng m(-3)), whereas SO42- concentrations were highest in the aerosols from China (mean: 3970 ng m(-3)) followed by marine- (2950 ng m(-3)) and biomass burning-influenced (1980 ng m(-3)) aerosols. Higher loadings of WSOC (2430 ng m(-3)) and OC (4360 ng m(-3)) were found in the fine mode, where biomass-burning products such as levoglucosan are abundant. This paper presents a case study of long-range transported aerosols illustrating that biomass burning episodes in the Siberian region have a significant influence on the chemical composition of carbonaceous aerosols in the western North Pacific rim.
